C++ 之引用

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;
}

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

// 2019-11-24日更新,对于引用作为返回值类型,可以通过汇编可知返回的是结果的地址值(引用视为弱化指针).在C++语法层面上考虑,就是首先,这个函数可以作为左值,是可以赋值的,如在重载[](下标索引)的时候,返回非应用是查值,返回引用,则可以进行赋值.其次,根据上面的说明,返回类型为引用的函数的一大作用就是给函数作用域外的值赋值.这个作用在我这个例子中几乎体现不出来,这是以前总结的时候忽略的,所以在此说明.

#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。

2016-5-21补充代码如下：

#include    <iostream>
using namespace std;


class    A {
public:
    int    a;
    A(int = 0);
    A(const    A &);
    A&    operator =    (const    A &);

    void    PV(A);
    void    PR(A&);
    A        RV(void);
    A&        RR(void);
};    //    A

A::A(int x) : a(x) { cout << "Constructor (value)" << endl; }
A::A(const    A & x) { cout << "Constructor (copy)-" << (void *)&x << endl;    a = x.a; }

A&    A::operator =    (const    A & x) { cout << "Operator (=) @ " << (void *)&x << endl;    a = x.a;    return *this; }

void    A::PV(A x) { cout << "Calling By Value @ " << (void *)&x << endl; }
void    A::PR(A &x) { cout << "Calling By Refe. @ " << (void *)&x << endl; }

A        A::RV(void) { cout << "Calling Rt Value" << endl;    return    *this; }
A&        A::RR(void) { cout << "Calling Rt Refe." << endl;    return    *this; }

int main(void) {
    A    oa, ob(3);

    cout << "oa @ " << (void *)&oa << endl;
    cout << "ob @ " << (void *)&ob << endl;


    cout << endl << "00000000000000000000000000000" << endl;
    oa.PR(ob);
    cout << endl << "11111111111111111111111111111" << endl;
    oa.PV(ob);
    cout << endl << "22222222222222222222222222222" << endl;

    ob = oa.RR();
    cout << endl << "33333333333333333333333333333" << endl;

    ob = oa.RV();
    cout << endl << "44444444444444444444444444444" << endl;
        return 0;
}

参数类型为class且为值传递的时，先调用对象的拷贝构造函数生成中间值，然后传入这个中间值到函数中；返回类型为值类型的时，会调用拷贝构造函数生成中间值，实际上返回的是这个中间值。

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