Question

除非定义了B0RKEN，否则以下编译（与命令行中的-DB0RKEN一样）：

#include <functional>
#include <boost/shared_ptr.hpp>
#include <boost/make_shared.hpp>

using boost::shared_ptr;
using boost::make_shared;

using my_fn = std::function<void()>;

void foo()
{
        my_fn fn = [](){};

#ifdef B0RKEN
        shared_ptr<my_fn> k = make_shared<my_fn>(fn);
#else
        shared_ptr<int> k = make_shared<int>(0);
#endif
}

似乎提升正在玩一些有趣的游戏，这可能就是为什么这个代码片段有这个问题。我不明白为什么它适用于shared_ptr<int>而不是shared_ptr<my_fn>。

我对辩论是否应该使用boost或std共享指针不感兴趣。

我从clang ++中得到以下错误：

foo.cpp:15:24: error: call to 'make_shared' is ambiguous
        shared_ptr<my_fn> k = make_shared<my_fn>(fn);
                              ^~~~~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:4670:1: note: candidate function [with _Tp =
      std::__1::function<void ()>, _Args = <std::__1::function<void ()> &>]
make_shared(_Args&& ...__args)
^
/opt/local/include/boost/smart_ptr/make_shared_object.hpp:246:87: note: candidate function [with T = std::__1::function<void ()>, Args =
      <std::__1::function<void ()> &>]
template< class T, class... Args > typename boost::detail::sp_if_not_array< T >::type make_shared( Args && ... args )
                                                                                      ^
1 error generated.

来自g ++：

foo.cpp: In function ‘void foo()’:
foo.cpp:15:45: error: call of overloaded ‘make_shared(my_fn&)’ is ambiguous
  shared_ptr<my_fn> k = make_shared<my_fn>(fn);
                                             ^
foo.cpp:15:45: note: candidates are:
In file included from PATH_TO_TOOLCHAIN/boost-1.59.0/include/boost/smart_ptr/make_shared.hpp:15:0,
                 from PATH_TO_TOOLCHAIN/boost-1.59.0/include/boost/make_shared.hpp:15,
                 from foo.cpp:3:
PATH_TO_TOOLCHAIN/boost-1.59.0/include/boost/smart_ptr/make_shared_object.hpp:246:87: note: typename boost::detail::sp_if_not_array<T>::type boost::make_shared(Args&& ...) [with T = std::function<void()>; Args = {std::function<void()>&}; typename boost::detail::sp_if_not_array<T>::type = boost::shared_ptr<std::function<void()> >]
 template< class T, class... Args > typename boost::detail::sp_if_not_array< T >::type make_shared( Args && ... args )
                                                                                       ^
In file included from PATH_TO_TOOLCHAIN/gcc-4.9.3/include/c++/4.9.3/memory:82:0,
                 from PATH_TO_TOOLCHAIN/boost-1.59.0/include/boost/config/no_tr1/memory.hpp:21,
                 from PATH_TO_TOOLCHAIN/boost-1.59.0/include/boost/smart_ptr/shared_ptr.hpp:23,
                 from PATH_TO_TOOLCHAIN/boost-1.59.0/include/boost/shared_ptr.hpp:17,
                 from foo.cpp:2:
PATH_TO_TOOLCHAIN/gcc-4.9.3/include/c++/4.9.3/bits/shared_ptr.h:600:5: note: std::shared_ptr<_Tp1> std::make_shared(_Args&& ...) [with _Tp = std::function<void()>; _Args = {std::function<void()>&}]
     make_shared(_Args&&... __args)
     ^

Answer 1

my_fn的类型为std::function<void()>;，位于名称空间std中。

当你试图调用make_shared<my_fn>(fn);时，它会看到两个boost版本（因为你写了using boost::make_shared;）和std版本凭借ADL。

int不属于std命名空间，不考虑make_shared的标准版本。

尽可能使用合格的名称来避免此类问题。

＆＃39; make_shared＆＃39;很暧昧

1 个答案: