Re: move constructors likely a requirement

[Carlo De Falco]

> Il giorno 5 set 2019, alle ore 16:36, Carlo De Falco <address@hidden> ha 
> scritto:
> 
> 
> 
>> Il giorno 5 set 2019, alle ore 16:20, John W. Eaton <address@hidden> ha 
>> scritto:
>> 
>> On 9/5/19 7:59 AM, Carlo De Falco wrote:
>>>> Il giorno 4 set 2019, alle ore 20:14, John W. Eaton <address@hidden> ha 
>>>> scritto:
>>>> 
>>>> If I understand correctly, -Wpessimizing-move is supposed to be implied 
>>>> when compiling C++ code with GCC and using -Wall.
>>> If I understand correctly, copy elision is mandated by the standard only 
>>> since c++14, while we are using c++11.
>>> Could it be possible that gcc and clang have different criteria to decide 
>>> whether to make the optimization?
>> 
>> I compiled the following program with g++ 9 (the version that introduced the 
>> -Wpessimizing-move warning option) using both -std=c++17 and no -std option 
>> (defaults to c++14) and the warnings are the same in both cases.
....
> 
> the -std option does not seem to make any difference but it seems g++ and 
> clang++ do behave differently ...
> 
> c.

If I compile and run the following

#include <iostream>

struct T {
  void show () { };
  T () { std::cout << "default constructor" << std::endl; }
  T (const T& a) { std::cout << "copy constructor" << __func__ << std::endl; }
  ~T () { std::cout << "destructor" << std::endl; }
  T (T&& a) { std::cout << "move constructor" << std::endl; }
};

T fn ()
{
 T t1;

 // No warning from GCC here.
 T t2 = (T ());

 t2.show ();

 // GCC does warn here.
 return (t1);
}

int
main ()
{
  T b = fn ();
  return 0;
}

I get 

$ ./a.out 
default constructor
default constructor
destructor
destructor


Am I right to infer that copy elision is actually being performed?
If I put back the std::move calls I get instead 

$ ./a.out 
default constructor
default constructor
move constructor
destructor
move constructor
destructor
destructor
destructor

c.