Question

我有一个模板类（Word）：

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

我想添加一个运算符：

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;
}

但是我得到了这个标记：

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

在你成为朋友之前首先声明函数函数。

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

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

然后，类中的friend声明应该没问题。

Answer 2

问题是由于您没有声明成员operator&重载，因此生成的声明不是模板化的。如果使用g++进行编译，则会看到与此相关的有用警告消息：

test4.cpp:4:60: warning: friend declaration ‘Word<C> operator&(Word<C>&, Word<C>&)’ declares a non-template function [-Wnon-template-friend]
test4.cpp:4:60: note: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) 
test4.cpp: In function ‘int main()’:

告诉你类中的朋友声明实际上没有声明模板函数。

要修复此operator&本身的模板。这是一个完整的例子（注意T != U）：

#include <iostream>

using namespace std;

template<int T>
class Word {
  public:
    template<int C>
    friend Word<C> operator&(Word<C>& word1, Word<C>& word2);
};

template<int C>
Word<C> operator&(Word<C>& word1, Word<C>& word2){
  Word<C> ans;
  std::cout << C << std::endl;
  // TODO add your stuff here
  return ans;
}

int main() {
  Word<8> a, b;
  Word<8> c = a & b;
}

编译并运行：

$ g++ test4.cpp && ./a.out
Inside operator &
8

如何在c ++中使用类模板声明new运算符

2 个答案: