std::map::插入麻烦

Question

我有非常简单和愚蠢的麻烦：

std::map<b2Vec2, b2Body*> mTakePoints;
mTakePoints.insert(std::make_pair(point, body));

编译器说：

In file included from /usr/include/c++/4.4/string:50,
                 from /usr/include/ClanLib-2.2/ClanLib/Display/../Core/Text/string_types.h:34,
                 from /usr/include/ClanLib-2.2/ClanLib/Display/display.h:35,
                 from /usr/include/ClanLib-2.2/ClanLib/display.h:40,
                 from /home/pfight/Themisto/include/World/Actions/Action.hpp:21,
                 from /home/pfight/Themisto/include/World/Actions/TakeAction.hpp:21,
                 from /home/pfight/Themisto/src/World/Actions/TakeAction.cpp:18:
/usr/include/c++/4.4/bits/stl_function.h: In member function ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = b2Vec2]’:
/usr/include/c++/4.4/bits/stl_tree.h:1170:   instantiated from ‘std::pair<typename std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(const _Val&) [with _Key = b2Vec2, _Val = std::pair<const b2Vec2, b2Body*>, _KeyOfValue = std::_Select1st<std::pair<const b2Vec2, b2Body*> >, _Compare = std::less<b2Vec2>, _Alloc = std::allocator<std::pair<const b2Vec2, b2Body*> >]’
/usr/include/c++/4.4/bits/stl_map.h:500:   instantiated from ‘std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename _Alloc::rebind<std::pair<const _Key, _Tp> >::other>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(const std::pair<const _Key, _Tp>&) [with _Key = b2Vec2, _Tp = b2Body*, _Compare = std::less<b2Vec2>, _Alloc = std::allocator<std::pair<const b2Vec2, b2Body*> >]’
/home/pfight/Themisto/src/World/Actions/TakeAction.cpp:43:   instantiated from here
/usr/include/c++/4.4/bits/stl_function.h:230: error: no match for ‘operator<’ in ‘__x < __y’
/usr/include/ClanLib-2.2/ClanLib/Display/../Core/Text/string_data16.h:383: note: candidates are: bool operator<(const CL_StringData16&, const wchar_t*)
/usr/include/ClanLib-2.2/ClanLib/Display/../Core/Text/string_data16.h:382: note:                 bool operator<(const wchar_t*, const CL_StringData16&)
/usr/include/ClanLib-2.2/ClanLib/Display/../Core/Text/string_data16.h:381: note:                 bool operator<(const CL_StringData16&, const CL_StringData16&)
/usr/include/ClanLib-2.2/ClanLib/Display/../Core/Text/string_data8.h:383: note:                 bool operator<(const CL_StringData8&, const char*)
/usr/include/ClanLib-2.2/ClanLib/Display/../Core/Text/string_data8.h:382: note:                 bool operator<(const char*, const CL_StringData8&)
/usr/include/ClanLib-2.2/ClanLib/Display/../Core/Text/string_data8.h:381: note:                 bool operator<(const CL_StringData8&, const CL_StringData8&)

TakeAction.cpp:43 这是带有插入调用的行。为了好玩，我接下来尝试了：

std::pair<b2Vec2, b2Body*> item(point, body);
mTakePoints.insert(item);

都一样。
我很困惑，请解释一下，哪里出了问题？

Answer 1

您是否定义了以下运算符：？

bool operator< (const b2Vec2&, const b2Vec2&)

std::map 要求为键定义小于运算符，或者提供小于运算符作为模板参数。特别是，std::map 被实现为（排序的）二叉树，并且需要运算符<;找出放置新物品的位置。

Answer 2

对于地图、集合等有序容器，必须告知在何处放置新项目。这是通过实现小于运算符来完成的：

bool operator< (const b2Vec2& first, const b2Vec2& second) 
{
   return first.some_attribute < second.some_attribute;
}

您可以在编译错误中看到它丢失了：

/usr/include/c++/4.4/bits/stl_function.h:230: error: no match for ‘operator<’ in ‘__x < __y’

输出的其余部分告诉您编译器尝试找到合适的运算符但失败了。