How to use noexcept in assignment operator with copy-and-swap idiom?

Question

The move assignment operator should often be declared noexcept (i.e. to store the type in STL containers). But the copy-and-swap idiom allows both copy- and move- assignment operators to be defined in a single piece of code. What to do with noexcept specifier in this case? The copy construction can throw, but I doubt whether it can violate the noexcept specifier.

// Is it correct considering that T copy constructor can throw?
T& operator=(T other) noexcept;

Answer 1

Since the copy is made on the caller's side of the call, it is not part of what your function does. It can therefore not be controlled by your function and consequently, you can not include this information in the noexcept specification.

The only thing you could do is to play it safe and add both options to your noexcept specification. Of course, that means you are getting some false-negatives.

Answer 2

As usual, Daniel Frey is correct. All I want is showing a piece of code that illustrates the point.

#include <iostream>

struct foo {

    foo() = default;

    foo(const foo&) {
        std::cout << "throw\n";
        throw 1;
    }

    foo& operator =(foo) noexcept {
        return *this;
    }
};

int main() {

    foo f, g;
    try {
        f = g; // throws
    }
    catch(int) {
        std::cout << "catch\n";
    }
}

When compiled with gcc 4.8.1 (-std=c++11 -Wall -Wextra -pedantic) it gives no warnings. Running the code produces the following output:

throw
catch

Therefore, the copy constructor does throw when called but that's no considered inside operator =() and, therefore, the noexcept promise was fulfilled. Otherwise, terminate would be called before catch could be printed out.