Introduction

Exception handling is an essential aspect of robust software development in C++. In my experience, well-structured exception handling can significantly improve a program’s reliability and maintainability, particularly when dealing with complex error scenarios. In this post, I examine the fundamentals of exception handling in C++17, discussing try-catch blocks, exception specifications, the standard exception hierarchy, and best practices for error handling. I also illustrate key concepts with code examples and references to relevant authoritative sources (Stroustrup, The C++ Programming Language, 4th ed.; ISO/IEC 14882:2017; cppreference.com).

1. What Are Exceptions and Why They Matter

From my perspective, exceptions provide a mechanism for signalling and handling error conditions outside the normal flow of a program. Rather than forcing every function to return an error code (which can be cumbersome to manage), exceptions allow us to separate the error-handling code from the primary logic. When an error occurs, we throw an exception, and the runtime system unwinds the stack until it finds a suitable handler (catch block) capable of dealing with the specific exception.

  1. Separation of concerns – Error handling does not obscure the main logic.
  2. Improved readability – Code becomes more readable when not cluttered with checks for return codes.
  3. Automatic cleanup – As the stack unwinds, objects with destructors are automatically destroyed (RAII principles).

(For a more detailed treatment of exceptions, see ISO/IEC 14882:2017 §15.)

2. Understanding try-catch Blocks

At the heart of exception handling in C++ is the try-catch construct, which I believe remains one of the most intuitive ways to handle unexpected runtime conditions. The general syntax is:

try {
    // Code that may throw an exception
} catch (const std::exception& e) {
    // Handler for std::exception and its subclasses
} catch (...) {
    // Handler for any exception type
}
  • try block: Encapsulates the code where exceptions might occur.
  • catch block(s): Defined to handle exceptions of specific types.
  • catch (...): A catch-all handler that intercepts exceptions of any type (though it is often discouraged to rely on this without logging or rethrowing, since diagnosis becomes harder).

I typically recommend catching exceptions by reference (especially const reference) to avoid slicing and unnecessary copies. For instance, catch (const std::exception& e) is preferred over catch (std::exception e).

3. Exception Specifications in C++17

In earlier versions of C++, dynamic exception specifications (e.g., throw(int, std::exception)) were used to declare which types of exceptions a function might throw. However, these were largely deprecated in C++11 and officially removed in C++17 (ISO/IEC 14882:2017). The modern approach is to use the noexcept specifier, which indicates whether a function is guaranteed not to throw exceptions:

int doSomething() noexcept {
    // Guaranteed not to throw
    return 42;
}
  • noexcept: Declares that a function does not throw exceptions (if an exception does escape, the program calls std::terminate).
  • Implicit exception specifications: Functions without explicit noexcept can potentially throw exceptions, yet the language does not require a declaration of which exceptions might be thrown.

From my standpoint, declaring functions as noexcept where possible aids both compiler optimisations and code clarity. Nonetheless, I caution that one should ensure a function truly cannot throw, or risk unexpected termination.

4. Standard Exceptions

The C++ Standard Library provides a robust hierarchy of exceptions, all derived from the base class std::exception. Some commonly used exceptions include:

  1. std::runtime_error – Signifies errors that occur during runtime (e.g., division by zero, illegal operations).
  2. std::logic_error – Represents errors in a program’s logic (e.g., violations of logical preconditions).
  3. std::out_of_range – Signals an out-of-bounds array or container access.
  4. std::bad_alloc – Thrown when memory allocation fails.
  5. std::bad_cast – Triggered by an invalid type cast (e.g., dynamic_cast failure).

I often rely on std::runtime_error (or a subclass thereof) for many general-purpose runtime issues, primarily because it is semantically clear. However, one can also create custom exception classes derived from std::exception if there is a need for domain-specific information (Stroustrup, 2013).

5. Best Practices for Error Handling

In my experience, handling errors effectively in C++ involves adhering to a set of best practices:

  1. Throw by value, catch by reference
    • Why? Throwing by value is concise; catching by reference prevents object slicing and unnecessary copying.
  2. Avoid using exceptions for control flow
    • Rationale: Exceptions are expensive in terms of performance and can make the code less predictable.
  3. Ensure exception safety
    • Method: Use strong exception guarantees (rollback on failure) or basic exception guarantees (no resource leaks).
  4. Prefer RAII (Resource Acquisition Is Initialisation)
    • Explanation: Bind resources (e.g., file handles, memory) to objects that automatically release those resources in their destructors (cppreference.com, n.d.).
  5. Use noexcept judiciously
    • Balance: While it can improve performance, incorrectly marking functions as noexcept that can throw leads to program termination.

6. Practical Examples

Below are a few concise examples demonstrating some of these principles in action.

6.1 Throwing and Catching Standard Exceptions

#include <iostream>
#include <stdexcept> // For standard exceptions

int main() {
    try {
        // Simulate an error condition
        throw std::runtime_error("Something went wrong!");
    } catch (const std::runtime_error& e) {
        std::cerr << "Runtime error caught: " << e.what() << std::endl;
    } catch (const std::exception& e) {
        // This block catches exceptions derived from std::exception
        std::cerr << "Standard exception caught: " << e.what() << std::endl;
    } catch (...) {
        // Catch-all for non-standard exceptions
        std::cerr << "Unknown exception caught." << std::endl;
    }
    return 0;
}
  • Explanation: In this snippet, I utilise multiple catch blocks to handle various types of exceptions. Once the throw statement is encountered, control transfers immediately to the matching catch block.

6.2 Custom Exceptions

#include <iostream>
#include <stdexcept>
#include <string>

class MyCustomError : public std::exception {
private:
    std::string message;
public:
    explicit MyCustomError(const std::string& msg) : message(msg) {}
    const char* what() const noexcept override {
        return message.c_str();
    }
};

void riskyFunction(bool errorOccured) {
    if (errorOccured) {
        throw MyCustomError("A custom error occurred!");
    }
}

int main() {
    try {
        riskyFunction(true);
    } catch (const MyCustomError& e) {
        std::cerr << "Caught MyCustomError: " << e.what() << std::endl;
    } catch (...) {
        std::cerr << "Caught unknown exception." << std::endl;
    }
    return 0;
}
  • Details: Here, I derive MyCustomError from std::exception. This class overrides the what() function to provide a descriptive error message, which can then be retrieved when the exception is caught.

6.3 Using noexcept

#include <iostream>

int doSomeComputation() noexcept {
    // Guaranteed not to throw
    return 42;
}

int main() {
    static_assert(noexcept(doSomeComputation()), "Function must be noexcept!");
    std::cout << "Computation result: " << doSomeComputation() << std::endl;
    return 0;
}
  • Commentary: Marking doSomeComputation() as noexcept informs both the compiler and other developers that the function will not throw, enabling further optimisations and clarity regarding error expectations.

Conclusion

Exception handling in C++17 remains a powerful feature for crafting robust and maintainable applications. By judiciously using try-catch blocks, leveraging standard exceptions, and employing modern practices such as noexcept, we can create programs that cleanly separate error handling from core functionality. It is my view that, as C++ continues to evolve, exceptions will remain relevant for dealing with both anticipated and unforeseen runtime issues. However, it is essential to adopt a disciplined approach—balancing performance, readability, and correctness.

In the future, we may see further refinements to exception mechanisms, potentially influencing how we manage concurrency or how the language handles exception guarantees in more advanced use cases (ISO/IEC 14882:2017). Still, the foundational principles outlined here will likely endure, guiding us toward writing safer, more expressive C++ code.

References

  • ISO/IEC 14882:2017 – Information Technology – Programming Languages – C++
  • Stroustrup, B. (2013). The C++ Programming Language (4th ed.). Addison-Wesley.
  • cppreference.com (n.d.). Exception handling. Retrieved from https://en.cppreference.com

Disclaimer: The code examples in this post are for illustrative purposes. Always test and adapt them to your specific use case.