重載<operator c ++

Question

我目前正試圖為一個我建立的存儲NFL戲劇的班級超過一個不足的操作員。 對於我的課程項目，我們將數據項插入到BST中。 我已經從我的班級中以這種方式重載了運算符：

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參數以紅色下划線標注，當我嘗試運行它時，我收到以下錯誤：

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

我從函數中的兩個參數中刪除了const，它不再強調我的任何代碼，但是當我編譯時，它給了我這些錯誤：

\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)'

我試圖弄清楚為什么這不起作用，我的BST是模板化的。

Answer 1

對於錯誤C2662： getOffenseTeam()需要標記為const ：

rettype_unclear getOffenseTeam() const {
}

因為你不能在const引用（你的lhs和rhs是）上調用非const方法。

對於錯誤C2228：確保getOffenseTeam()返回的內容具有compare方法。