According to the C++23 standard, what is the output of this program?
#include <iostream>
struct X {
X() { std::cout << "a"; }
X(const X &x) { std::cout << "b"; }
const X &operator=(const X &x) {
std::cout << "c";
return *this;
}
};
int main() {
X x;
X y(x);
X z = y;
z = x;
}What is the difference between direct-initialization and copy-initialization? Do they in any way relate to the copy constructor and copy assignment operator?
abbc
The first line in main(), X x; is straightforward, it calls the default constructor.
The next two lines is the heart of the question: The difference between X y(x) and X z = y is not that the first calls the copy constructor, and the second calls the copy assignment operator. The difference is that the first is direct-initialization (§[dcl.init.general]¶15 in the standard) and the second is copy-initialization (§[dcl.init.general]¶14).
Otherwise, if the initialization is direct-initialization, or if it is copy-initialization where the cv-unqualified version of the source type is the same class as (...) the class of the destination, constructors are considered.
So both our cases use the copy constructor.
Not until z = x; do we have an actual assignment that uses the assignment operator.
See https://stackoverflow.com/a/1051468 for a more detailed discussion of direct- vs copy-initialization.