Static keyword

This keyword is in C There are two main functions in language ,C++ It is extended .

Modifying variables

The first function ： Modifying variables . Variables are divided into local and global variables , But they all exist in static areas of memory .

Static global variables

Scope is limited to the file in which the variable is defined , Other documents are used extern The statement can't use him . To be precise, scope starts from where it is defined , End at the end of the file , It cannot be used in the first lines of code where it is defined . If you want to use it, you have to add extern , Want to be easy to use , It's simple , Just define it directly at the top of the file .

Static local variables

Defined in the function body , It can only be used in this function , Other functions in the same file cannot be used . Because of being static Modified variables always exist in the static area of memory , So even if this function ends , The value of this static variable will not be destroyed , This value can still be used next time the function is used .

i and j What are the values of , Why? ？

static int j;

void fun1（void）
{
    
	static int i = 0;
	i++;
}

void fun2（void）
{
    
	j = 0;
	j++;
}

int main()
{
    
	for(k=0; k<10; k++)
	{
    
		fun1();
		fun2();
	}
	return 0; 
}

i The value of is 10.i Is a local static variable , Store in local static area , Its value will not be cleared as the function call ends , Life cycle still exists , When the function is called again , Its value is the value after the last call .
j The value of is 1.j Is a static global variable , A global variable originally has an external link attribute , But be static After modification , The external link attribute becomes the internal link attribute , You can only use it inside your own source file , Cannot be used inside other files . It has no effect on the variables used later in the definition of this document , stay for Cyclic invocation fun2（） when ,fun2（） It will be assigned as 0.

Modify function

The second function ： Modify function . Add... Before the function static Make the function static . But here, “static” It's not about storage , It means that the scope of the function is limited to this file ( So it is also called internal function ). The advantage of using intrinsic functions is ： When different people write different functions , Don't worry about your own functions , Whether it will have the same name as the function in other files .

static The third function of is default initialization to 0

In fact, global variables also have this property , Because global variables are also stored in the static data area . In the static data area , The default value of all bytes in memory is 0x00

static What's the difference between global variables and ordinary global variables ?

Global variables ( External variables ) Before the description of static It's a static global variable .
Global variables themselves are static storage , Static global variables are also static storage . There is no difference between the two in the way of storage .

Changing a global variable to a static variable changes its scope , It limits its use .
The difference between the two is that the scope of non static global variables is the whole source program , When a source program consists of multiple source files , Non static global variables are valid in all source files . Static global variables limit their scope , That is, it is only valid in the source file where the variable is defined , It cannot be used in other source files of the same source program . Because the scope of static global variables is limited to one source file , Can only be used for functions within the source file , So you can avoid causing errors in other source files .

static Global variables are initialized only once , Prevent references in other file units ;

static What's the difference between local variables and ordinary local variables ？

Changing a local variable to a static variable changes its storage mode, that is, its lifetime .

static Local variables are initialized only once , Next time according to the last result value ;

static What is the difference between ordinary functions and ordinary functions ？

But here, “static” It's not about storage , It means that the scope of the function is limited to this file ( So it is also called internal function ).static Functions are different in scope from ordinary functions , Only in this document . Functions used only in the current source file should be described as internal functions (static Modified function ), Internal functions should be described and defined in the current source file .
For functions that can be used outside of the current source file , It should be stated in a header file , The source file to use these functions should contain this header file .

summary ： In fact, the most important thing is to distinguish the scope and storage mode