使用std :: reference_wrapper作为std :: map中的键 [英] Using std::reference_wrapper as the key in a std::map

问题描述

我有一个类层次结构中的一堆对象，并希望使用对这些对象的引用作为地图中的键的 std :: map 。它看起来像 std :: reference_wrapper 将是正是这需要的，但我似乎无法使它的工作。到目前为止我尝试了：

  class Object {//我的层次结构的基类
 //最详细的unimportant 
 public 
 virtual bool operator< （const Object&）const; //比较运算符
}; 
 
 std :: map< std :: reference_wrapper< const Object>，int>表; 
 
 auto it = table.find（object）; 
 
 table [object] = 42; 
 
 table [object] ++ 
  

但是，我总是有些模糊来自编译器的错误：

  / usr / include / c ++ / 4.5.3 / bits / stl_function.h： bool std :: less< _Tp> :: operator（）（const _Tp& const _Tp&）const [with _Tp = std :: reference_wrapper< const Object>]'：
 /usr/include/c++/4.5 .3 / bits / stl_tree.h：1522：38：从'std :: _ Rb_tree< _Key，_Val，_KeyOfValue，_Compare，_Alloc> :: iterator std :: _ Rb_tree< _Key，_Val，_KeyOfValue，_Compare，_Alloc> ：find（const _Key&）[with _Key = std :: reference_wrapper< const Object>，_Val = std :: pair< const std :: reference_wrapper< const Object>，int>，_KeyOfValue = std :: _ Select1st& pair< const std :: reference_wrapper< const Object>，int> >，_Compare = std :: less< std :: reference_wrapper< const Object> >，_Alloc = std :: allocator< std :: pair< const std :: reference_wrapper< const Object>，int> >，std :: _ Rb_tree< _Key，_Val，_KeyOfValue，_Compare，_Alloc> :: iterator = std :: _ Rb_tree_iterator< std :: pair< const std :: reference_wrapper< const Object> >]'
 /usr/include/c++/4.5.3/bits/stl_map.h:697:29：从'std :: map< _Key，_Tp，_Compare，_Alloc> :: iterator std实例化： ：map< _Key，_Tp，_Compare，_Alloc> :: find（const key_type&）[with _Key = std :: reference_wrapper< const Object>，_Tp = int，_Compare = std :: less< std :: reference_wrapper& ; >，_Alloc = std :: allocator< std :: pair< const std :: reference_wrapper< const Object>，int> >，std :: map< _Key，_Tp，_Compare，_Alloc> :: iterator = std :: _ Rb_tree_iterator< std :: pair< const std :: reference_wrapper< const Object> >，key_type = std :: reference_wrapper< const Object>]'
 testfile.cpp：39：31：从这里实例化
 /include/c++/4.5.3/bits/stl_function.h:230 ：22：error：没有匹配'operator'<'in'__x< __y'
  

错误似乎是说它不能比较两个 std :: reference_wrapper< const Object> 对象，但它似乎应该是可能的 - std :: reference_wrapper 有一个转换操作符可以隐式将它转换为 T& （ const Object& code>有运算符< ，为什么它不工作？

这只是g ++中的一个错误？

解决方案

似乎如果你让比较运算符是一个自由函数调用虚拟成员函数）。

如果它是一个成员函数， a< b 真的表示 a.operator<（b）; ，并且左侧参数不考虑隐式转换。 $ b

I have a bunch of objects in a class hierarchy and would like to make a std::map using references to those objects as the keys in the map. Its seems like std::reference_wrapper would be exactly what is needed for this, but I can't seem to make it work. What I've tried so far:

class Object { // base class of my hierarchy
    // most details unimportant
public
    virtual bool operator< (const Object &) const;  // comparison operator
};

std::map<std::reference_wrapper<const Object>, int> table;

auto it = table.find(object);

table[object] = 42;

table[object]++

However, I always get somewhat obscure errors from the compiler:

/usr/include/c++/4.5.3/bits/stl_function.h: In member function ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = std::reference_wrapper<const Object>]’:
/usr/include/c++/4.5.3/bits/stl_tree.h:1522:38:   instantiated from ‘std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::find(const _Key&) [with _Key = std::reference_wrapper<const Object>, _Val = std::pair<const std::reference_wrapper<const Object>, int>, _KeyOfValue = std::_Select1st<std::pair<const std::reference_wrapper<const Object>, int> >, _Compare = std::less<std::reference_wrapper<const Object> >, _Alloc = std::allocator<std::pair<const std::reference_wrapper<const Object>, int> >, std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const std::reference_wrapper<const Object>, int> >]’
/usr/include/c++/4.5.3/bits/stl_map.h:697:29:   instantiated from ‘std::map<_Key, _Tp, _Compare, _Alloc>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::find(const key_type&)[with _Key = std::reference_wrapper<const Object>, _Tp = int, _Compare = std::less<std::reference_wrapper<const Object> >, _Alloc = std::allocator<std::pair<const std::reference_wrapper<const Object>, int> >, std::map<_Key, _Tp, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const std::reference_wrapper<const Object>, int> >, key_type = std::reference_wrapper<const Object>]’
testfile.cpp:39:31:   instantiated from here
/include/c++/4.5.3/bits/stl_function.h:230:22: error: no match for ‘operator<’ in ‘__x < __y’

The error seems to be saying it can't compare two std::reference_wrapper<const Object> objects, but it seems like it should be possible -- std::reference_wrapper has a conversion operator that can implicitly convert it to a T& (const Object & here), and Object has a operator <, so why doesn't it work?

Should it work and this is merely a bug in g++? Or is something else going on?

解决方案

It seems that it would work if you made the comparison operator a free function (that perhaps calls a virtual member function).

If it is a member function, a < b really means a.operator<(b); and implicit conversions are not considered for the left-side argument.

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