c++ - 重载 < 运算符 C++

Question

我目前正在尝试为我构建的存储 NFL Plays 的类重载小于运算符。对于我的类(class)项目，我们将数据项插入到 BST 中。我在我的类(class)中以这种方式重载了运算符:

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，lhs 和 rhs 参数在 Visual Studio 2010 中带有红色下划线，当我尝试运行它时，出现以下错误:

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

我从函数的两个参数中删除了 const，它不再强调我的任何代码，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 4draft\nfldata.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 4draft\bst.h(79) : while compiling class template member function 'void BST<T>::insert(const T &,BST<T>::TreeNode *&) const'
         with
          [
              T=NFLData
         ]
         c:\project 4draft\test.cpp(35) : see reference to class template instantiation 'BST<T>' being compiled
          with
         [
              T=NFLData
          ]
c:\project 4draft\bst.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 4draft\nfldata.h(104): could be 'bool operator <(NFLData &,NFLData &)' [found using argument-dependent lookup]
1>          while trying to match the argument list '(NFLData, const NFLData)'

我想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:getOffenseTeam() 需要标记为 const:

rettype_unclear getOffenseTeam() const {
}

因为您不能在常量引用(您的 lhs 和 rhs 是)上调用非常量方法。

对于错误 C2228:确保 getOffenseTeam() 返回的任何内容都有一个 compare 方法。