Re: [boost] [move] Implicit conversion to rvalues and move ctors.

27 Aug 2014


      On Wed, 27 Aug 2014 16:17:06 -0700, Vicente J. Botet Escriba  
 wrote:
...
Le 26/08/14 02:59, Mostafa a écrit :
...
Here's another use case that fails to compile in C++03 but compiles  
successfully in C++11. (Note, it fails to compile in VS2005 and g++  
4.8.2 -std=c++98, and compiles successfully with g++ 4.8.2 -std=c++11)
MOC is a BOOST_MOVABLE_BUT_NOT_COPYABLE type. Foo is an empty with an  
implicit conversion function to MOC. "MOC m(Foo());" compiles  
successfully but "MOC m2(makeFoo());" fails to compile. Is this a bug  
or a limitation of Boost.Move in C++03?
//----------
//Start Code
//----------
#include 
#include 
struct MOC
{
  MOC() : value(121) {}
  MOC(BOOST_RV_REF(MOC) rhs) : value(rhs.value) {}
  int value;
private:
  BOOST_MOVABLE_BUT_NOT_COPYABLE(MOC)
};
struct Foo
{
  operator MOC ()
  {
    return MOC();
  }
};
Foo makeFoo()
{
  return Foo();
}
int main()
{
  // (1) Successfully Compiles.
  MOC m(Foo());
// (2) Fails to Compile.
  MOC m2(makeFoo());
return 0;
}
I'm getting
../example/lambda_future.cpp:32:8: warning: parentheses were  
disambiguated as a function declaration [-Wvexing-parse]
   MOC m(Foo());
        ^~~~~~~
../example/lambda_future.cpp:32:9: note: add a pair of parentheses to  
declare a variable
   MOC m(Foo());
         ^
         (    )
After adding the parentheses I get
../example/lambda_future.cpp:32:7: error: no matching constructor for  
initialization of 'MOC'
   MOC m((Foo()));
       ^ ~~~~~~~
Yes, you're right. That would explain the different compiler behaviors  
between (1) and (2). Adding double parenthesis to both (1) and (2) now  
gives me the same error messages in g++ 4.8.2 -std=c++98. But they both do  
compile under C++11 mode. FWIW, I have an internal workaround, I'm just  
wondering if this is another undocumented limitation of Boost.Move in  
C++98.

(What's strange is that on my machine -Wall -pedantic doesn't even catch  
that MVPE you noted above.)