c++ - 如何在 C++ 中使用类模板声明新运算符

Question

我有一个模板类(Word):

template<int C>
class Word {
...
}

我想添加一个运算符(operator):

friend Word<C> operator&(Word<C>& word1, Word<C>& word2);   // & operator between all bits

所以现在我有:

template<int C>
class Word {
   ...
   friend Word<C> operator&(Word<C>& word1, Word<C>& word2);    // & operator between all bits
}

template<int C>
Word<C> operator&(Word<C>& word1, Word<C>& word2) {
    Word<C> ans = new Word<C>;

    for (int i = 0; i < C; i++) {
        ans[i] = word1[i] & word2[i];
    }
    return ans;
}

但我得到了这个标记:

Multiple markers at this line
    - (if this is not what you intended, make sure the function template has already been declared and add <> after the function name 
     here)
    - friend declaration 'Word<C> operator&(Word<C>&, Word<C>&)' declares a non-template function [-Wnon-template-friend]

使用运算符时出现此错误:

undefined reference to `operator&(Word<8>&, Word<8>&)'

使用代码:

Word<C> tmp1 = (words[i] & mask);

Answer 1

先声明函数 function，然后再将其设为友元。

// Forward declare the class.
template<int C> class Word;

// Declare the function.
template<int C> Word<C> operator&(Word<C>& word1, Word<C>& word2);

那么，类中的friend声明就OK了。