c++代码备忘录-reference

系列文章目录

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前言

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一、引用

函数引用，与变量名指向相同的内存，不需要向指针那样需要申请内存
但引用必须要进行初始化

函数代码如下：

#include <iostream>
#include <cmath>
using namespace std;

float norm_l1(float x, float y); //declaration
float norm_l2(float x, float y); //declaration
float (&norm_ref)(float x, float y) = norm_l1; //norm_ref is a function reference

int main()
{
    cout << "L1 norm of (-3, 4) = " << norm_ref(-3, 4) << endl;
    return 0;
}

float norm_l1(float x, float y)
{
    return fabs(x) + fabs(y);
}

float norm_l2(float x, float y)
{
    return sqrt(x * x + y * y);
}

2.简单示例

代码如下：

#include <iostream>
using namespace std;

int main()
{
    int num = 0;
    // 定义变量名的别名-引用
    int & num_ref = num;
    cout << "num = " << num << endl;

    num_ref = 10;
    cout << "num = " << num << endl;

    return 0;
}
// num = 0
// num = 10

2.参数引用

代码如下（示例）：

#include <iostream>
#include <float.h>

struct Matrix
{
    int rows;
    int cols;
    float * pData;
};

float matrix_max(const struct Matrix & mat)
{
    float max = FLT_MIN;
    //find max value of mat
    for(int r = 0; r < mat.rows; r++)
        for (int c = 0; c < mat.cols; c++)
        {
            float val = mat.pData[ r * mat.cols + c];
            max = ( max > val ? max : val);
        }
    return max;
}

int main()
{
    using namespace std;

    Matrix matA = {3,4};
    matA.pData = new float[matA.rows * matA.cols]{1.f, 2.f, 3.f};

    Matrix matB = {4,8};
    matB.pData = new float[matB.rows * matB.cols]{10.f, 20.f, 30.f};

    Matrix matC = {4, 2};
    matC.pData = new float[matC.rows * matC.cols]{100.f, 200.f, 300.f};

    // some operations on the matrices

    float maxa = matrix_max(matA);
    float maxb = matrix_max(matB);
    float maxc = matrix_max(matC);

    cout << "max(matA) = " << maxa << endl;
    cout << "max(matB) = " << maxb << endl;
    cout << "max(matC) = " << maxc << endl;


    delete [] matA.pData;
    delete [] matB.pData;
    delete [] matC.pData;

    return 0;
}

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总结

// 引用是变量的别名，对应同一个数据；
// 指针与引用的区别，指针创建变量、申请内存，引用声明时必须初始化；
// 引用更安全，具体到特定对象，不会乱指；没有copy数据，通过引用来访问数据内存
// 会修改外部变量，加const 避免数据修改