C++ 之引用

Posted 2020-07-08 技术笔记

1. int argc ，char * argv[] - argument count & argument vector
   

   argc - 命令行参数个数，argv[]依次指向每一个命令行参数，其中argv[0]为程序名字：下面的程序包含了完整的程序路径。

   #include <iostream>
   
   int main(int argc, char *argv[])
   {     int i = 0;                 // begin with 0
         while (i < argc)
             std::cout << argv[i++] // output string
                  << " "            // output SPACE
                  << std::endl;     // terminate output line 
         return 0;
   }

    

2. 引用
   

   1）作为返回值，不需要产生临时对象，然后拷贝这个对象，（见下面代码的注释），目前不要深究引用机制，目前记住的返回引用的场景：输入流、 //发现其他的再补充

   #include    <iostream>
   using namespace std;
   
   
   int & rfun(int &);                                                     //don‘t dig more,ref func saves time.
   int fun(int);
   
   void  main(void) {
       int  a = 7, b = 9; 
       int &ra = a, &rb = b;                                          //Treat ra as another name of a
       int *pa = &ra, *pb = &rb;
   
       cout << pa << "  " << (void *)pa << endl;
       cout << pb << "  " << (void *)pb << endl;
   
       cout << a << "  " << b << endl;
       cout << rb << "  " << (void *)&rb << endl;                      //&rb == pb always.
       rb = rfun(ra);                                 //b=ra;  rb=ra;  -->  int &rd=a;
                                                   //rb=fun(ra);
       cout << rb << "  " << (void *)&rb << endl;
   
       cout << a << "  " << b << endl;
       rb = 8;
       cout << a << "  " << b << endl;
   }
   
   int & rfun(int & r) {
       return  r;        //  b=fun(a)  -->  b=a;
   };
   
   int fun(int r) {
       return    r;        //  b=fun(a)  -->  r=a;  b=r;
   };

   2）引用类型参数

   对于class，vector等类型的参数，引用 避免了值拷贝，提升效率，使用引用，函数可以改变实参的值，如果在调用过程中实参值不会发生变化，那么添加 const 修饰。

         

//call by ref can change the value after func called.
#include    <iostream>

using    namespace    std;

void  funr(const int &, int);
int&  rfun(const int &, int);

void  main(void) {
    int a = 100;
    int& b = a;
    const  int& c = 20;

    const int & ca = a;

    a = 200;

    const int cb = 300;

    cout << &a << "  " << a << endl;
    cout << &b << "  " << b << endl;
    cout << &c << "  " << c << endl;

    cout << endl << endl;

    funr(a, a);                           //first a,like above;second a, int tmp = a;
    funr(2, 2);
    cout << endl << endl;

    int& rf = rfun(c, c);
    cout << &rf << "  " << rf << endl;

}

void  funr(const int &  ra, int  pa) {    
    cout << &ra << "  " << ra << endl;
    cout << &pa << "  " << pa << endl;    //int tmp = pa, &pa is addr of tmp
}

int&  rfun(const int &  ra, int  pa) {    // b = rfun(ra,pa)-->b = ra

    int  x = 200, &d = x;
    cout << &ra << "  " << ra << endl;
    cout << &pa << "  " << pa << endl;
    return  (int&)ra;
}

 以上则是目前想到的关于引用的知识点小结，以前曾经写过这话题的博文，今天讲解细致些，看注释，运行程序，更快速理解引用，VS2015社区版。