C++中的朋友和模板 [英] friend and template in C++

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本文介绍了C++中的朋友和模板的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我的 C++ 代码示例中有一个大问题.朋友"和模板"有问题.

错误消息:
Matrix.h:26:79:警告:

<块引用>

友元声明'std::ostream&matrixClass::operator<<(std::ostream&, const matrixClass::Matrix&)'声明一个非模板函数 [-Wnon-template-friend]朋友 std::ostream &operator<<(std::ostream&, const Matrix &matrix);

Matrix.h:26:79:注意:

<块引用>

(如果这不是您想要的，请确保函数模板

已经声明并在此处的函数名后添加<>)

Matrix.h:28:77:警告:

<块引用>

 友元声明 'matrixClass::Matrix*

matrixClass::operator*(const matrixClass::Matrix&, constmatrixClass::Matrix&)' 声明了一个非模板函数[-Wnon-template-friend]朋友矩阵*操作符*(const Matrix &m1, const Matrix &m2);

Matrix.cpp:1:0:

<块引用>

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&)'声明一个非模板函数 [-Wnon-template-friend]朋友 std::ostream &operator<<(std::ostream&, const Matrix &matrix);

Matrix.h:26:79:注意:

<块引用>

(如果这不是您想要的，请确保函数模板

已经声明并在此处的函数名后添加<>)

Matrix.h:28:77:警告:

<块引用>

 友元声明 'matrixClass::Matrix*

matrixClass::operator*(const matrixClass::Matrix&, constmatrixClass::Matrix&)' 声明了一个非模板函数[-Wnon-template-friend]朋友矩阵*操作符*(const Matrix &m1, const Matrix &m2);

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

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

代码:Matrix.h

#ifndef MATRIX_H_#define MATRIX_H_#include 命名空间矩阵类{模板类矩阵{私人的:整数维度；Tm值;民众:矩阵(int d);矩阵(const Matrix & original);~矩阵()；void set(int x, int y, T 值);T get(int x, int y) const;int getDimension() const;朋友 std::ostream &operator<<(std::ostream&, const Matrix<T> &matrix);朋友矩阵<T>*运算符*(const Matrix<T> &m1, const Matrix<T> &m2);};}#万一

Matrix.cpp

#include "Matrix.h"使用命名空间矩阵类；模板矩阵::Matrix(int d): 尺寸{d}, m{new T *[d]} {//m = 新 T*[d];for (int i = 0; i < d; i++) {m[i] = 新 T[d];}}//复制构造函数模板矩阵<T>::Matrix(const Matrix &original):尺寸{原始尺寸}，m{new T *[original.dimension]} {for (int i = 0; i <维数; i++) {*(m + i) = *(original.m + i);}}//析构函数模板矩阵::~矩阵(){for (int i = 0; i <维数; i++) {删除[] m[i];}删除[]米；}模板void Matrix::set(int x, int y, T 值) {m[x][y] = 值；}模板T 矩阵::get(int x, int y) const {返回 m[x][y];}模板int Matrix::getDimension() const {返回维度；}模板std::ostream&运算符<<(std::ostream&输出，const矩阵&矩阵){int 维度 = matrix.getDimension();for(int x = 0; x <维度; x++) {for(int y = 0; y <维度; y++) {输出<矩阵<T>*运算符*(const Matrix<T>& m1, const Matrix<T>& m2){int 维度 = m1.getDimension();矩阵*m=新矩阵(维度)；for(int x = 0; x <维度; x++) {for(int y = 0; y <维度; y++) {T值=0；for(int i = 0; i <维数; i++) {值 += m1.get(x, i) * m2.get(i, y);}m->set(x, y, value);}}返回 m;}

main.cpp

#include #include "Matrix.h"使用命名空间矩阵类；使用命名空间标准；int main() {矩阵米(2)；m.set(0, 0, 1);m.set(0, 1, 2);m.set(1, 0, 3);m.set(1, 1, 4);cout<<米＜＜*"<<结束<<米＜＜="<<结束;返回0；}

解决方案

在 friend 声明中 operator<< 指的是一个非模板函数，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 声明内联定义它(作为非模板函数):

template类矩阵{……朋友 std::ostream&运算符<<(std::ostream&输出，const矩阵&矩阵){int 维度 = matrix.getDimension();for(int x = 0; x <维度; x++) {for(int y = 0; y <维度; y++) {输出<

或者使 friend 声明引用函数模板:

//类声明模板类矩阵；//函数声明模板std::ostream&运算符<<(std::ostream&输出，const矩阵&矩阵)；//类定义模板类矩阵{……朋友 std::ostream&运算符<<<T>(std::ostream&输出，const矩阵<T>&矩阵);……};//函数定义模板std::ostream&运算符<<(std::ostream&输出，const矩阵&矩阵){int 维度 = matrix.getDimension();for(int x = 0; x <维度; x++) {for(int y = 0; y <维度; y++) {输出<

关于未定义的引用错误，参见为什么模板只能在头文件中实现?

I have a big problem in my C++ Code example. There is something wrong with 'friend' and the 'template'.

Error Messages:
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: note:

  (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: warning:

  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);

Matrix.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: note:

  (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: warning:

  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): In function `main':

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

Code: 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

#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;
}

解决方案

In friend declaration operator<< refers to a non-template function, while its definition says it's a template function; they don't match.

You can define it inline with the friend declaration (as non-template function):

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;
        }
    }
    ... ...
};

Or make the friend declaration referring to the function template:

// 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;
    }
}

And about the undefined reference error, see Why can templates only be implemented in the header file?

这篇关于C++中的朋友和模板的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！

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