constructors#2

PHOTO EMBED

Fri Mar 11 2022 09:07:32 GMT+0000 (Coordinated Universal Time)

Saved by @gtsekas #c++ 

#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;
}
content_copyCOPY

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]¶16 in the standard) and the second is copy initialization ([dcl.init]¶15). §[dcl.init]¶17 says: "If the initialization is direct-initialization, or if it is copy-initialization where 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 http://stackoverflow.com/questions/1051379/is-there-a-difference-in-c-between-copy-initialization-and-direct-initializati/1051468#1051468 for a more detailed discussion of direct vs. copy initialization.

https://cppquiz.org/