Exception Specifications | Exception Handling

Introduction

Exception specifications in C++ provide a way to declare which exceptions a function might throw. This is part of the broader topic of exception handling, which allows programs to manage and respond to errors and exceptional conditions in a structured manner.

Basic Exception Specification

In C++, you can specify the exceptions that a function might throw using the throw keyword followed by a list of exception types. Here is a basic example:


                    void func() throw(int, std::string);

This means that the function func can throw either an int or a std::string exception.

Dynamic Exception Specification

C++ introduced dynamic exception specifications, allowing you to specify a list of exception types a function might throw. However, this feature has been deprecated in C++11 and removed in C++17. Here's an example of how it was used:


                    void func() throw(int, double);

This would indicate that func might throw exceptions of type int or double. If an exception of a different type is thrown, the unexpected function is called, which by default calls terminate.

noexcept Specification

In modern C++ (C++11 and later), the noexcept specifier is used to indicate that a function does not throw any exceptions. This can help with optimizations and better error handling. Here's an example:


                    void func() noexcept;

This means that func is not expected to throw any exceptions. If it does throw an exception, terminate will be called. You can also conditionally specify noexcept:


                    void func() noexcept(true); // Equivalent to noexcept
                    void func2() noexcept(false); // Equivalent to not using noexcept

noexcept Operator

The noexcept operator can be used to check if an expression is declared to throw exceptions. It returns true if the expression is declared noexcept, otherwise it returns false. Here's an example:


                    bool isNoexcept = noexcept(func());

If func is declared with noexcept, then isNoexcept will be true.

Practical Example

Let's put it all together with a practical example:


#include <iostream>

#include <stdexcept>



void might_throw() noexcept(false) {

    throw std::runtime_error("Error!");

}



void wont_throw() noexcept {

    // No exceptions here

}



int main() {

    try {

        might_throw();

    } catch (const std::exception& e) {

        std::cout << "Caught exception: " << e.what() << std::endl;

    }



    if (noexcept(wont_throw())) {

        std::cout << "wont_throw is noexcept" << std::endl;

    }



    return 0;

}

Caught exception: Error!
wont_throw is noexcept

Conclusion

Exception specifications in C++ are a powerful way to declare and manage the exceptions that functions might throw. While dynamic exception specifications have been deprecated in favor of noexcept, understanding both can help in maintaining and upgrading legacy code. Proper use of noexcept can lead to more optimized and reliable code.

Exception Specifications in C++