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

Subject: Re: [boost] [move] Implicit conversion to rvalues and move ctors.
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2014-08-27 19:17:06

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 <boost/move/core.hpp>
> #include <boost/move/utility.hpp>
>
> 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()));
       ^ ~~~~~~~

In case this helps,
Vicente