Strange behavior of copy-initialization, doesn't call the copy-constructor!

Question

I was reading the difference between direct-initialization and copy-initialization (§8.5/12):

T x(a);  //direct-initialization
T y = a; //copy-initialization         


        

Answer 1

C++ explicitly allows several optimizations involving the copy constructor that actually change the semantics of the program. (This is in contrast with most optimizations, which do not affect the semantics of the program). In particular, there are several cases where the compiler is allowed to re-use an existing object, rather than copying one, if it knows that the existing object will become unreachable. This (copy construction) is one such case; another similar case is the "return value optimization" (RVO), where if you declare the variable that holds the return value of a function, then C++ can choose to allocate that on the frame of the caller, so that it doesn't need to copy it back to the caller when the function completes.

In general, in C++, you are playing with fire if you define a copy constructor that has side effects or does anything other than just copying.