What happens if an exception is thrown in a function marked as noexcept?

A call to std::terminate will occur. The standard guarantees that, prior to calling std::terminate, std::unexpected will be called in C++17 and earlier versions, but this behavior was removed in C++20. Therefore, in modern standards (C++20 and above), when an exception is thrown from a function marked noexcept, std::terminate is called immediately.

By default, std::terminate calls abort. The behavior of std::terminate can be modified by calling std::set_terminate.

Example:

#include <iostream>
#include <exception>

void my_terminate_handler() {
    std::cerr << "Custom terminate handler called!\n";
    std::abort(); // You can call abort or exit
}

void function_noexcept() noexcept {
    std::cout << "Inside noexcept function. Throwing exception...\n";
    throw std::runtime_error("Something went wrong!"); // Throwing an exception
    std::cout << "This line will not be reached.\n";
}

int main() {
    std::set_terminate(my_terminate_handler); // Setting custom terminate handler

    try {
        function_noexcept();
    } catch (...) {
        // This catch block will not be executed because exception from noexcept
        // leads directly to calling terminate.
        std::cerr << "Caught an exception (this should not happen for noexcept).\n";
    }

    std::cout << "End of main (should not be reached).\n";

    return 0; // This return will not be reached because main will end via terminate/abort
}

The program output will be approximately:

Inside noexcept function. Throwing exception...
Custom terminate handler called!