C language: deep analysis of const keyword

const It's a C Language （ANSI C） Key words of , Has a pivotal position . It defines a variable that cannot be changed , Produce a static effect . Use const To some extent, it can improve the security and reliability of the program . in addition , When watching other people's code , Understand clearly const What it does , It is also helpful to understand each other's procedures . in addition CONST It also appears in other programming languages , for example Pascal、C++、PHP5、B#.net、HC08 C、C# etc. .

1. Definition

const The type of decoration is the pointing type , The value of a constant type variable or object cannot be updated .

2. Purpose

const The initial purpose of keyword launch , To replace precompiled instructions , Eliminate its shortcomings while inheriting the advantages of precompiled instructions .

3. effect

1. Can define const Constant , Have no variability
2. Easy to type check , Let the compiler know more about processing , Eliminated some hidden dangers .
3. It can avoid the appearance of fuzzy numbers , It is also convenient to adjust and modify parameters . Same as macro definition , If it can be done, it will not change , Change all the time ！
4. Can protect things that are decorated , Prevent accidental modification , Enhance the robustness of the program .
5. You can save space , Avoid unnecessary memory allocation .
6. Improved efficiency

3.1const Modifying variables

Usually we write an integer variable a, We can change by assignment a The corresponding value , If we add one in front of it const What will happen ？

like this , In the face of const An error will be reported when modifying variables to assign values .
So we can come to a conclusion ：const Decorated variable , Cannot be modified directly .
But there is no way to change the value of variables , We can introduce and modify variables through pointers a

#include<stdio.h>

int main()
{
    
	const int a = 0;
	int* p = &a;// Defining pointer variables p To hold the a The address of 
	printf(" Before the change ：%d\n", a);
	*p = 20;// Give... By dereferencing a assignment 
	printf(" After change ：%d\n", a);
	return 0;
}

This shows that const Variables can be modified indirectly .

that const What's the use of this keyword ？

effect ：

1. Let the compiler do modified checks （ Grammar detection ）.
   2. Tell the code reader not to change this variable , It's one of them ” Self describing meaning “.（ At the grammatical level, the relatively weak binding effect ）.

that const Can the decorated variable be part of the array ？

Conclusion ：const Decorated variables cannot be part of an array .

3.2const Modify the array

const Modify the array , and const Modifier constants are the same , Array elements cannot be assigned twice , Let's look at a piece of code ：

When we define a const Decorated array , When it is assigned twice , compiler , An error has occurred .

int const a[5] = {
     1, 2, 3, 4, 5};
count int a[5] = {
     1, 2, 3, 4, 5};
// These two arrays are essentially no different .
//const  No matter what  int  In fact, both the left and the right are ok .
// But we are usually used to  const  Put it on the left side. .

3.3const Modify a pointer

First, let's understand the difference between pointer and pointer variable ：
1. The pointer is the address .
2. Pointer variables are used to store addresses .

Constant pointer ：
The value of the variable pointed to by the pointer cannot be modified through the pointer . But pointers can point to other variables .

pointer to const （ constant pointer ）：
The value of a pointer constant cannot be modified , That is, you cannot save a new address , Cannot point to other variables . But you can modify the value of the variable it points to through the pointer .

Here we give a more common example to deepen our understanding ：
If p It's a girl ,m It's the boy 1,n It's the boy 2, Now the boy 1 Yes 10 Yuan , The boy 2 Yes 100 Yuan , Girls are boys 1 My girlfriend , So there is int *p = &m; But boy 1 Don't want girls to spend their money , So he int const *p = &m; So the girl can't spend the boy's money to make *p = 0 了 ; The girl p Think boys 1 Too tight , So with the boy 1 break up , Choose a boy 2 Be your girlfriend , So there is p = &n; This is OK .

This time the boy 1 Panic again , You can't change your boyfriend , So he just int * const p = &m; So he allows the girl to spend his money * p = 0; But you can't change your boyfriend p = &n;

And then one day , The boy 1 Another idea ：int const * const p = &m; At this time, the girl can't spend money , You can't change your boyfriend .

If you still don't understand , You can go to b Take a look at brother bitpeng's explanation of this part of knowledge ： link : Brother bitpeng said const Modifier pointer is an example of popular understanding
stay 90P Of 46:56 At the time of , You can jump to study ！

3.4const Modify function

3.4.1.const Modifies the function parameter

const Modifiers can also modify the parameters of a function , When you don't want this parameter value to be accidentally changed in the function body, you can use const To modify .

const int fun（const int a）const;

#include<stdio.h>

void fun(const int* p)// add const Modifier means a Can't be changed directly 
{
    
	printf("%d\n", *p);
	*p = 20;// Report errors , because const modification a Cannot be changed directly 
}

int main()
{
    
	int a = 0;
	int* p = &a;

	fun(p);// Call function 
	return 0;
}

3.4.2.const Decorated return value

const Modifiers can also modify the return value of a function , The return value cannot be changed .