const pointer and pointer to const in Cpp

March 6, 2017
C++

中文翻译好像有常量指针和指针常量这两种说法。然而翻译过来让我更加费解。还是英文更加直白。

pointer to const

const int b = 30;
const int * bptr = &b;

bptr是一个指针，它指向的是一个类型为const int的对象。可以对bptr本身的内容（保存的地址）进行修改，但是不能对它指向的对象进行修改：

const int c = 31;
bptr = &c;

这样，bptr就指向了另外一个const对象。

有个例外：可以让pointer to const指向一个nonconst对象。

int d = 32;
bptr = &d;

需要注意的是，即使d变量是nonconst，仍然不能通过bptr这个指针对b进行修改：

// *bptr = 33; // error!

总之，

我可以指向const对象，也可以指向nonconst对象，你也可以修改我。但是，你不能修改我指向的*对象*。 — pointer to const

const pointer

int a = 3;
int * const aptr = &a;

aptr是一个常量，同时是一个指针。不能对aptr里面保存的地址进行修改。但是可以对aptr所指向的对象进行修改：

*aptr = 4;

const reference

一个例子：

const int i = 42;
const int &ri = i;

例外：const reference 可以引用一个nonconst对象：

int j = 10;
const int &rj = &j;

这样做唯一的作用就是，不能够通过rj这个引用来改变j变量的值。这里和pointer to const是一样的。

但是不能让一个nonconst reference引用一个const对象：

const int k = 30;
// int &rk = &k; // error!

总之指向const的指针或者引用可以指向（引用）nonconst对象。但是指向nonconst的指针或者引用不可以指向（const）对象。

这个问题以前就遇到过也弄明白了，然而过了很长时间又忘了。所以记在这里。

一段完整的测试程序：

// 2017/03/06 08:57:48

/*
const: const.cpp
	g++ -Wall -std=gnu++11 const.cpp -o const
*/

#include <bits/stdc++.h>

// Pointers and const

using namespace std;

int main(int argc, char *argv[])
{
  // we may store the address of a const object ONLY in a pointer to const
  const double pi = 3.14;
  // double *ptr = &pi;            // error!
  const double *cptr = &pi;
  // *cptr = 10;                   // error: cannot assign to *cptr

  // But there are two exceptions to the rule that the type of a pointer
  // and the object to which it points must match:
  // 1) we can use a pointer to const to point to a nonconst object:
  double a = 2.17;
  cptr = &a;                    // we can not change a through cptr
  // pointer to const: I think I point to a const though I am not sure. T_T
  // Maybe it is nonconst. But I won't (cannot) change its value. ^_^ Besides,
  // You can change myself, but you can not change the OBJECT I point to.

  int b = 0;
  int *const bptr = &b;         // bptr will ALWAYS point to b
  // but you can change the object it points if that is not a const object:
  *bptr = 1;

  const double * const ppi = &pi; // ppi is a const pointer to const object

  // //////////////////////////////
  // References
  const int c = 8;
  const int & cc = c;
  // int & c1 = c;                 // error! we cannot assign directly to c,
  // we also should not be able to use a reference to change c.

  // But there is an exception: a reference to const may refer to an object
  // that is not const
  int i = 42;
  const int &r1 = i;
  int &r2 = i;
  r2 = 43;
  // r1 = 44;                      // error: can not change i through r1

  // c++ programmers tend to abbreviate `reference to const` as `const reference`. There are non const references. A reference is not an object, so we cannot make a reference itself const. Because there is no way to make a reference refer to a different object, in some sense all references are const.

  return 0;
}

pointer to const

const pointer

const reference

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