error: declaration of ‘operator*’ as non-function

Question

我在使用模板类的绑定模板友元函数时遇到了一些问题。我参考了与我有类似和简单问题的页面：vect.hpp:13:33: error: declaration of ‘operator<<’ as non-function ，但我还是一头雾水，问题无法解决。这是我的代码：

#include<iostream>  
#include<valarray>

using namespace std;

//forward declaration
template<typename ElemType, size_t row, size_t col = row>
class matrix;

template<typename ET, size_t R, size_t C>
matrix<ET, R, C> operator*(ET multier, matrix<ET, R, C> mattem);

template<typename ElemType, size_t row, size_t col>
class matrix
{
    private:
        valarray<ElemType> mat;
    public:
        matrix(): mat(row*col) {}
        matrix(initializer_list<ElemType>);

        matrix operator*(ElemType multier) const
        {
            matrix mattem;
            mattem.mat = this->mat*multier;
            return mattem;
        }
        
        //friend function prototype
        friend matrix<ElemType, row, col> operator*<>(ElemType multier, matrix<ElemType, row, col> mattem);

        void show() const
        {
            for (ElemType Elem : this->mat)
                cout << Elem << " ";
            cout << endl;
        }
};

template<typename ElemType, size_t row, size_t col>
matrix<ElemType, row, col>::matrix(initializer_list<ElemType> ini)
{
    try
    {
        if (ini.size() != row*col)
            throw "Inappropriate length!";
        this->mat = ini;
    }
    catch (const char * str)
    {
        cout << str << endl;
    }
}

//friend function definition
template<typename ElemType, size_t row, size_t col>
matrix<ElemType, row, col> operator*<>(ElemType multier, matrix<ElemType, row, col> mattem)
{
    return mattem*multier;
}

int main(void)
{
    matrix<int, 2, 3> test = {1, 3, 5, 8, 9, 7};
    matrix<int, 2, 3> abc =2*test;
    abc.show();
}

我使用 gcc version 9.3.0 在 WSL2 (Ubuntu 9.3.0-17ubuntu1~20.04) 中编译并运行它，我得到了如下一系列错误：

test.cpp:28:51: error: declaration of ‘operator*’ as non-function
   28 |         friend matrix<ElemType, row, col> operator*<>(ElemType multier, matrix<ElemType, row, col> mattem);
      |                                                   ^
test.cpp:28:51: error: expected ‘;’ at end of member declaration
   28 |         friend matrix<ElemType, row, col> operator*<>(ElemType multier, matrix<ElemType, row, col> mattem);
      |                                                   ^
      |                                                    ;
test.cpp:28:52: error: expected unqualified-id before ‘<’ token
   28 |         friend matrix<ElemType, row, col> operator*<>(ElemType multier, matrix<ElemType, row, col> mattem);
      |                                                    ^
test.cpp:54:91: error: template-id ‘operator*<>’ in declaration of primary template
   54 | matrix<ElemType, row, col> operator*<>(ElemType multier, matrix<ElemType, row, col> mattem)
      |                                                                                           ^
test.cpp: In function ‘int main()’:
test.cpp:62:29: error: conversion from ‘matrix<[...],[...],2>’ to non-scalar type ‘matrix<[...],[...],3>’ requested
   62 |     matrix<int, 2, 3> abc =2*test;
      |                            ~^~~~~

Answer 1

您不需要在该声明中使用 <>。令人恼火的是，第一个错误并没有告诉你这个，而是由于编译器不知道你的意思而导致的其他错误。

您可以将声明简化为

friend matrix operator*(ElemType multier, matrix mattem);

定义仍然需要完整的模板参数列表

template<typename ElemType, size_t row, size_t col>
matrix<ElemType, row, col> operator*(ElemType multier, matrix<ElemType, row, col> mattem)
{
    return mattem*multier;
}

Answer 2

我发现在模板 classes 中处理友元运算符的最可靠方法是将完整的定义放在 class 本身中：

//forward declaration
template<typename ElemType, size_t row, size_t col = row>
class matrix;
/*
template<typename ET, size_t R, size_t C>
matrix<ET, R, C> operator*(ET multier, matrix<ET, R, C> mattem);
*/
template<typename ElemType, size_t row, size_t col>
class matrix
...
        //friend function prototype
        friend matrix<ElemType, row, col> operator *  (ElemType multier, matrix<ElemType, row, col> mattem){
            return mattem * multier;
        }
...
/*
//friend function definition
template<typename ElemType, size_t row, size_t col>
matrix<ElemType, row, col> operator*(ElemType multier, matrix<ElemType, row, col> mattem)
{
    return mattem*multier;
}
*/

error: declaration of ‘operator*’ as non-function

error: declaration of ‘operator*’ as non-function

c++

class

templates

friend-function

valarray