指针和引用的比较

Comparing Pointers and References

指针和引用的比较

While both references and pointers are used to indirectly access another value, there are two important differences between references and pointers. The first is that a reference always refers to an object: It is an error to define a reference without initializing it. The behavior of assignment is the second important difference: Assigning to a reference changes the object to which the reference is bound; it does not rebind the reference to another object. Once initialized, a reference always refers to the same underlying object.

虽然使用引用（reference）和指针都可间接访问另一个值，但它们之间有两个重要区别。第一个区别在于引用总是指向某个对象：定义引用时没有初始化是错误的。第二个重要区别则是赋值行为的差异：给引用赋值修改的是该引用所关联的对象的值，而并不是使引用与另一个对象关联。引用一经初始化，就始终指向同一个特定对象（这就是为什么引用必须在定义时初始化的原因）。

 

1

 

#include <cstdlib>
#include <iostream>

using namespace std;

int main(int argc, char *argv[])
{
    int a = 1024;
    int *pi =&a; 
    int **ppi = &pi;
   
    cout << a <<"/n"     //输出1024
         << *pi<<"/n"   //输出1024
         << *ppi       //输出一个内存地址
         << **ppe     //输出1024 
    system("pause");
   
    return EXIT_SUCCESS;
}

 

 

C++ 使用 ** 操作符指派一个指针指向另一指针。这些对象可表示为：

 

对 ppi 进行解引用照常获得 ppi 所指的对象，在本例中，所获得的对象是指向 int 型变量的指针 pi：

          int *pi2 = *ppi; // ppi points to a pointer

To actually access ival, we need to dereference ppi twice:

为了真正地访问到 ival 对象，必须对 ppi 进行两次解引用。

 

 

2

 

在表达式中使用数组名时，该名字会自动转换为指向数组第一个元素的指针：

          int ia[] = {0,2,4,6,8};
          int *ip = ia; // ip points to ia[0]

如果希望使指针指向数组中的另一个元素，则可使用下标操作符给某个元素定位，然后用取地址操作符 & 获取该元素的存储地址：

          ip = &ia[4];    // ip points to last element in ia

使用指针的算术操作在指向数组某个元素的指针上加上（或减去）一个整型数值，就可以计算出指向数组另一元素的指针值：
          ip = ia;            // ok: ip points to ia[0]
          int *ip2 = ip + 4;  // ok: ip2 points to ia[4], the last element in ia
When we add 4 to the pointer ip, we are computing a new pointer. That new pointer points to the element four elements further on in the array from the one to which ip currently points.
在指针 ip 上加 4 得到一个新的指针，指向数组中 ip 当前指向的元素后的第 4 个元素。
More generally, when we add (or subtract) an integral value to a pointer, the effect is to compute a new pointer. The new pointer points to the element as many elements as that integral value ahead of (or behind) the original pointer.
通常，在指针上加上（或减去）一个整型数值 n 等效于获得一个新指针，该新指针指向指针原来指向的元素之后（或之前）的第 n 个元素。
 
Pointer arithmetic is legal only if the original pointer and the newly calculated pointer address elements of the same array or an element one past the end of that array. If we have a pointer to an object, we can also compute a pointer that points just after that object by adding one to the pointer.
指针的算术操作只有在原指针和计算出来的新指针都指向同一个数组的元素，或指向该数组存储空间的下一单元时才是合法的。如果指针指向一对象，我们还可以在指针上加1从而获取指向相邻的下一个对象的指针。