Re: move constructors likely a requirement

[Carlo De Falco]

> Il giorno 5 set 2019, alle ore 17:04, Carlo de Falco <address@hidden> ha 
> scritto:
> 
> 
> 
>> 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.


BTW, if I use g++ from gcc9 :

$ g++-mp-9 --version
g++-mp-9 (MacPorts gcc9 9.2.0_0) 9.2.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

I see the same output as for clang++ when running the code but one less warning 
when compiling

$ g++-mp-9 -Wall prova.cc 
prova.cc: In function 'T fn()':
prova.cc:21:19: warning: moving a local object in a return statement prevents 
copy elision [-Wpessimizing-move]
   21 |  return std::move (t1);
      |         ~~~~~~~~~~^~~~
prova.cc:21:19: note: remove 'std::move' call

c.