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

28 Aug 2014

      Le 28/08/14 01:49, Mostafa a écrit :
...
On Wed, 27 Aug 2014 16:17:06 -0700, Vicente J. Botet Escriba 
<vicente.botet@wanadoo.fr> 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 <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()));
       ^ ~~~~~~~
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.
There is something specific to conversions and move semantics that seems 
to don't work. return an rvalue :(

#include <boost/move/core.hpp>
#include <boost/move/utility.hpp>

struct Bar
{};
struct MOC
{
   MOC() : value(121) {}
   MOC(Bar) : value(122) {}
   MOC(BOOST_RV_REF(MOC) rhs) : value(rhs.value) {}
   int value;
private:
   BOOST_MOVABLE_BUT_NOT_COPYABLE(MOC)
};

struct Foo
{
   operator MOC () const
   {
     return MOC();
   }
};

Foo makeFoo()
{
   return Foo();
}
Bar makeBar()
{
   return Bar();
}

MOC makeMOC()
{
   return MOC();
}

void f(MOC) {}
void g(MOC&) {}
void h(MOC const&) {}

int main()
{
   // (1) Fails  to Compile..
   //MOC m(Foo());
   //MOC m((Foo()));
   // (1a) Compile.
   MOC m((Bar()));

   // (2) Fails to Compile.
   //  MOC m2(makeFoo());
   // (2a) Compiles.
   MOC m3(makeMOC());
   MOC m4(makeBar());

   // (3) Compiles.
   f(MOC());
   // (3.a) Fails to Compile.
   //f(Bar());
   //f(Foo());

   // (4) Compiles.
   MOC x;
   g(x);
   // (4.a) Fails to Compile.
   //Bar y;
   //g(y);
   //Foo z;
   //g(z);

   // (5) Fails to Compile.
   //h(MOC()); // Normal is not MOC is not CopyConstructible
   // (5.a) Compiles.
   h(Bar()); // MOC is not Constructible from Bar
   h(Foo()); // // Foo is convertible to MOC

   return 0;
}
...
(What's strange is that on my machine -Wall -pedantic doesn't even 
catch that MVPE you noted above.)
Here it is my cl

"/Users/viboes/clang/clang+llvm-3.2-x86_64-apple-darwin11/bin/clang++" 
-x c++ -Wextra -Wno-long-long -Wunused-function -Wno-unused-value 
-Wno-c99-extensions -Wno-variadic-macros -O0 -g -O0 -fno-inline -Wall -g 
-DBOOST_ALL_NO_LIB=1 -DBOOST_CHRONO_STATIC_LINK=1 
-DBOOST_SYSTEM_NO_DEPRECATED -DBOOST_SYSTEM_STATIC_LINK=1 
-DBOOST_THREAD_BUILD_LIB=1 -DBOOST_THREAD_POSIX 
-DBOOST_THREAD_THROW_IF_PRECONDITION_NOT_SATISFIED 
-DBOOST_THREAD_USE_LIB=1 -I"../../.." -c -o 
"../../../bin.v2/libs/thread/test/ex_lambda_future2_lib.test/clang-darwin-3.2/debug/threading-multi/lambda_future.o" 
"../example/lambda_future.cpp"

Vicente