重载运算符 C++ <

Question

我目前正在尝试为我构建的存储 NFL 比赛的类重载小于运算符。对于我的课堂项目，我们将数据项插入到 BST 中。我以这种方式从我的类中重载了运算符：

friend bool operator< (const NFLData& lhs, const NFLData& rhs){
    if((lhs.getOffenseTeam().compare(rhs.getOffenseTeam())) == 0 ){//They're equal, go left
        return true;
    }
    else{
        return false;
    }
}

但是，在Visual Studio 2010中，lhs和rhs参数以红色下划线显示，当我尝试运行它时，我收到以下错误：

c:project 4draftnfldata.h(105): error C2228: left of '.compare' must have class/struct/union
c:project 4draftnfldata.h(105): error C2662: 'NFLData::getOffenseTeam' : cannot convert 'this' pointer from 'const NFLData' to 'NFLData &' Conversion loses qualifiers

我

从函数中的两个参数中删除了 const，它不再为我的任何代码添加下划线，但是当我开始编译时，它给了我这些错误：

bst.h(82): error C2678: binary '<' : no operator found which takes a left-hand operand of type 'const NFLData' (or there is no acceptable conversion)
          c:project 4draftnfldata.h(104): could be 'bool operator <(NFLData &,NFLData &)' [found using argument-dependent lookup]
         while trying to match the argument list '(const NFLData, NFLData)'
          c:project 4draftbst.h(79) : while compiling class template member function 'void BST<T>::insert(const T &,BST<T>::TreeNode *&) const'
         with
          [
              T=NFLData
         ]
         c:project 4drafttest.cpp(35) : see reference to class template instantiation 'BST<T>' being compiled
          with
         [
              T=NFLData
          ]
c:project 4draftbst.h(84): error C2679: binary '<' : no operator found which takes a right-hand operand of type 'const NFLData' (or there is no acceptable conversion)
1>          c:project 4draftnfldata.h(104): could be 'bool operator <(NFLData &,NFLData &)' [found using argument-dependent lookup]
1>          while trying to match the argument list '(NFLData, const NFLData)'

我试图弄清楚为什么这不起作用，我的 BST 是模板化的。

Answer 1

对于错误 C2662：需要将getOffenseTeam()标记为const：

rettype_unclear getOffenseTeam() const {
}

因为您不能在 const 引用（您的 lhs 和 rhs 是）上调用非 const 方法。

对于错误 C2228：确保getOffenseTeam()返回的任何内容都有compare方法。