The noexcept Specifier

The noexcept specifier in C++ is used to indicate that a function does not throw any exceptions. This specifier serves as a promise to the compiler that a particular function, or set of operations within a function, will not result in exceptions being thrown. By marking functions as noexcept, you can optimize performance, particularly in contexts where exception handling incurs overhead.

Why Use noexcept?

1. Performance Optimization: When a function is marked with noexcept, the compiler can make optimizations because it no longer needs to generate code to handle exceptions for that function. This is especially useful in scenarios like move constructors, destructors, and other critical performance paths.
2. Improving Code Safety: By using noexcept, you can explicitly state that certain functions are guaranteed to not throw exceptions, making it clearer to users of the function and improving the overall robustness of your code.
3. Exception-Safe Code: In certain contexts, such as destructors, throwing an exception is generally undesirable because it can lead to program termination if not handled. The noexcept specifier ensures that such functions do not propagate exceptions.

Syntax

To mark a function as noexcept, use the specifier directly in the function declaration:

void myFunction() noexcept {
    // Function body
}

This indicates that myFunction will not throw any exceptions.

Example of noexcept:

#include <iostream>
#include <utility> // for std::move

class MyClass {
public:
    MyClass() {
        std::cout << "Constructor called" << std::endl;
    }

    // Move constructor marked noexcept
    MyClass(MyClass&&) noexcept {
        std::cout << "Move constructor called" << std::endl;
    }

    // Destructor also marked noexcept
    ~MyClass() noexcept {
        std::cout << "Destructor called" << std::endl;
    }
};

int main() {
    MyClass obj1;
    MyClass obj2 = std::move(obj1); // Calls move constructor

    return 0;
}

In this example, the move constructor and destructor of MyClass are marked noexcept. This allows the compiler to optimize move operations without worrying about exception handling overhead.

Conditional noexcept

You can also use conditional noexcept to make the exception guarantee dependent on the behavior of operations within the function. This is useful when you’re not sure whether the code you’re calling will throw exceptions or not.

Example of Conditional noexcept:

#include <iostream>
#include <type_traits>

template <typename T>
void myFunction(T&& arg) noexcept(std::is_nothrow_move_constructible<T>::value) {
    T local = std::move(arg); // Conditional noexcept based on whether T is noexcept move-constructible
}

class SafeClass {
public:
    SafeClass(SafeClass&&) noexcept = default; // noexcept move constructor
};

class RiskyClass {
public:
    RiskyClass(RiskyClass&&) { // Not marked as noexcept, might throw
        throw std::runtime_error("Move constructor threw an exception!");
    }
};

int main() {
    SafeClass safe;
    myFunction(std::move(safe)); // No exceptions

    RiskyClass risky;
    try {
        myFunction(std::move(risky)); // May throw an exception
    } catch (const std::runtime_error& e) {
        std::cout << e.what() << std::endl;
    }

    return 0;
}

In this example:

• myFunction has a conditional noexcept based on whether the type T‘s move constructor is marked as noexcept.
• If T is a class that guarantees no exceptions during move (like SafeClass), the function will be noexcept.
• If T may throw during the move (like RiskyClass), the function is not marked as noexcept.

Benefits of Using noexcept:

Move Semantics: When using the Standard Template Library (STL), containers like std::vector can make optimizations when moving elements if the move constructor of the element type is marked as noexcept. If the move constructor is not noexcept, the container may fall back to copying elements, which is slower.

For example:

std::vector<MyClass> vec1;
std::vector<MyClass> vec2 = std::move(vec1); // Faster if MyClass's move constructor is noexcept

Exception Guarantees: Functions that guarantee they do not throw exceptions are easier to reason about, and it reduces the risk of uncaught exceptions leading to undefined behavior. For instance, destructors and move constructors are good candidates for being marked noexcept.

Improved Performance in Destructors: By marking destructors noexcept, you can avoid unnecessary overhead from exception handling when cleaning up resources.

Safety with std::terminate: If a noexcept function throws an exception, the program will call std::terminate() because it violates the promise that no exceptions will be thrown. This forces you to carefully design such functions, ensuring their safety and stability.

When Not to Use noexcept:

• Exception-Safe Code: You should avoid marking functions noexcept if they might potentially throw exceptions. For instance, if a function allocates memory, accesses files, or performs any operation that might fail, you shouldn’t mark it noexcept unless you’re certain such failures are handled internally.
• Third-Party Code: Be cautious when calling third-party code that may not guarantee noexcept behavior. If the function you’re calling can throw, it’s better to avoid using noexcept in your own code to prevent program termination due to unhandled exceptions.