如何使用以std :: weak_ptr为键的std :: map? [英] How can I use a std::map with std::weak_ptr as key?

问题描述

如何使用std::weak_ptr作为std::map的键，如以下代码所示?

How can I use std::weak_ptr as key for a std::map as shown in the following code?

#include <map>
#include <memory>

int main()
{
    std::map< std::weak_ptr<int>, bool > myMap;

    std::shared_ptr<int> sharedptr(new int(5));
    std::weak_ptr<int> weakptr = sharedptr;

    myMap[weakptr] = true;

    return 0;
}

上面的程序没有构建，并且尝试编译它会产生许多错误消息，例如:

The above program doesn't build and trying to compile it gives many error messages such as:

1>C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\xfunctional(125): error C2784: 'bool std::operator <(const std::_Tree<_Traits> &,const std::_Tree<_Traits> &)' : could not deduce template argument for 'const std::_Tree<_Traits> &' from 'const std::tr1::weak_ptr<_Ty>'
1>          with
1>          [
1>              _Ty=int
1>          ]
1>          C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\xtree(1885) : see declaration of 'std::operator <'
1>          C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\xfunctional(124) : while compiling class template member function 'bool std::less<_Ty>::operator ()(const _Ty &,const _Ty &) const'
1>          with
1>          [
1>              _Ty=std::tr1::weak_ptr<int>
1>          ]
1>          C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\map(71) : see reference to class template instantiation 'std::less<_Ty>' being compiled
1>          with
1>          [
1>              _Ty=std::tr1::weak_ptr<int>
1>          ]
1>          C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\xtree(451) : see reference to class template instantiation 'std::_Tmap_traits<_Kty,_Ty,_Pr,_Alloc,_Mfl>' being compiled
1>          with
1>          [
1>              _Kty=std::tr1::weak_ptr<int>,
1>              _Ty=bool,
1>              _Pr=std::less<std::tr1::weak_ptr<int>>,
1>              _Alloc=std::allocator<std::pair<const std::tr1::weak_ptr<int>,bool>>,
1>              _Mfl=false
1>          ]

由于以下行而发生问题:

The problem occurs due to the following line:

myMap[weakptr] = true;

错误消息似乎与operator<有关.我是否需要为weak_ptr定义operator<?确实，需要定义哪些运算符才能将数据类型用作std::map的键?

The error messages seem to be related to operator<. Do I need to define operator< for weak_ptr? Exactly, what operators need to be defined in order to use a data type as a key for a std::map?

(我应该注意，我已经在std名称空间中定义了operator==.此外，我打算将<​​c4>用作自定义类类型，而不是int.)

(I should note that I have already defined operator== in the std namespace. Also, I plan to use a weak_ptr to a custom class type and not an int.)

推荐答案

C ++ 11提供了用于比较std::weak_ptr的适当机制，即:std::owner_less.

C++11 provides appropriate mechanisms for comparisons of std::weak_ptr, namely: std::owner_less.

这应该是地图和集合的默认设置.如果您正在使用的C ++编译器遇到困难，请尝试使用std::owner_less(如果可用).如果不可用，则需要提供与std::owner_less类似的机制，以便可以适当地比较std::weak_ptr对象.

This should be the default for maps and sets. If the C++ compiler you are using is having a hard time, try using std::owner_less if it is available. If it is not available, you will need to provide a similar mechanism as std::owner_less so you can appropriately compare std::weak_ptr objects.

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