Modern C++ has placed increasing emphasis on type safety and expressiveness. C++17 introduced three powerful utilities – std::optional, std::variant, and std::any – that enhance the language’s flexibility while making code safer and more self-documenting. These facilities allow programmers to represent “optional” values, to use type-safe unions for variant types, and to perform type erasure for arbitrary types, respectively, all with the rigour of compile-time type checking or controlled runtime checks. In this article, we delve into each of these C++17 features in detail, examining how they improve upon pre-C++17 techniques, demonstrate their usage with practical code examples, discuss real-world applications, and consider edge cases, performance implications, and pitfalls. We also touch on how C++20/C++23 have further refined these utilities, all while keeping a focus on C++17.

Type Safety and Flexibility in Modern C++

One of the guiding principles of modern C++ design is to make incorrect code harder to write. Features like std::optional, std::variant, and std::any embody this principle by encoding intent in the type system. Instead of relying on raw pointers, primitive unions, or void pointers (which can lead to undefined behaviour or runtime errors), these abstractions provide explicit, expressive, and safe mechanisms for common programming scenarios:

  • std::optional expresses the optional presence or absence of a value of a given type. It forces the programmer to handle the “no value” case, rather than using ad-hoc signals like nullptr or sentinel values.
  • std::variant enables a variable to hold one of several alternative types in a type-safe way. It’s essentially a discriminated union that knows which type is active, preventing mistakes like accessing the wrong member of a union.
  • std::any provides a container for a value of any type, with runtime type checking. This is a type-safe alternative to void* for scenarios where the type might only be known at runtime (using type erasure).

By leveraging these, C++17 code becomes more robust (less prone to type errors), more readable (the code clearly states intent, such as “this function may not return a value”), and often more efficient at runtime than naive approaches (due to avoiding unnecessary allocations or branches, as we’ll discuss). Let’s explore each type in turn.

std::optional – Expressing Optional Values Safely

std::optional<T> is a class template (defined in the <optional> header) that manages an optional contained value of type T. In simple terms, an optional<T> either contains a value of type T or it is empty (containing no value). This allows us to represent the concept of “maybe a T” explicitly in the type system. As the C++ reference states, “The class template std::optional manages an optional contained value, i.e. a value that may or may not be present.” In practice, this is often used for the return value of functions that may fail or otherwise not produce a value. Instead of resorting to output parameters, special return codes, or exceptions, a function can return std::optional<T> to indicate “I either have a result of type T, or I have nothing.”

Type Safety and Clarity: By using std::optional, we make the presence/absence of a value explicit. Callers must check whether the optional has a value before using it, typically via optional::operator bool() or optional::has_value(). This prevents errors like accidentally dereferencing a null pointer. It also improves readability: the intent is clear without needing to resort to comments or sentinel conventions. Indeed, std::optional is more expressive and safer than alternatives like a std::pair<T, bool> or a special out-of-band value, and it handles expensive-to-construct objects more gracefully. If an optional is disengaged (empty) and one tries to access the value, the library throws a std::bad_optional_access exception, rather than yielding undefined behaviour. This makes errors easier to catch and diagnose.

Basic Usage Example: Consider a configuration lookup function that tries to find a configuration value by key and returns an integer if found. Prior to C++17, one might return a special value (e.g. -1) or use a pointer to indicate “not found”. With std::optional<int>, the function can directly express the possibility of no result:

#include <optional>
#include <string>
#include <unordered_map>

// A simple configuration map for illustration
std::unordered_map<std::string, int> config = {
    {"max_retries", 5},
    {"timeout_seconds", 60}
};

std::optional<int> getConfigInt(const std::string& key) {
    auto it = config.find(key);
    if (it != config.end())
        return it->second;           // return a value
    return std::nullopt;             // return an "empty" optional
}

int main() {
    auto retries = getConfigInt("max_retries");
    if (retries) {
        // Safe to dereference or access the value
        std::cout << "Max retries: " << *retries << "\n";
    } else {
        std::cout << "Max retries not set.\n";
    }

    // Using value_or to provide a default if not present
    int timeout = getConfigInt("timeout_seconds").value_or(30);
    std::cout << "Timeout: " << timeout << " seconds\n";
}

In this example, getConfigInt returns an std::optional<int>. The caller checks if (retries) to see if a value is present. The *retries syntax uses optional::operator*() to retrieve the contained value when it exists. This operator is provided for convenience and behaves like dereferencing a pointer (and indeed will throw bad_optional_access if no value is present, similar to how dereferencing a null pointer would be undefined). Alternatively, value_or(30) was used to get either the stored value or a default (30 in this case) if the optional is empty. This style is clear and prevents common errors. The code reads almost like natural language: “get config… if (it has value) use it, otherwise use default.” The explicitness eliminates the need for magic numbers or separate flags.

Memory and Performance: std::optional<T> typically holds a T object in-place (without additional allocation) and a small flag to indicate whether a value is present. The size of an optional<T> is roughly sizeof(T) + 1 (with some padding for alignment). For instance, an optional<double> will hold the double directly and a boolean state. This makes it efficient – there’s no heap allocation and no indirection needed to access the value (unlike using a pointer). Additionally, an empty optional does not construct a T until a value is assigned, so it can represent “nothing” without incurring the cost of a default-constructed T. This is particularly useful for types that are expensive to initialise. In fact, the standard ensures that if an optional is disengaged, the storage for T is uninitialised, and only when a value is emplaced or assigned is the T constructed (and correspondingly destroyed on reset or destruction of the optional). Thus, optional “handles expensive-to-construct objects well” by only creating them when needed.

Edge Cases and Pitfalls: While std::optional is straightforward, there are a few things to be mindful of:

  • An optional must be checked for a value before using. If you call .value() or dereference an empty optional, a std::bad_optional_access will be thrown. This is safer than a null pointer dereference (which would be undefined behaviour), but it’s still a runtime error that should be avoided by proper checking (or using value_or, etc.). Make it a habit to handle the empty case, either with if (opt) or with the provided utility functions.
  • std::optional<T> is not a drop-in replacement for T* in all cases. Notably, optional does not allow references or array types as the template parameter. You cannot have optional<SomeClass&> directly (since references are not assignable or destructible in a way optional can manage). If you truly need an optional reference, you can use something like std::optional<std::reference_wrapper<T>> or simply a pointer. Usually, though, if you find yourself wanting an optional reference, it may indicate a design that can be refactored.
  • Copying or moving an optional will copy/move the contained T if there is one. This is usually fine, but keep in mind if T is a heavy object, copying an optional<T> involves copying that object. In such cases, consider passing references to optionals or using std::optional<some_ptr> to large structures to avoid deep copies. On the other hand, if T is cheap to move, optional is an excellent way to avoid heap allocations that a pointer might cause.
  • std::optional provides relational operators (==, <, etc.), which perform comparisons in a way that an empty optional is considered less than one with a value, etc. These are handy but be aware of the semantics (e.g., two disengaged optionals are equal). C++20 even added three-way comparison (<=>) support for optionals.
  • Monadic operations (in C++23): While C++17’s optional is somewhat minimal, C++23 introduced monadic operations like optional::and_then, transform, and or_else. These allow chaining operations on optional values in a functional style. For example, opt.and_then(f) executes a function f if the optional has a value, passing the contained value, and propagates an empty optional otherwise. These enhancements (available if you’re using C++23) can make handling optional values more concise, but even in C++17 one can achieve similar effects with manual checks or by utilising value_or and ternary operators.

Real-World Uses: std::optional is broadly useful wherever a value may be contextually optional. Common scenarios include configuration settings (a setting might not be provided, so use an optional), parsing (if a parse might fail, return an optional instead of a special value), and interactions with hardware or OS (where an API might return a value or indicate the absence of it). For instance, the filesystem library in C++17 uses std::optional<uintmax_t> for file_size() – it returns a size if available, but for directories (where size is not applicable) it returns an empty optional, rather than overloading the function or using an out parameter. Overall, std::optional increases clarity: the programmer and the compiler know when a value might not be there, and the code must handle that possibility.

std::variant – Type-Safe Unions for Heterogeneous Data

While std::optional deals with presence or absence of one type, std::variant (defined in <variant>) deals with one of many types. It is often described as a “type-safe union” – indeed, the C++ standard reference defines std::variant as “represents a type-safe union”. A std::variant<...> can hold a value that alternates among a fixed set of types (known at compile time). For example, std::variant<int, std::string> can hold either an int or a std::string at any given time (one or the other, but not both). Unlike a traditional C union, the variant knows which type is currently active, and it will only allow you to access that active type, enforcing correctness at compile time or by throwing an exception if you attempt the wrong type access.

Type Safety and Alternatives: std::variant addresses a common need: sometimes a variable or return type can naturally be one of a few alternatives. Prior to C++17, one might use a raw union plus a manual tag variable to track the current type, or use a base class with derived types (polymorphism) and dynamic_cast to figure out the actual type, or even abuse void*. All those approaches are error-prone or heavyweight:

  • A raw union in C++ does not keep track of which member is active, and reading the wrong member is undefined behaviour. Moreover, unions cannot have non-trivial types without manual constructor/destructor management – they “won’t call destructors of the underlying types”, requiring the programmer to explicitly destroy and construct members when switching the active type. This is complex and bug-prone (leaking memory or calling the wrong destructor are common pitfalls). For example, if a union holds a std::string and a std::vector<int>, the programmer must manually std::destroy the active member and placement-new the new one when changing the type. Failing to do so correctly leads to memory leaks or double-destruction.
  • Using class inheritance (polymorphism) to allow a variable to take multiple forms requires heap allocation (for polymorphic objects) and incurs virtual dispatch. It also moves the type determination to runtime (via dynamic_cast or virtual methods) and typically cannot easily return stack-allocated alternatives. It’s often overkill when the set of types is known and limited.
  • void* (or even std::any) could be used to store “something of unknown type”, but then it’s on the programmer to remember what actual type was stored and to cast it back. With void* there’s no checking at all – a wrong cast is undefined behaviour, likely crashing the program.

Enter std::variant: it solves these issues by providing an enumerated set of types with strictly enforced access. When you define std::variant<T1, T2, ..., Tn>, that variant can hold exactly one value, which is either a T1 or a T2 or … Tn. The active type is tracked internally (typically by storing an index of which alternative is active). The variant will call the constructor of the stored value when you assign, and call the destructor when the variant changes to hold a different type or is destroyed. This means all the manual work required with unions is handled for you – the proper constructor/destructor is invoked as needed, and you always know which type is active. In fact, if you try to access the wrong type, std::get will throw std::bad_variant_access if you guessed incorrectly, and std::get_if will return null – you have mechanisms to check safely.

Basic Usage Example: Let’s illustrate std::variant with a practical scenario: suppose we have a configuration system where a configuration value could be one of a few types (say int, bool, or std::string). We can use a variant to represent such a value in a type-safe way:

#include <variant>
#include <string>
#include <iostream>
#include <unordered_map>

using ConfigValue = std::variant<int, bool, std::string>;

// A map of configuration keys to variant values
std::unordered_map<std::string, ConfigValue> config = {
    {"max_connections", 12}, 
    {"enable_logging", true}, 
    {"log_file", std::string("logs/output.txt")}
};

void printConfigValue(const std::string& key) {
    const auto it = config.find(key);
    if (it == config.end()) {
        std::cout << key << " is not set.\n";
        return;
    }
    const ConfigValue& val = it->second;
    // Visit the variant to handle each possible type
    std::visit([](auto&& arg){
        using T = std::decay_t<decltype(arg)>;
        if constexpr (std::is_same_v<T, int>)
            std::cout << "int: " << arg << "\n";
        else if constexpr (std::is_same_v<T, bool>)
            std::cout << "bool: " << std::boolalpha << arg << "\n";
        else if constexpr (std::is_same_v<T, std::string>)
            std::cout << "string: " << arg << "\n";
    }, val);
}

int main() {
    printConfigValue("max_connections");
    printConfigValue("enable_logging");
    printConfigValue("log_file");
}

In this example, ConfigValue is defined as std::variant<int, bool, std::string>. The config map holds values that can be any one of those three types. The function printConfigValue demonstrates accessing the variant: it uses std::visit with a lambda that has an auto parameter. This C++17 feature (std::visit) will call the lambda with the currently held value. Here we use a C++17 fold of if constexpr to detect the type of arg and act accordingly. The output will vary based on the type, e.g.:

max_connections -> int: 12
enable_logging -> bool: true
log_file -> string: logs/output.txt

This approach is fully type-safe: if we add another type to the variant, the code inside visit will fail to compile if we don’t handle it (in the above generic lambda approach, it actually will handle any type due to the auto and conditional, but one can also use overloaded function objects or explicit lambdas for each type). We can also access the variant in other ways:

  • std::get<T>(variant) returns a reference to the value if the variant currently holds type T, or throws std::bad_variant_access if not.
  • std::get<I>(variant) returns the value by index (0-based index of the type in the template parameters), which can be useful in some generic contexts.
  • std::get_if<T>(&variant) returns a pointer to the value of type T if it’s active, or nullptr otherwise (this is a non-throwing way to query the active type).
  • variant.index() tells you which alternative is active (by index), and std::holds_alternative<T>(variant) returns true if the variant currently holds type T.

Active State and Default Construction: A std::variant must always hold a value of one of its alternatives (except for a special valueless state discussed shortly). Unlike std::optional, a variant cannot be “empty” (no value) by design – there is no variant::has_value() because one of the types is always active. When you default-construct a variant, it default-initialises the first alternative type (index 0). If that type is not default-constructible, the variant itself won’t be default-constructible. To allow a variant to have an “empty” state, a common trick is to include an alternative like std::monostate (a trivial empty struct type provided by <variant>) as the first alternative. std::monostate is useful as a kind of “null” alternative – it’s a distinct type that carries no data. For example, std::variant<std::monostate, int, std::string> default-constructs to monostate (index 0, representing “no value”), and you can assign an int or std::string later. In many cases, though, you might not need monostate because you might always have a meaningful default. If not, monostate is a handy tool to explicitly represent a valueless initial state.

Valueless-by-Exception State: Although a variant generally always holds one of its types, there is an exceptional situation where it can become valueless. If an exception is thrown during a type assignment (for instance, during a converting assignment where the new type’s constructor throws after the old value has been destroyed), the variant is left without any active member. This is called the valueless_by_exception state. It’s rare and hard to achieve in practice, but it is detectable via variant.valueless_by_exception() returning true. If a variant is valueless, any attempt to get the value will throw bad_variant_access. One should generally design types and assignments such that this state is avoided (e.g., provide nothrow move constructors for types, or handle exceptions appropriately), but it’s good to know the possibility exists. In well-behaved code, you might never encounter a valueless variant.

Memory and Performance: A std::variant does not allocate on the heap. Internally, it can be thought of as a union of all the alternative types, plus a small discriminator (typically an index of type size_t or uint8_t depending on number of alternatives). The size of a variant is therefore at least the size of the largest alternative type, plus space for the index. There may also be alignment padding. For instance, if you have variant<int, double>, the size will likely be sizeof(double) + some bytes for the index (since double is larger than int). One of the advantages of variant over approaches like any is that this memory is statically determined – no dynamic memory allocation is needed for contained values. Accessing the value involves a type check, but because the set of types is known at compile time, tools like std::visit are able to use compile-time dispatch (often implemented with a table of function pointers or if-else chain under the hood). This means accessing a variant is generally a constant-time operation that is very efficient (comparable to a switch on an enum). The overhead is the switch on the discriminator and the cost of calling the visitor. In most cases, this overhead is negligible, and certainly far less than the cost of misused unions or the dynamic allocation overhead of polymorphic pointers. Also, since the variant alternatives are known, the compiler can often optimise away some checks or inline the visitation.

Pitfalls and Gotchas: When using std::variant, keep the following in mind:

  • Exhaustive Visiting: If you use std::visit with a visitor that has an overload for each variant type, the compiler will ensure you handled all types. However, if your visitor is not exhaustive (say you used a lambda that takes auto to catch multiple types generically or you missed a type), you must be careful to handle all cases. Using std::visit with a structured binding of lambdas (an “overloaded” functor of lambdas for each type) is a common pattern to ensure completeness – if you miss an alternative, the code simply won’t compile. This is a strength of variant: it forces you (at compile time) to consider all possible types, preventing latent bugs. If you choose to use index-based access or get_if, be mindful to check the index or pointer result.
  • Type Conversions: Variants have rules for converting constructors and assignments. For example, if you assign a value of a type that is not exactly one of the alternatives but convertible to one, it might invoke a converting constructor for an unexpected alternative. A classic surprise was std::variant<std::string, bool> v = "hello"; – in C++17, this could actually select the bool alternative, because string literals can convert to bool (the pointer-to-char literal converted to bool yields true). C++20/23 resolved this with a more sane converting constructor resolution, so now "hello" will construct the std::string alternative as expected. It’s worth knowing that if an input is convertible to multiple variant alternatives, there might be ambiguity or unexpected resolution, so sometimes an explicit std::variant constructor call with the desired type is clearer.
  • No References or Arrays: Like optional, variant cannot directly hold reference types or C-style arrays. All alternatives must satisfy the requirements of Destructible (and must be object types). If you need to variant-hold a reference, you can wrap it in std::reference_wrapper or use pointers. Typically, value types are stored in variant.
  • Duplicate Types: Interestingly, std::variant can hold the same type more than once (e.g., std::variant<int, int> is allowed). This is not often useful, but if it happens, note that the two alternatives are treated as distinct by index (so get<0> vs get<1> would refer to the first or second int). Avoid such design unless it’s really necessary for some meta-programming reason.
  • Alternative Order Matters: The index order of types is determined by how you list them. This can affect which type is the “first” (for default construction) and also affects converting constructor resolution priority. It’s usually intuitive (list types from most to least specific, perhaps), but be mindful that variant<int, double> is not the same type as variant<double, int> and their behaviour on default-init (and converting assignment from, say, 0) will differ.
  • Performance considerations: If your variant holds alternatives where one is significantly larger than the others, the variant’s size will be that of the largest. This could mean wasted space when holding a smaller alternative. In extreme cases (many large alternatives), this could impact memory usage. But in typical use (a handful of types of reasonable size), this is not an issue. Also, each access has to check a discriminator – in tight loops where millions of variant accesses occur, this overhead could be measurable. If you have such a scenario, you might reconsider the design (maybe separate vectors for each type, etc.), but for most purposes variants are plenty efficient.

Real-World Uses: std::variant shines in scenarios such as message handling systems or state machines. For example, if you are writing an event processing loop, you might have a std::variant that can hold any of several event structures. By using std::visit, you can dispatch the correct handler for each event type without needing a big chain of dynamic_casts or manual tag comparisons. Another example is a compiler’s abstract syntax tree node: it could be a variant of different node type structs (literal, binary_op, variable, etc.), allowing pattern-matching on node types with visitors. The C++ community has embraced variant for implementing algebraic data types – essentially, std::variant is a closed set sum type, akin to std::optional being a special case of a sum type (with two cases: value or none). A concrete illustration is implementing a finite state machine: one can define a variant of state types (as structs for each state) and then use visitors to transition between states based on events. This leads to very readable code that clearly enumerates possible states and transitions. In fact, one could model a simple traffic light or door lock state machine with a variant of states and visitors representing events (open, close, lock, unlock) that each produce a new state. This is typesafe – impossible to transition from a state that doesn’t support a given event because the visitor won’t have a matching overload, catching errors at compile time.

In summary, std::variant provides flexible polymorphism without polymorphic classes. It gives the benefits of a union (no heap allocations, contiguous storage) while eliminating the unsafe parts (active member tracking and destruction are handled). The code is often more explicit than inheritance hierarchies, and certainly much safer and clearer than manual unions.

std::any – Type-Erased Containers for Arbitrary Values

There are situations where you genuinely need to store “any type” of data, without knowing the type at compile time. This is where std::any (from the <any> header) comes into play. std::any is a type-safe container for single values of arbitrary type. You can store a value of any copyable type in an std::any object, and later retrieve it (if you know the correct type to cast to). It employs type erasure: internally, it will allocate memory (if needed) to hold the object and remember the type information so that it can enforce correct casting. In essence, it’s like a safer alternative to void* that eliminates the risk of mismatched types – if you attempt to retrieve the wrong type, it will throw an exception rather than corrupt memory.

Type Safety at Runtime: Unlike optional and variant, the types that std::any might hold are not fixed at compile time – it truly can be anything. This gives ultimate flexibility, but shifts type checking to runtime. When you store a value in an any, the object is type-erased (the any doesn’t expose what it is holding except via the type() method or by trying a cast). To get the value out, you use std::any_cast<T>(any) which will either return the value (or reference) if the stored object is of type T, or throw std::bad_any_cast if the types don’t match. This is type-safe in the sense that you won’t accidentally interpret data as the wrong type – you’ll get a runtime error instead.

Basic Usage Example: Consider a scenario where we want a container that can hold heterogenous types – perhaps a vector that can hold ints, strings, doubles, etc., determined at runtime (maybe parsed from a scripting language or configuration where types vary). std::any makes this easy:

#include <any>
#include <vector>
#include <iostream>
#include <typeinfo>
#include <string>

int main() {
    std::vector<std::any> values;
    values.push_back(42);                   // int
    values.push_back(std::string("hello")); // string
    values.push_back(3.14);                 // double (actually a double literal)

    for (const std::any& v : values) {
        if (v.type() == typeid(int)) {
            std::cout << "int: " << std::any_cast<int>(v) << "\n";
        } else if (v.type() == typeid(std::string)) {
            std::cout << "string: " << std::any_cast<std::string>(v) << "\n";
        } else if (v.type() == typeid(double)) {
            std::cout << "double: " << std::any_cast<double>(v) << "\n";
        }
    }

    // Example of incorrect any_cast
    try {
        // v[1] holds a string, so casting to int should throw
        int n = std::any_cast<int>(values[1]);
    } catch (const std::bad_any_cast& e) {
        std::cerr << "Bad cast: " << e.what() << "\n";
    }
}

In this snippet, we create a std::vector<std::any> and push different typed values into it. We then iterate and use any.type() (which returns a std::type_info) to identify the type of each element in order to cast appropriately. This is admittedly not as neat as using std::variant with a fixed set of types (where std::visit would do it in one go), but in a scenario where you truly don’t know the set of possible types (for example, a plugin system where plugins can insert arbitrary types), std::any is a suitable choice. The example also shows a failed std::any_cast – attempting to get an int out of an any that holds a std::string throws a std::bad_any_cast. Unlike a raw pointer cast, which would yield nonsense or crash, std::any detects the mismatch and safely alerts you.

Storage and Performance: Under the hood, an std::any typically holds a pointer to a heap-allocated copy of the object you put into it (for larger objects), or it might use a small internal buffer for small objects (small object optimisation). The type information is stored as a std::type_info (from RTTI) or similar identifier. Because of this, there are some performance considerations:

  • Copying an std::any requires copying the contained object (via its copy constructor). Therefore, the type you store in an any must be copy constructible (this is a requirement: only copyable types can be stored, since std::any itself is copyable). If you attempt to store a non-copyable type (like std::unique_ptr<T>), it won’t compile. The design is such that std::any has value semantics – if you copy the any, you get an independent copy of the held object.
  • Move-only types: Because of the above, move-only types can’t be directly stored by copying. However, you can still move them in (with any.emplace or constructing the any with std::move(obj)), but once in the any, you cannot copy the any (as it would attempt to copy the non-copyable content and fail at compile or runtime). In C++17, std::any’s constructor template and emplace require the type to be CopyConstructible (there was discussion on possibly loosening this, but as of C++17 it’s a limitation).
  • Each any carries a type id and possibly a pointer, so it’s larger than a raw pointer. Typically an any might be the size of three pointers (one for type info, one for small buffer or pointer to object, etc.). This is not huge, but if you use a lot of them, be mindful of memory overhead.
  • Accessing an any via any_cast involves checking the stored type id against the requested type. This is a runtime comparison (basically comparing typeid(T) of the template against the stored typeid of content). This is quite fast (essentially pointer comparison or string comparison of type info names, depending on implementation), but it’s not zero-cost. So, using any in performance-critical code (e.g., inner loops where millions of any_casts occur) would be ill-advised. It’s meant more for flexibility at boundaries (like scripting interfaces, configuration storage, etc.), not for heavy computation on each element repeatedly.

Pitfalls and Best Practices:

  • Use std::any sparingly. Because it defers type checking to runtime, heavy use of any can make code less transparent. If you do know a fixed set of types, prefer std::variant for that situation. Reserve std::any for cases where the type list truly cannot be known or is open-ended (for example, a GUI property bag that can hold values of any type, or an instrumentation system carrying payloads of various plugin-defined types).
  • Always retrieve with the correct type. This sounds obvious, but it means the onus is on the programmer to keep track of what an any is supposed to hold. Misuses might not be caught until runtime (throwing an exception). Organise your code such that the logic “knows” the type before calling any_cast. Sometimes this involves storing type tags in parallel or encoding the information in the context (like using a key that implies the type).
  • If an any is empty (default constructed or after a reset), any_cast will throw as well. You can check any.has_value() before casting, similar to optional’s has_value (and there’s any.reset() to clear it).
  • No direct comparison: Unlike optional and variant, std::any does not provide comparison operators or a way to directly compare stored values (since the type is erased, you’d have to know the type to compare meaningfully). You also cannot directly serialize an any without knowing what’s inside. Essentially, any is a type-erased box – you need additional knowledge to use what’s inside. Keep that in mind when designing APIs: an any is best used at the boundaries of a system (e.g., passing data through interfaces where compile-time coupling is not possible), and then quickly converted to a known type for use.
  • If you find yourself putting the same few types into an any repeatedly, reconsider using a variant. std::any is more for when you truly have completely varied types or want to allow future extension without changing code (at the cost of needing runtime checks).
  • Exception safety: In theory, constructing or copying the contained object could throw, leaving the any in an empty state or propagating the exception. Typically this is not a problem unique to any – it just forwards exceptions from the type’s constructors. std::any provides the basic exception guarantee: if a throw happens during an any assignment, the any is left unchanged.

Real-World Uses: A classic use-case for std::any is in implementing generic data storage or messaging where the set of types is open-ended. For example, consider an event system where events can carry a payload of arbitrary type, and listeners are expecting specific types. The event could store its payload in an std::any. Listeners would retrieve it with any_cast to the type they expect. Another example is a configuration system where config values might be of various types not known in advance – you could use std::any to hold the value, alongside maybe a description string of what type is expected. (If the types are known in advance, std::variant would be preferred, as discussed in the config example earlier, but if a plugin can introduce a new type of config value unknown to the core program, std::any could accommodate it.) The C++17 introduction of std::any was largely motivated by the existence of boost::any and the need for a safer alternative to void pointers in generic code. GUI frameworks, serialization libraries, and scripting language embeddings are all areas where std::any might appear.

To sum up, std::any offers maximum flexibility at the cost of compile-time type knowledge. It should be used when that flexibility is needed, and with discipline to manage types. It complements std::optional and std::variant – if optional is a maybe-value and variant is a one-of-several, then any is an anything. Each has its place in the C++ toolbox for achieving type-safe programming without sacrificing too much performance or clarity.

Practical Examples and Applications

To see these utilities in action together, let’s sketch a couple of real-world inspired scenarios:

  • Configuration Parsing: Imagine a configuration file where each setting has a name and a value, and values can be of different types. Using std::variant or std::any can greatly simplify the design. As shown earlier, we could use std::variant<int, bool, std::string> for a config system with a fixed set of value types. If a certain configuration entry is optional, we can make the config map return an optional<ConfigValue> – for example, a function std::optional<ConfigValue> getConfig(const std::string& name) that returns a value variant if the key exists, or nullopt if not. This explicitly forces the caller to handle the “setting not found” case, instead of, say, returning a default or sentinel. If the config values are very heterogeneous and not known ahead (perhaps user-defined types allowed), one could choose std::map<std::string, std::any> to store them. Then, each consumer of a particular config key would any_cast to the expected type. The combination of optional and variant here gives both safety and flexibility: optional for presence, variant for multiple known types, any for truly dynamic cases. In fact, using std::variant or std::any is a marked improvement over older techniques where people might have stored config values as void* along with a type tag, or used a union of possible types without safety. It was observed that even void* was used to hold such unknown-type values, but now one can “improve the design by using std::variant if we know all the possible types, or leverage std::any”.

  • Message Handling System: Consider a distributed message bus or event system where messages of different types (with different payload structures) are passed through a single channel. Prior to C++17, one might define a base Message class and subclass it for each message type, using dynamic_cast or an enum to distinguish them. With C++17, you can define using Message = std::variant<MsgA, MsgB, MsgC> for all your message types. A consumer can use std::visit to handle each message type in a separate branch, and the compiler will ensure you covered all message variants. For optional parts of a message (say a field that may or may not be present), std::optional can be used within the message struct. For example, struct MsgA { std::optional<std::string> comment; /*...*/ }; clearly indicates that comment might be absent. In more dynamic systems where new message types can be added at runtime (imagine a plugin introducing a new message), std::any might be used to carry the payload, with some external mechanism (like a message type registry) to know what type to cast to. However, in most systems, the set of message types is known at compile time, and std::variant excels there by giving compile-time type checking for the handlers. The result is code that is both efficient (no heap allocation for the polymorphism, unlike virtual inheritance) and safe (no bad casts – visitors ensure proper type handling).

  • Data Processing Pipeline: In some data processing or ETL pipelines, data might be represented in a union-like structure as it flows between stages, especially if the pipeline can handle different data shapes (for example, a field that might be int or string depending on context). std::variant can represent this intermediate data. Each stage of the pipeline can pattern-match on the variant and process accordingly. If a stage optionally produces a result (maybe a filtering stage can drop data), it could return std::optional<DataVariant>. If a pipeline stage can yield a result in an unpredictable type (maybe scriptable stage), std::any might be used to hold that result until the next stage casts it to something expected. Designing such pipelines with these abstractions makes the flow of types explicit and eliminates classes of errors (e.g., one stage forgetting to check that the previous stage provided a value, or interpreting bytes as the wrong type).

In all these scenarios, the overarching theme is that expressing the intent in the type system leads to clearer and safer code. By using optional, variant, and any, we document the shape of our data: whether it can be missing, whether it can take multiple forms, or whether it’s completely dynamic. This moves potential errors from runtime to compile time (in the case of optional and variant) or at least to controlled runtime (in the case of any, where a wrong cast throws an exception instead of corrupting memory).

Edge Cases, Performance, and Pitfalls

To summarise the caveats and special considerations of each of these C++17 features, below is a breakdown of edge cases, performance implications, and potential pitfalls for each:

  • std::optional: An optional is pretty lightweight (usually just one extra boolean flag). Performance is generally a non-issue, but be wary of copying large objects in optionals. Always check for presence before accessing; a mistaken access when empty throws a bad_optional_access – which is better than undefined behaviour, but still something to avoid in normal logic. Remember that optional cannot hold reference types or array types. If you need an optional reference, consider redesign or use pointer/reference_wrapper. Another subtle pitfall is that optional<bool> is not a substitute for the “tri-state boolean” (some languages have a nullable boolean); while it works for that, note that optional<bool> will be larger than a plain bool and you must still check it (so, no, you can’t directly do if(optional_bool) to mean true/false/missing without a check – if(opt_bool) only tells you if it’s present, not its value). Also, be mindful that optional’s implicit conversion to bool (for checking) can sometimes lead to slight ambiguities in overload resolution if you also have a bool overload; but those cases are rare. In terms of exception safety, optional is strong – if a constructor throws while emplacing a value in an optional, the optional remains empty. Optional’s comparison operators make it easy to sort containers of optionals, but ensure you understand that an empty optional is treated as < any value.

  • std::variant: The main edge case is the valueless by exception state as discussed – rare, but if one of your variant’s types can throw during move or copy, be aware of exception safety. You can check valueless_by_exception() if needed. In practice, many variants hold simple types or types with nothrow moves, so this isn’t encountered. Another consideration is visitor runtime cost: using std::visit introduces an indirection (essentially a switch or jump) which is extremely fast, but if you have deeply nested variants or a chain of visitors it could add up. Still, it’s usually negligible compared to any real work done inside the visitor. A pitfall to watch out for is using variant for types that don’t naturally belong together – if you find yourself with a variant of 10+ types that are unrelated, you might be abusing it and making the code harder to maintain. Each addition of a type forces all visitations to handle it, which can increase code complexity. Try to keep variants to a reasonable number of alternatives that make sense in context. Also, as noted, if your variant alternatives have a large size disparity, you pay the cost in space for each variant instance. For example, std::variant<std::array<int,1000>, int> will be roughly the size of that 1000-int array (plus overhead) even when holding an int. If memory is a concern and usage of the large alternative is rare, an alternative design might be to use a pointer or a separate structure. But often this is not a major issue unless you have thousands of variant objects. Finally, be careful with variant’s converting constructors: C++17 has a certain set of rules that might surprise, though C++20 made them more intuitive. When in doubt, explicitly construct the variant with the type you want (variant<Ts...> var(std::in_place_type<T>, value)) to avoid ambiguity.

  • std::any: The biggest pitfall is loss of compile-time checking. Overuse of any can lead to code where you only find out at runtime that you mixed up types. As a rule of thumb, confine std::any to the boundaries of your system. For instance, if you have a plugin API where plugins can return data of arbitrary type, accept std::any there, but internally, downcast to known types as soon as possible and work with those. Performance-wise, each any might incur a dynamic allocation (except for small trivially moveable objects where small-object optimization kicks in). This means storing large objects in an any could be costly. Also, any requires types to be copyable, which means you cannot directly store move-only types. If you must store something like a unique_ptr or a move-only lambda, you might wrap it in a std::shared_ptr or similar to satisfy copyability (or store a wrapper that holds it). This is a limitation to be aware of: std::any was intentionally designed this way to avoid complex behaviour on copying. Another issue is that any’s type erasure uses RTTI (typeid), which is typically available in C++ builds, but if you disabled RTTI, std::any might not function (or you lose any_cast ability). This is rare, but worth noting in some embedded or strict environments. In terms of exceptions, incorrect any_cast throws bad_any_cast – always handle or avoid that by logic. And as with variant, storing extremely large disparate types in a single any and then copying it frequently can hurt performance due to allocations and copies. If you find yourself doing heavy operations on any in tight loops, consider refactoring the design (perhaps using templates or overloading to regain compile-time types).

In summary, each of these features is designed to make common patterns safer. The trade-offs are usually minor – a small constant overhead for the safety they provide. By understanding their behaviour and limitations, one can use them effectively without performance surprises or bugs.

Comparison to Pre-C++17 Approaches

It’s instructive to briefly contrast how one would achieve similar goals before C++17 (or in lower-level C-style code) versus using optional, variant, and any:

  • Without std::optional: One might use a raw pointer or a special flag/value to indicate “no value”. For example, returning a pointer that is nullptr if not found, or returning an int where -1 means “not found”. Pointers introduce ambiguity (is a nullptr an error or just a null object?) and potential for misuse (forgetting to check for null). Sentinel values (like -1) can be even worse if that value might also be a valid data (leading to bugs where -1 could be mistaken for actual data). Alternatively, a common pattern was to return a std::pair<T, bool> or an std::tuple<ErrorCode, T> to signal success or failure. These work but are less clear; for instance, auto [val, ok] = func(); if (!ok) { ... } is more cumbersome than auto opt = func(); if (!opt) { ... }. Boost.Optional was available and widely used in pre-C++17 code as a stop-gap; it offered nearly the same interface and semantics as std::optional. Now with the standard optional, there is no need for custom solutions – we have a clear, language-supported way to represent an optional value.

  • Without std::variant: C++11 and earlier didn’t have a standard discriminated union. You either used a C-style union (with all the attendant problems discussed – manual tracking of active member, inability to hold complex types without manual construction, etc.) or you used polymorphic classes (an abstract base with derived types for each alternative). Polymorphism can be overkill if value semantics are desired, and it complicates object lifetimes (objects typically allocated on heap). Another approach was an ad-hoc struct that had all possible fields and an enum indicating which is valid, which is essentially a poor man’s variant. Maintaining such code is error-prone (someone might set the enum to one thing but put data in another field, etc.). Boost.Variant existed to fill this gap pre-C++17 and had similar functionality (though without std::visit – Boost.Variant used either visitors via boost::apply_visitor or boost::get). Boost.Variant also lacked some of the constexpr capabilities that std::variant now has. In modern C++, there’s almost no reason to use raw unions for variant-like purposes – std::variant covers the need in nearly all cases, except maybe very low-level bit reinterpretation tricks (and even those have safer alternatives).

  • Without std::any: The old approach for “hold anything” was typically void* or something like a union of common types (which still fails for custom types) or using a base class Any with virtual cloning (a manual type erasure). void* is extremely unsafe because you lose all type information – the burden is entirely on the programmer to cast to the correct type, and if they get it wrong, it’s undefined behaviour (likely a crash or memory corruption). Debugging such issues is hard. Some frameworks introduced their own any-like containers; for instance, Qt has a QVariant (which despite the name, is more like a type-safe union of a fixed set of Qt types) and Boost introduced boost::any. Boost.Any was essentially the reference implementation that inspired std::any. It provided type erasure and boost::any_cast. The standard std::any is very similar but benefits from being in <any> and carefully specified. Using std::any over raw void* is hugely beneficial in terms of safety – you get a runtime check on type mismatch and you avoid the immediate need for heap allocations or custom new/delete for arbitrary data (since any takes care of that internally with correct copying semantics). In performance-sensitive scenarios, templates (to keep types known) or variants (if types are limited) are still preferable, but when those aren’t feasible, std::any is the go-to.

In essence, C++17 saved us from a lot of boilerplate and hazards. Where we used to have to choose between safety and convenience, we now have tools that offer both. Code that uses optional/variant/any is often shorter, clearer, and safer than the pre-C++17 equivalents. These features also integrate well with other modern C++ techniques – for instance, you can combine std::optional with ranges or algorithms (C++20 ranges even have .transform() for optionals to apply a function if present), and std::variant can be used in constexpr context from C++20 onwards, which makes it usable in compile-time computation or constexpr if/visit. The language and standard library are moving toward giving us higher-level abstractions that cost nothing at runtime but prevent entire classes of errors.

Enhancements in C++20 and C++23

While our focus is on C++17, it’s worth noting that subsequent standards have extended these facilities:

  • std::optional in C++20/C++23: C++20 added comparisons (operator<=>) for optional, making them easily sortable. C++23 introduced the aforementioned monadic operations and_then, transform, and or_else, which were inspired by functional programming (they simplify chaining operations on optionals without deeply nested if statements). These improve ergonomics but do not change the fundamental performance or semantics of optional. Optional remains a straightforward struct with a value-or-not.

  • std::variant in C++20/C++23: C++20 made variants constexpr-friendly, meaning you can create and manipulate variants in constant expressions. This is useful for compile-time computations and metaprogramming. Additionally, as mentioned, C++20 fixed the converting constructor behaviour to be more intuitive. C++23 introduced some helper features, like visitation of variant types that are themselves variants (to ease handling nested variants), and a proposal to propagate triviality (so that a variant is trivially copyable if all alternatives are trivially copyable), which helps optimisers. Another C++23 improvement was allowing std::visit to accept lambdas that take base classes of variant types (if one alternative is derived from another’s type). These are relatively minor refinements – the core functionality of variant as introduced in C++17 remains the same. It’s simply becoming more powerful and easier to use in more contexts.

  • std::any in C++20/C++23: There have been fewer changes here. std::any is largely unchanged since C++17. One notable addition in the standard library, however, is std::any_cast gaining support for arrays (C++23) in a limited form, and std::make_any factory function (added in C++17 after initial, and improved later). But fundamentally, any remains as it was. If anything, the community has realised that std::any should be used with care, and alternatives like using concepts or variants are preferred when possible.

It’s also worth acknowledging that C++23 introduced std::expected, which is a type for error handling that combines the idea of an optional value or an error code (much like a simplified variant of value or error). This is another step in the direction of type-safe programming – giving a structured way to handle errors instead of relying on exceptions or output parameters. While std::expected is beyond our main topic, it fits the narrative: modern C++ continues the trend of providing strong, type-safe abstractions for scenarios that used to rely on convention or non-typesafe idioms.

The evolution of these features from Boost libraries to C++17 and minor tweaks in later standards shows a commitment in the C++ committee to type safety, clarity, and expressiveness without compromising performance. With each revision, they integrate more smoothly with the language’s other features (e.g., constexpr, concepts, ranges).

Conclusion

C++17’s std::optional, std::variant, and std::any represent significant strides toward more expressive and type-safe C++ programming. They enable us to model optional values, variant types, and dynamic types in a way that is clear in intent and safe in usage. By leveraging these constructs, developers can avoid a whole class of bugs – no more unchecked null pointers or unsafe unions or mysterious void pointer casts. The code becomes self-documenting: an optional<T> shouts “this T might not be here,” a variant<A,B> declares “it’s either A or B,” and an any signals “this could be anything, handle with care.”

We have seen through examples how these types can be applied to real-world tasks like configuration handling, message dispatch, and state management, leading to solutions that are both elegant and robust. We also discussed how they improve upon pre-C++17 techniques: where older code might rely on conventions and extensive comments to explain that “we use -1 to mean not valid” or maintain parallel enums alongside unions, modern C++ code can encode that directly in the type system, making the compiler our ally in catching mistakes.

In terms of performance, these features are crafted to impose minimal overhead. std::optional typically has the same performance as using a flag and separate value, but with nicer syntax. std::variant avoids allocations and uses only a small constant time dispatch, giving you the speed of a union with far better safety. std::any does have some overhead due to type erasure, but it’s a conscious trade-off for flexibility; and even then, it provides safety checks that raw approaches lack. For most applications, these are small costs for the huge gains in correctness and maintainability.

As C++ moves forward (with C++20, C++23, and beyond), the trend is clearly toward stronger static typing with more expressive power. Features like concepts, constexpr if, structured bindings, and these type-safe containers all reinforce the idea that we can write code that is both high-level and efficient. We can expect future proposals to continue in this vein – perhaps pattern matching in the language will make variants even easier to use, and more monadic utilities might appear for optional-like types. The direction is set: C++ is embracing techniques long proven in functional and high-level languages, but adapting them to the C++ ethos of zero-overhead abstractions.

In conclusion, mastering std::optional, std::variant, and std::any is essential for intermediate and advanced C++ programmers looking to write modern, robust software. These constructs allow us to express our intent directly in code, reduce bugs, and produce maintainable designs. By using them appropriately, we align with C++’s philosophy of making the type system work for us – resulting in programs that are not only safer and more correct, but also easier to understand. The inclusion of these facilities in C++17 was a big leap forward in type-safe programming, and using them will undoubtedly become second nature as we continue to modernise our C++ codebases.

Sources:

  • C++ reference for std::optional – definition and properties
  • C++ reference for std::variant – definition and detailed behavior
  • C++ reference for std::any – definition and usage notes
  • Bjarne Stroustrup, The C++ Programming Language (4th Edition) – discussions on variant and optional in modern C++ design (for conceptual background).
  • CppCoreGuidelines C.183 – “Don’t use a union for type punning” (and related discussion on union safety).
  • Andrzej Krzemienski, “Everything You Need to Know About std::variant from C++17” – C++ Stories blog (Jan 2023 update), which provides insightful examples (state machines, config parsing) and covers variant’s design and usage in depth.
  • Richard Smith, et al. – ISO C++ proposal papers P2231 (constexpr for variant) and others (for improvements in C++20/23).
  • Boost documentation for boost::optional, boost::variant, boost::any – as historical reference points for these utilities prior to standardisation.