C++中的朋友和模板

Question

我的C++代码示例中存在一个大问题."朋友"和"模板"有问题.

错误消息:
Matrix.h:26:79:警告:

friend声明'std :: ostream&matrixClass :: operator <<(std :: ostream&,const matrixClass :: Matrix&)'声明一个非模板函数[-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

已声明并在此处添加<>后的函数名称)

Matrix.h:28:77:警告:

  friend declaration 'matrixClass::Matrix<T>*

matrixClass :: operator*(const matrixClass :: Matrix&,const matrixClass :: Matrix&)'声明一个非模板函数[-Wnon-template-friend]朋友Matrix*operator*(const Matrix&m1,const Matrix&m2);

Matrix.cpp:1:0:

C:\ Users\Peter\CLIONProjects\PK\untitled76\Matrix.h:26:79:警告:朋友声明'std :: ostream&matrixClass :: operator <<(std :: ostream&,const matrixClass :: Matrix&)'声明非模板函数[-Wnon-template-friend]朋友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

已声明并在此处添加<>后的函数名称)

Matrix.h:28:77:警告:

  friend declaration 'matrixClass::Matrix<T>*

matrixClass :: operator*(const matrixClass :: Matrix&,const matrixClass :: Matrix&)'声明一个非模板函数[-Wnon-template-friend]朋友Matrix*operator*(const Matrix&m1,const Matrix&m2);

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

main.cpp:8:对main.cpp的未定义引用:8:对matrixClass::Matrix<int>::Matrix(int)'<br> main.cpp:10: undefined reference tomatrixClass的未定义引用:: Matrix :: set(int,int,int)'
main.cpp:11:对matrixClass::Matrix<int>::set(int, int, int)'<br> main.cpp:12: undefined reference tomatrixClass :: Matrix :: set的未定义引用( int,int,int)'
main.cpp:13:对matrixClass::Matrix<int>::set(int, int, int)'<br> main.cpp:15: undefined reference tomatrixClass :: operator <<(std :: ostream&,matrixClass :: Matrix const&)'
main.cpp的未定义引用:15:对matrixClass::operator<<(std::ostream&, matrixClass::Matrix<int> const&)'<br> main.cpp:8: undefined reference tomatrixClass :: Matrix ::的未定义引用~Matrix()'
main.cpp:8:未定义引用`matrixClass :: Matrix :: ~Matrix()'

代码: Matrix.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

Matrix.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中

#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<<指非模板函数,而它的定义说,这是一个模板函数; 他们不匹配.

您可以使用friend声明内联定义它(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friend使用函数模板进行声明:

// 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;
    }
}

关于未定义的引用错误,请参阅为什么模板只能在头文件中实现？