Re: move constructors likely a requirement

[Carlo De Falco]

> 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.
> 
> #include <utility>
> 
> struct T {
>  void show () { }
> };
> 
> T fn ()
> {
>  T t1;
> 
>  // No warning from GCC here.
>  T t2 = std::move (T ());
> 
>  t2.show ();
> 
>  // GCC does warn here.
>  return std::move (t1);
> }
> 
> jwe
> 

Here's the results  clang++ (Apple LLVM version 10.0.1 (clang-1001.0.46.4)):

------------------------------------------
## no -std option (defaults to gnu++14)
$ clang++ -Wall -c prova.cc 
prova.cc:12:9: warning: moving a temporary object prevents copy elision 
[-Wpessimizing-move]
 T t2 = std::move (T ());
        ^
prova.cc:12:9: note: remove std::move call here
 T t2 = std::move (T ());
        ^~~~~~~~~~~    ~
prova.cc:17:9: warning: moving a local object in a return statement prevents 
copy elision [-Wpessimizing-move]
 return std::move (t1);
        ^
prova.cc:17:9: note: remove std::move call here
 return std::move (t1);
        ^~~~~~~~~~~  ~
2 warnings generated.

------------------------------------------
## -std=c++11 
$ clang++ -std=c++11 -Wall -c prova.cc 
prova.cc:12:9: warning: moving a temporary object prevents copy elision 
[-Wpessimizing-move]
 T t2 = std::move (T ());
        ^
prova.cc:12:9: note: remove std::move call here
 T t2 = std::move (T ());
        ^~~~~~~~~~~    ~
prova.cc:17:9: warning: moving a local object in a return statement prevents 
copy elision [-Wpessimizing-move]
 return std::move (t1);
        ^
prova.cc:17:9: note: remove std::move call here
 return std::move (t1);
        ^~~~~~~~~~~  ~
2 warnings generated.

------------------------------------------
## -std=c++14
$ clang++ -std=c++14 -Wall -c prova.cc 
prova.cc:12:9: warning: moving a temporary object prevents copy elision 
[-Wpessimizing-move]
 T t2 = std::move (T ());
        ^
prova.cc:12:9: note: remove std::move call here
 T t2 = std::move (T ());
        ^~~~~~~~~~~    ~
prova.cc:17:9: warning: moving a local object in a return statement prevents 
copy elision [-Wpessimizing-move]
 return std::move (t1);
        ^
prova.cc:17:9: note: remove std::move call here
 return std::move (t1);
        ^~~~~~~~~~~  ~
2 warnings generated.

------------------------------------------
## -std=c++17
$ clang++ -std=c++17 -Wall -c prova.cc 
prova.cc:12:9: warning: moving a temporary object prevents copy elision 
[-Wpessimizing-move]
 T t2 = std::move (T ());
        ^
prova.cc:12:9: note: remove std::move call here
 T t2 = std::move (T ());
        ^~~~~~~~~~~    ~
prova.cc:17:9: warning: moving a local object in a return statement prevents 
copy elision [-Wpessimizing-move]
 return std::move (t1);
        ^
prova.cc:17:9: note: remove std::move call here
 return std::move (t1);
        ^~~~~~~~~~~  ~
2 warnings generated.

------------------------------------------

the -std option does not seem to make any difference but it seems g++ and 
clang++ do behave differently ...

c.