c++ - C++ 中的 friend 和模板

Question

我的 C++ 代码示例中有一个大问题。 “ friend ”和"template"有问题。

错误信息:
Matrix.h:26:79: 警告:

friend declaration 'std::ostream& matrixClass::operator<<(std::ostream&, const matrixClass::Matrix&)' declares a non-template function [-Wnon-template-friend] friend std::ostream &operator<<(std::ostream&, const Matrix &matrix);

Matrix.h:26:79: 注意:

  (if this is not what you intended, make sure the function template

has already been declared and add <> after the function name here)

Matrix.h:28:77: 警告:

  friend declaration 'matrixClass::Matrix<T>*

matrixClass::operator*(const matrixClass::Matrix&, const matrixClass::Matrix&)' declares a non-template function [-Wnon-template-friend] friend Matrix* operator*(const Matrix &m1, const Matrix &m2);

矩阵.cpp:1:0:

C:\Users\Peter\CLionProjects\PK\untitled76\Matrix.h:26:79: warning: friend declaration 'std::ostream& matrixClass::operator<<(std::ostream&, const matrixClass::Matrix&)' declares a non-template function [-Wnon-template-friend] friend std::ostream &operator<<(std::ostream&, const Matrix &matrix);

Matrix.h:26:79: 注意:

  (if this is not what you intended, make sure the function template

has already been declared and add <> after the function name here)

Matrix.h:28:77: 警告:

  friend declaration 'matrixClass::Matrix<T>*

matrixClass::operator*(const matrixClass::Matrix&, const matrixClass::Matrix&)' declares a non-template function [-Wnon-template-friend] friend Matrix* operator*(const Matrix &m1, const Matrix &m2);

CMakeFiles\untitled76.dir/objects.a(main.cpp.obj):在函数“main”中:

main.cpp:8: undefined reference main.cpp:8: 对 matrixClass::Matrix<int>::Matrix(int)'<br>
main.cpp:10: undefined reference to 的 undefined reference matrixClass::Matrix::set(int, int, int)'
main.cpp:11: 对 matrixClass::Matrix<int>::set(int, int, int)'<br>
main.cpp:12: undefined reference to 的 undefined reference matrixClass::Matrix::set(int, int, int)'
main.cpp:13: 对 matrixClass::Matrix<int>::set(int, int, int)'<br>
main.cpp:15: undefined reference to 的 undefined reference matrixClass::operator<<(std::ostream&, matrixClass::Matrix const&)'
main.cpp:15: 未定义对 matrixClass::operator<<(std::ostream&, matrixClass::Matrix<int> const&)'<br>
main.cpp:8: undefined reference to 的引用矩阵类::矩阵::~矩阵()'
main.cpp:8: 未定义对`matrixClass::Matrix::~Matrix()'的引用

代码:矩阵.h

#ifndef MATRIX_H_
#define MATRIX_H_

#include <iostream>

namespace matrixClass {

    template<class T>
    class Matrix {
    private:
        int dimension;
        T **m;
    public:
        Matrix(int d);

        Matrix(const Matrix &original);

        ~Matrix();

        void set(int x, int y, T value);

        T get(int x, int y) const;

        int getDimension() const;

        friend std::ostream &operator<<(std::ostream&, const Matrix<T> &matrix);

        friend Matrix<T>* operator*(const Matrix<T> &m1, const Matrix<T> &m2);
    };
}

#endif

矩阵.cpp

#include "Matrix.h"

using namespace matrixClass;

template<class T>
Matrix<T>::Matrix(int d)
        : dimension{d}, m{new T *[d]} {
    //m = new T*[d];

    for (int i = 0; i < d; i++) {
        m[i] = new T[d];
    }
}

// COPY-CONSTRUCTOR
template<class T>
Matrix<T>::Matrix(const Matrix &original)
        : dimension{original.dimension},
          m{new T *[original.dimension]} {
    for (int i = 0; i < dimension; i++) {
        *(m + i) = *(original.m + i);
    }
}

// DESTRUCTOR
template<class T>
Matrix<T>::~Matrix() {
    for (int i = 0; i < dimension; i++) {
        delete[] m[i];
    }
    delete[] m;
}

template<class T>
void Matrix<T>::set(int x, int y, T value) {
    m[x][y] = value;
}

template<class T>
T Matrix<T>::get(int x, int y) const {
    return m[x][y];
}

template<class T>
int Matrix<T>::getDimension() const {
    return dimension;
}

template<class T>
std::ostream& operator<<(std::ostream& output, const Matrix<T>& matrix) {
    int dimension = matrix.getDimension();

    for(int x = 0; x < dimension; x++) {
        for(int y = 0; y < dimension; y++) {
            output << matrix.get(x, y) << " ";
        }
        return output;
    }
}

template<class T>
Matrix<T>* operator*(const Matrix<T>& m1, const Matrix<T>& m2) {
    int dimension = m1.getDimension();
    Matrix<T>* m = new Matrix<T>(dimension);

    for(int x = 0; x < dimension; x++) {
        for(int y = 0; y < dimension; y++) {
            T value = 0;
            for(int i = 0; i < dimension; i++) {
                value += m1.get(x, i) * m2.get(i, y);
            }
            m->set(x, y, value);
        }
    }
    return m;
}

main.cpp

#include <iostream>
#include "Matrix.h"

using namespace matrixClass;
using namespace std;

int main() {
    Matrix<int> m(2);

    m.set(0, 0, 1);
    m.set(0, 1, 2);
    m.set(1, 0, 3);
    m.set(1, 1, 4);

    cout << m << "*" << endl << m << "=" << endl;

    return 0;
}

Answer 1

在friend声明operator<<引用一个非模板函数，而它的定义说它是一个模板函数；他们不匹配。

您可以使用 friend 内联定义它声明(作为非模板函数):

template<class T>
class Matrix {
    ... ...
    friend std::ostream& operator<<(std::ostream& output, const Matrix<T>& matrix) {
        int dimension = matrix.getDimension();

        for(int x = 0; x < dimension; x++) {
            for(int y = 0; y < dimension; y++) {
                output << matrix.get(x, y) << " ";
            }
            return output;
        }
    }
    ... ...
};

或者制作friend引用函数模板的声明:

// class declaration
template<class T>
class Matrix;

// function declaration
template<class T>
std::ostream& operator<<(std::ostream& output, const Matrix<T>& matrix);

// class definition
template<class T>
class Matrix {
    ... ...
    friend std::ostream& operator<< <T>(std::ostream& output, const Matrix<T>& matrix);
    ... ...
};

// function definition
template<class T>
std::ostream& operator<<(std::ostream& output, const Matrix<T>& matrix) {
    int dimension = matrix.getDimension();

    for(int x = 0; x < dimension; x++) {
        for(int y = 0; y < dimension; y++) {
            output << matrix.get(x, y) << " ";
        }
        return output;
    }
}

关于 undefined reference 错误，参见Why can templates only be implemented in the header file?