What is noexcept in C++: Explanation and Usage

noexcept in C++ is a keyword used to specify that a function does not throw exceptions. It helps the compiler optimize code and signals to programmers that the function is guaranteed not to fail with an exception.

How It Works

Imagine you tell your friend that a certain task will never fail no matter what. In C++, noexcept is like that promise for functions: it guarantees that the function will not throw any exceptions. This helps the compiler make better decisions, like skipping extra checks for errors, which can make your program faster.

If a function marked noexcept does throw an exception, the program will call std::terminate(), which usually ends the program immediately. So, it's a strong promise that the function won't fail in that way.

Example

This example shows a function marked with noexcept that adds two numbers. Since it cannot throw exceptions, the compiler can optimize calls to it.

cpp

#include <iostream>

int add(int a, int b) noexcept {
    return a + b;
}

int main() {
    std::cout << add(3, 4) << "\n";
    return 0;
}

Output

7

When to Use

Use noexcept when you are sure a function will not throw exceptions. This is common for simple functions like getters, setters, or utility functions that do not perform operations that can fail.

Marking functions noexcept helps improve performance and makes your code safer by clearly communicating your intent. It is especially useful in move constructors and move assignment operators to enable optimizations in standard containers like std::vector.

Key Points

• noexcept tells the compiler a function won't throw exceptions.
• If an exception is thrown inside a noexcept function, the program ends immediately.
• It helps the compiler optimize code and improves program safety.
• Commonly used in move constructors and simple utility functions.

Key Takeaways

noexcept guarantees a function will not throw exceptions, enabling compiler optimizations.

If a noexcept function throws, the program calls std::terminate() and ends.

Use noexcept for simple, safe functions and move operations to improve performance.

Marking functions noexcept clearly communicates your code's behavior to others.