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

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) {}
};

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 statement:

gcc -o test -I. test.t.cpp -lstdc++

But it produces this error:

test.t.cpp:18:41: error: no matching function for call to 'make_shared'
 std::shared_ptr<SimpleClass> spSc3 = std::make_shared<SimpleClass> ( 10, 16 );
                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:4708:1: 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/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:4692:1: note:
      candidate function template not viable: requires 0 arguments, but 2 were provided
make_shared()
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:4700:1: note:
      candidate function template not viable: requires single argument '__a0', but 2 arguments were provided
make_shared(_A0& __a0)
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:4716:1: 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 my 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 pass constants to make_shared().

4

1 回答 1

3

您正在使用 C 编译器编译 C++ 代码，请g++改用。此外，std::shared_ptr亲戚是 C++11 功能，需要在 G++ 编译器上启用。所以你的命令行将是g++ -o test -I. test.t.cpp -std=c++11（不需要链接到 libstdc++，g++ 会自动链接）

编辑： 此外，G++ 4.2 不支持 C++11 功能。在 OSX 上，请clang++改用

于 2016-11-03T18:41:57.583 回答