Detailed explanation of pointer constant and constant pointer

title ： Pointer constants and constant pointers

1. Constant pointer ：

int a,b;
const int *p1=&a;
int const *p2=&b;

1.1 Memory method ：
const stay *p Let's read out const（ Constant ） Then read the pointer , Together, they are constant pointers .
1.2 understand ：
We can understand it as a " Constant " The pointer to , There is no rule on which one to point to " Constant ", So the direction of the pointer can be changed . go back to " Constant " The word ," Constant " It's an unchangeable quantity, so we You can't go through the pointer To modify this " Constant " Value . The constant here is quoted , Next, we will explain clearly in the notes .
1.3 matters needing attention ：
Constant pointers cannot be modified by pointers a Value , But we can modify it directly a Value .
1.3.1 The first way to write it ：

int a=10;
	const int *p=&a;
	a=20;
	cout<<"a="<<a<<","<<"*p="<<*p; 

There is no mistake in this way , I'll explain later .
1.3.2 The second way ：

int a=10;
	const int *p=&a;
	*p=20
	cout<<"a="<<a<<","<<"*p="<<*p; 

The compiler will report an error for the above writing ：[Error] assignment of read-only location ‘* p’. Let's translate this sentence , Roughly meaning ：*p Allocated in the read-only area . Therefore, writing , Why do you explain it like this ？ Is it in conflict with the above understanding . Does not , Because we can understand that ：a Itself is a variable , It's just that we started with const int *p, Look in this direction , I just put a As a constant , This is how I look at compilers in this direction . But in fact a Itself is a variable and cannot pass * p Can also be changed in other ways . For example ：

float num=3.14;
	cout<<"num="<<(int)num;

A simple example ,num It's obviously a float, But we just want to treat him as a int.

2. constant pointer ：

int c;
int * const p3 =&c;

2.1 Memory method ：
You can see from the expression that int * stay const front ,int* Can represent pointer , and const For constant , Together, we read it as a pointer constant .
2.2 understand ：
We know that constants are immutable quantities . From the literal meaning, we can see that constants modify the two words pointer . So we can't change the value of the pointer , The value of the pointer is another address , Push it, we can't point to the pointer （ That's the value of the pointer ）. But you can modify variables c Value , Because there is no regulation c Is a constant .

3.
Of course const int * const p That is, the pointer cannot be changed and the value of the variable cannot be modified through the pointer . But it can still be accessed directly through a To modify the a Value . For explanation, see 1.3

int a=10;
	const int * const p=&a;
	a=20;
	cout<<"a="<<a<<","<<"*p="<<*p; 

4. summary ：
This article explains my simple connection between pointer and Chang from my personal point of view , If you have any questions, please leave a message in the comment area or send a private message to me . Views for reference , If there are mistakes, you are welcome to correct .