[c++] const pointer

Reference : "C++ from entry to mastery" People's Posts and Telecommunications Publishing House

The quantity represented by const is a constant. Can the data of the pointer type also use const?

of course can. The keyword const can be used before or after a pointer type. A const pointer has the following two forms:

const int *p1 =&a; //Pointer to an integer constant whose value cannot be changed

int* const p2 =&b; //Pointer to an integer constant whose address cannot be changed

Among them, p1 is a pointer to an integer constant, and the value it points to (*p1) cannot be changed, but the address it points to and the value of the variable (the value of a) can be changed; p2 points to a pointer to an integer constant, which points to The address (&b) cannot be changed, but the value it points to (*p2) can be changed.

Here's a programming example:

original:

#include<iostream>
using namespacestd;
intmain()
{
       int a=1,b=2,c=3;
       const int *p1; //correct
       int* const p2; // will report an error! ! ! Uninitialized
     
       p1=&a;
       p2=&b; //Error! The address pointed to by p2 cannot be changed
       cout<<"*p1:"<<*p1<<endl;
       cout<<"*p2:"<<*p2<<endl;
       return 0;
}

Among them, int* const p2; // uninitialized, an error will be reported. This is because the address pointed to by p2 cannot be changed, so it needs to be initialized.

After modification:

int* constp2=&b; //Initialization, via

Modify again:

int* constp2=&b; //Initialization, via
p2=&c; //error

The second sentence will report an error because the address pointed to by p2 cannot be changed, it can only point to b, and cannot point to other variables (addresses). But you can change the value pointed to by *p2, as follows:

*p2=3; //correct

After modification:

const int *p1; //Pointer to an integer constant, the integer pointed to cannot be changed
p1=&a;
*p1=5; //Add

Among them, *p1=5 will report an error, because p1 is a pointer to an integer constant and the value it points to cannot be changed .

Modify again:

const int *p1; //Pointer to an integer constant, the integer pointed to cannot be changed
p1=&a;
//*p1=5; //Error will be reported
a=5; //Add, pass

Among them, *p1=5 will report an error, and a=5 will pass, because the value pointed to by p1 cannot be changed, but the value of variable a can be changed directly .

Overall comparison:

p1=&c; //Correct, p1 can change the address pointed to

p2=&c; //Error, p2 cannot change the address pointed to

*p1=10; //Error, the value pointed to by p1 cannot be changed

*p2=10; //Correct, the value pointed to by p2 can be changed

so:

const int *p1 =&a; //The value pointed to by p1 cannot be changed, but the address pointed to is variable

int* const p2 =&b; //The address pointed to by p2 cannot be changed, but the value it points to can be changed

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[c++] const pointer

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