笔记：C++学习之旅---引用

笔记：C++学习之旅---引用

什么是引用？
引用就是别名，引用并非对象，相反的，他只是为一个已经存在的对象所起的另外一个名字。
/*引用就是别名*/
#include <iostream>
using namespace std;

int main()
{
             int num;
             int &mum = num; //mum是num的别名，这两个变量是一个变量，只不过名字不同而已。对num的操作就是对mum的操作，这里‘&’不是取地址运算符，而是引用运算符，虽然符号一样，但是功能却不一样。
            num = 999;
            cout<< "mum:" <<mum<<endl; //结果为999
            mum = 0;
            cout<< "num:" <<num<<endl; //结果为0
    return 0;
}

/*引用就是别名变量*/
#include <iostream>
using namespace std;

int main()
{
             int a;
             int &ra = a;
            a = 999;
            cout<< "&a:" <<&a<<endl;
            cout<< "&ra:" <<&ra<<endl;
             int b = 888;
            ra = b;
            cout<< "&a:" <<&a<<endl; //ra和a地址和先前一样
            cout<< "&ra:" <<&ra<<endl;
            cout<< "&b:" <<&b<<endl; //b的地址不同
            cout<< "a:" <<a<<endl; //a = 888，同样ra是a的引用，a受到了牵连，所以a也等于888；
            cout<< "ra:" <<ra<<endl; //ra = 888,b的值赋值给了ra，所以ra值发生了改变；
            cout<< "b:" <<b<<endl; //b = 888
            ra = 1;
            cout<< "a:" <<a<<endl; //a = 1,由于ra = 1了，ra是a的引用，a受到牵连，所以a = 1；
            cout<< "ra:" <<ra<<endl; //ra = 1
            cout<< "b:" <<b<<endl; //b = 888,
            a = 666;
            cout<< "a:" <<a<<endl; //a = 666
            cout<< "ra:" <<ra<<endl; //ra = 666
            cout<< "b:" <<b<<endl; //b = 888
    return 0;
}
在上面例子中，我们将ra定义为a的别名，这样ra这个别名就只属于变量a，它不会变成b的别名。变量b只能将自己的值赋给这个ra，它不能改变ra的地址，比如说它不能让ra变成自己的别名。因此这个ra又可以看作是个别名常量，它是a的别名这个身份我们无法更改，我们能改变的只是他它引用的值。

引用对象
引用就是常量，在对其引用的同时要进行初始化
#include <iostream>
using namespace std;
class Human
{
public :
             void set( int x )
            {
                 i = x ;
            }
             int get()
            {
                return i;
            }
private :
             int i;
};
int main()
{
             Human Mike;
             Human &rMike = Mike; //定义一个对象的别名
             //Human &rHuman = Human;//不能定义一个类的别名，因为Human是一个类型，他没有具体的内存地址，所以我们不能定义它的别名；
    rMike.set(123); //通过引用来访问Human类
            cout<<rMike.get()<<endl;
             int a;
             int &ra = a;
             //int &ra;//这两行是错误的，引用就是常量，只能对其初始化，不能赋值
             //ra = a;  所以引用时一定要对该引用进行初始化
             return 0;
}

值传递、指针传递、引用传递
/*值传递*/
#include <iostream>
using namespace std;

void swap( int a , int b )
{
    int t;
            cout<< "swap函数中交换前a：" << a << ",b：" << b <<endl; //a = 3,b = 4
            t = a ;
             a = b ;
             b = t;
            cout<< "swap函数中交换后a：" << a << ",b：" << b <<endl; //a = 4,b = 3;
}
int main()
{
    int a = 3;
             int b = 4;
            cout<< "main函数中交换前a：" <<a<< ",b：" <<b<<endl; //a = 3,b = 4;
            swap(a,b);
            cout<< "main函数中交换后a：" <<a<< ",b：" <<b<<endl; //a = 3,b = 4，并没有发生改变，交换的只是a和b的副本；

             return 0;
}
a和b按值传递给swap函数，那么便一起会自动在栈中创建a和b的拷贝，然后将a和b的拷贝传递给swap函数。在swap函数中对a和b的拷贝进行交换。因此我们看到的输出语句，a和b确实进行了交换，只不过交换的是a和b的副本。所以并不是a和b的本身，所以在swap函数结束后，输出显示的main函数中的a和b并没有改变。

按址传递（指针传递）
/*按址传递（指针传递）*/
#include <iostream>
using namespace std;
void swap( int * a , int * b )
{
    int t;
            cout<< "swap函数中交换前a：" <<* a << ",b：" <<* b <<endl; //a = 3,b = 4;
            t = * a ;
            * a = * b ;
            * b = t;
            cout<< "swap函数中交换后a：" <<* a << ",b：" <<* b <<endl; //a = 4,b = 3;
}
int main()
{
             int a = 3;
             int b = 4;
            cout<< "main函数中交换前a：" <<a<< ",b：" <<b<<endl; //a = 3,b = 4;
            swap(&a,&b);
            cout<< "main函数中交换后a：" <<a<< ",b：" <<b<<endl; //a = 4,b = 3;
    return 0;
}
将a和b的地址传递给swap函数

引用传递
/*引用传递*/
#include <iostream>
using namespace std;
void swap( int & a , int & b ) //‘&’引用运算符,传递a和b的别名
{
    int t;
            cout<< "swap函数中交换前a：" << a << ",b：" << b <<endl; //a = 3,b = 4;
            t = a ;
             a = b ;
             b = t;
            cout<< "swap函数中交换后a：" << a << ",b：" << b <<endl; //a = 4,b = 3;
}
int main()
{
             int a = 3;
             int b = 4;
            cout<< "main函数中交换前a：" <<a<< ",b：" <<b<<endl; //a = 3,b = 4;
            swap(a,b);
            cout<< "main函数中交换后a：" <<a<< ",b：" <<b<<endl; //a = 4,b = 3;
    return 0;
}

/*利用指针返回多值*/
#include <iostream>
using namespace std;
int func( int a, int *b, int *c);
int main()
{
             int a = 1;
             int b = 2;
             int c = 3;
            cout<< "main函数调用func函数前...\n" ;
            cout<< "a:" <<a<<endl<< "b:" <<b<<endl<< "c:" <<c<<endl; //a = 1,b = 2,c = 3;
            func(a,&b,&c);
            cout<< "main函数调用func函数后...\n" ;
            cout<< "a:" <<a<<endl<< "b:" <<b<<endl<< "c:" <<c<<endl; //a = 1,b = 4,c = 27;a作为局部变量已经被释放所以打印的还是原先a = 1的值；

             return 0;
}
int func( int a , int * b , int * c )
{
    cout<< "func函数中，计算前...\n" ;
            cout<< "a:" << a <<endl<< "b:" <<* b <<endl<< "c:" <<* c <<endl; //a = 1,b = 2,c = 3;
             a = a + 1;
            * b = (* b )*(* b );
            * c = (* c )*(* c )*(* c );
            cout<< "func函数中，计算后...\n" ;
            cout<< "a:" << a <<endl<< "b:" <<* b <<endl<< "c:" <<* c <<endl; //a = 2,b = 4,c = 27;
             return a ;
}

/*利用引用传递返回多值*/
#include <iostream>
using namespace std;
int func( int a, int &b, int &c);
int main()
{
             int a,b,c;
             int check;
            cout<< "请输入一个数字，作为园的半径和正方形的边长\n" ;
            cin>>a;
            check = func(a,b,c);
    if (check)
            {
                cout<< "您输入的数字穿过计算范围\n" ;
            }
             else
            {
                cout<< "圆的面积为：" <<b<<endl;
                        cout<< "正方形的面积为：" <<c<<endl;
            }
             return 0;
}
int func( int a , int & b , int & c )
{
    if ( a > 20000)
            {
               a = 1;
            }
             else
            {
                b = a * a *3.14;
                         c = a * a ;
                         a = 0;
            }
             return a ;
}
posted @ 2016-08-24 23:08  奔涌吧，后浪  阅读(18)  评论(0)    收藏  举报