jeudi 3 novembre 2016

Compiler complains make_shared() expects l-value

I have a simple class. One of its constructors takes two ints as arguments:

Dino@MacDino:~/code/test$ cat simple_class.h

class SimpleClass {
   private:
      int d_ii;
      int d_jj;
   public:
      SimpleClass() : d_ii(40), d_jj(10) {}
      SimpleClass( const int ii, const int jj ) : d_ii(ii), d_jj(jj) {}
};

Dino@MacDino:~/code/test$ cat test.t.cpp

//----------------------------------------------------------------//
//
//  Test Program
#include <memory>
#include <simple_class.h>

int main ( int argc, char * argv[] )
{
   SimpleClass sc1;
   SimpleClass sc2( 10, 20 );

   std::shared_ptr<SimpleClass> spSc1( new SimpleClass(10,12) );

   int ii = 10;
   int jj = 16;
   std::shared_ptr<SimpleClass> spSc2 = std::make_shared<SimpleClass> ( ii, jj );

   std::shared_ptr<SimpleClass> spSc3 = std::make_shared<SimpleClass> ( 10, 16 );

   return 0;
}

Running on a macbook. Here is my compile:

Dino@MacDino:~/code/test$ gcc -o test -I. test.t.cpp -lstdc++

test.t.cpp:18:41: error: no matching function for call to 'make_shared' std::shared_ptr spSc3 = std::make_shared ( 10, 16 ); ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~ /Applications/http://ift.tt/2esnRqv: note: candidate function [with _Tp = SimpleClass, _A0 = int, _A1 = int] not viable: expects an l-value for 1st argument make_shared(_A0& __a0, _A1& __a1) ^ /Applications/http://ift.tt/2flU4xA: note: candidate function template not viable: requires 0 arguments, but 2 were provided make_shared() ^ /Applications/http://ift.tt/2eslnZj: note: candidate function template not viable: requires single argument '__a0', but 2 arguments were provided make_shared(_A0& __a0) ^ /Applications/http://ift.tt/2flTDmX: note: candidate function template not viable: requires 3 arguments, but 2 were provided make_shared(_A0& __a0, _A1& __a1, _A2& __a2) ^ 1 error generated.

So the question is, why doesn't this usage of make_shared work?

std::shared_ptr<SimpleClass> spSc2 = std::make_shared<SimpleClass>( 10, 16 );

Notice that the version with the l-values passed does compile:

   int ii = 10;
   int jj = 16;
   std::shared_ptr<SimpleClass> spSc2 = std::make_shared<SimpleClass> ( ii, jj );

Can anyone explain why this is? My constructor declares the arguments as const (although that should not be necessary). And the examples given here http://ift.tt/2fgs6qD pass constants to make_shared. Any ideas? Thanks.

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