c++ - 错误 C2678：二进制“<”：未找到采用左操作数的运算符...（或没有可接受的转换）

Question

这是我在地图中查找值的代码：

bool myclass::getFreqFromCache( plVariablesConjunction& varABC, vector<plFloat>& freq )
{
std::map<plVariablesConjunction, std::vector<plFloat>>::iterator freqItr;
    freqItr = freqCache.find(varABC);

    if (freqItr != freqCache.end())
        {
        freq = freqItr->second;
        return true;
        }
 }

“PlVariablesConjunction”是 ProBT 库数据类型。它包含运算符“==”，如果发现两个变量相同，则返回 true，否则返回 false。

这是错误：

C:\Program Files\Microsoft Visual Studio 10.0\VC\include\xfunctional(125): error C2678: binary '<' : no operator found which takes a left-hand operand of type 'const plVariablesConjunction' (or there is no acceptable conversion)
1>          E:\ProBT22\probt-spl-2.2.0-expires-20121130-vc10-dynamic-release\include\plSymbol.h(71): could be 'bool operator <(const plSymbol &,const plSymbol &)' [found using argument-dependent lookup]
1>          while trying to match the argument list '(const plVariablesConjunction, const plVariablesConjunction)'
1>          C:\Program Files\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=plVariablesConjunction
1>          ]
1>          C:\Program Files\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=plVariablesConjunction
1>          ]
1>          C:\Program Files\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=plVariablesConjunction,
1>              _Ty=std::vector<plProbValue>,
1>              _Pr=std::less<plVariablesConjunction>,
1>              _Alloc=std::allocator<std::pair<const plVariablesConjunction,std::vector<plProbValue>>>,
1>              _Mfl=false
1>          ]
1>          C:\Program Files\Microsoft Visual Studio 10.0\VC\include\xtree(520) : see reference to class template instantiation 'std::_Tree_nod<_Traits>' being compiled
1>          with
1>          [
1>              _Traits=std::_Tmap_traits<plVariablesConjunction,std::vector<plProbValue>,std::less<plVariablesConjunction>,std::allocator<std::pair<const plVariablesConjunction,std::vector<plProbValue>>>,false>
1>          ]
1>          C:\Program Files\Microsoft Visual Studio 10.0\VC\include\xtree(659) : see reference to class template instantiation 'std::_Tree_val<_Traits>' being compiled
1>          with
1>          [
1>              _Traits=std::_Tmap_traits<plVariablesConjunction,std::vector<plProbValue>,std::less<plVariablesConjunction>,std::allocator<std::pair<const plVariablesConjunction,std::vector<plProbValue>>>,false>
1>          ]
1>          C:\Program Files\Microsoft Visual Studio 10.0\VC\include\map(81) : see reference to class template instantiation 'std::_Tree<_Traits>' being compiled
1>          with
1>          [
1>              _Traits=std::_Tmap_traits<plVariablesConjunction,std::vector<plProbValue>,std::less<plVariablesConjunction>,std::allocator<std::pair<const plVariablesConjunction,std::vector<plProbValue>>>,false>
1>          ]
1>          e:\probt22\work\yasin\testmmhcfinalversion\testmmhc_mi_probt_sw\mmhc\slidingWindow.h(55) : see reference to class template instantiation 'std::map<_Kty,_Ty>' being compiled
1>          with
1>          [
1>              _Kty=plVariablesConjunction,
1>              _Ty=std::vector<plProbValue>
1>          ]

Answer 1

std::map（通常）实现为二叉搜索树，最常见的是红黑树。它需要为键值定义线性顺序才能在树中找到正确的位置。这就是为什么std::map尝试调用operator<插入的键值。

您的课程不提供operator<. 为您的班级定义operator<或为模板提供比较功能：std::map<plVariablesConjunction, std::vector<plFloat>, my_comparison_function>.

Answer 2

要使用地图类，模板需要两种，也可能是三种类型：

std::map <key_type, data_type, [comparison_function]>

您需要提供比较函数或重载键类中的 < 运算符。

请注意，比较函数在括号中，表明它是可选的，只要您的 key_type 定义了小于运算符 <

Answer 3

map<> 不operator==用于检查插入的值。它需要operator<对键值进行比较。