noexcept

Keyword: noexcept

The noexcept specifier in C++ is used to indicate whether a function is guaranteed not to throw exceptions. It was introduced in C++11 to help developers optimize code and improve performance by allowing the compiler to make certain assumptions about functions that do not throw exceptions.

Key Concepts

• Declaring noexcept: You can declare a function as noexcept to tell the compiler that the function will not throw any exceptions. This can lead to optimizations, such as the removal of unnecessary exception-handling code.
• Conditional noexcept: The noexcept specifier can also be used conditionally, allowing a function to be noexcept only if certain conditions are met.
• noexcept Operator: The noexcept operator can be used to determine if an expression is noexcept, which can be useful in templates and conditional code.

Example 1: Declaring a Function noexcept

Here’s a simple example demonstrating how to declare a function as noexcept:

include <iostream>

void safeFunction() noexcept {
    std::cout << "This function is noexcept and cannot throw exceptions." << std::endl;
}

void riskyFunction() {
    std::cout << "This function may throw an exception." << std::endl;
    throw std::runtime_error("Error in riskyFunction");
}

int main() {
    try {
        safeFunction();
        riskyFunction();
    } catch (const std::exception& e) {
        std::cerr << "Caught exception: " << e.what() << std::endl;
    }

    return 0;
}

Expected Output

This function is noexcept and cannot throw exceptions.
This function may throw an exception.
Caught exception: Error in `riskyFunction`

Explanation

• void safeFunction() noexcept: The safeFunction is declared as noexcept, meaning it guarantees not to throw any exceptions. The compiler can optimize calls to this function, knowing that it doesn’t need to generate exception-handling code.
• void riskyFunction(): This function is not marked as noexcept, so it can throw exceptions. In this case, it throws a std::runtime_error, which is then caught in the main function.

Example 2: Conditional noexcept

You can also make noexcept conditional, meaning a function is noexcept depending on the conditions in its definition. This is particularly useful in templates where the noexcept status might depend on the template parameters.

include <iostream>
include <type_traits>

template<typename T>
void conditionalFunction(T&& t) noexcept(std::is_nothrow_move_constructible<T>::value) {
    T temp = std::move(t);
    std::cout << "Conditional noexcept: " << std::boolalpha << noexcept(T(std::move(t))) << std::endl;
}

int main() {
    int x = 10;
    conditionalFunction(x);

    std::string str = "Hello";
    conditionalFunction(str);

    return 0;
}

Expected Output

Conditional noexcept: true
Conditional noexcept: false

Explanation

• noexcept(std::is_nothrow_move_constructible<T>::value): The conditionalFunction is noexcept only if the type T is nothrow move constructible. For fundamental types like int, this is true, but for types like std::string, which might allocate memory during a move, it is false.
• noexcept(T(std::move(t))): This expression checks whether the move constructor for T is noexcept and prints the result.

Benefits of Using noexcept

• Optimization: Functions marked as noexcept can be optimized better by the compiler, leading to potentially faster code.
• Clarity: It makes the code's intent clearer by explicitly stating that a function will not throw exceptions.
• Move Semantics: noexcept is particularly important in move constructors and move assignment operators, as it can affect the performance of standard library containers.

Example 3: Using noexcept with Move Constructors

noexcept is especially significant when writing move constructors and move assignment operators. If these operations are not marked noexcept, some standard library containers might fall back to copying instead of moving, which can be less efficient.

include <iostream>
include <vector>

class MyClass {
public:
    MyClass() = default;
    MyClass(const MyClass&) = default;
    MyClass(MyClass&&) noexcept = default;  // Move constructor marked noexcept
};

int main() {
    std::vector<MyClass> vec;
    vec.push_back(MyClass());  // Efficient move operation because MyClass's move constructor is noexcept

    return 0;
}

When to use noexcept

• Move Constructors/Operators: Always consider marking move constructors and move assignment operators as noexcept if they do not throw exceptions.
• Performance-Critical Code: In performance-sensitive code, marking functions as noexcept can lead to better optimization by the compiler.
• Self-Documenting Code: Using noexcept makes it clear to other developers that certain functions are guaranteed not to throw exceptions.

Conclusion

The noexcept specifier is an important feature in modern C++ that allows developers to optimize and clarify their code. By marking functions as noexcept, you can help the compiler make better optimization decisions and avoid unnecessary exception-handling overhead. It’s particularly important in move semantics, where it can significantly impact the performance of standard library containers.