C language explanation series -- understanding of functions (3) formal parameters, arguments, nested calls and chain access

Review knowledge

In the previous function chapter, we shared with you two cases of using functions to solve , One is to obtain the maximum value of two integer variables through a function

#include<stdio.h>
int get_max(int x, int y)
{
    
	return (x > y ? x : y);
}

int main()
{
    
	int a = 10;
	int b = 20;
	int max = get_max(a, b);
	printf("%d", max);
	return 0;
}

The other is to exchange the values of two integer variables through a function

void exchange_num(int* pa, int* pb)
{
    
	int tmp = *pa;
	*pa = *pb;
	*pb = tmp;
}


int main()
{
    
	int a = 10;
	int b = 20;
	
	printf(" Exchange before : a = %d,b = %d\n", a, b);
	exchange_num(&a,&b);
	printf(" After exchanging ：a = %d,b = %d", a, b);
}

Through these two codes, we learned what is Value transfer call What is? Address call , When we need to make certain connections inside and outside the function, we need to use Address call .

Value transfer call
The formal and actual parameters of a function occupy different memory blocks , Modification of a parameter does not affect the argument

Address call
Address calling is a way to call a function by passing the memory address of the variable created outside the function to the function parameters .
This method of parameter transfer can establish a relationship between the function and the variables outside the function , In other words, the variables outside the function can be directly manipulated inside the function

From the above knowledge, we can sum up a sentence ： When a function is called , When an argument is passed to a formal parameter , The formal parameter will be a temporary copy of the argument , Therefore, the modification of formal parameters does not affect the arguments . This is the knowledge we shared with you before , Here is a simple review , It may be difficult for us to understand the words on the yellow background , What is formal parameter ？ What is real parameter ？ How to define ？ Listen to me .

Formal parameter and actual parameter

The actual parameter is the actual parameter
The parameters actually passed to the function are called arguments
The argument can be ： Constant 、 Variable 、 expression 、 Functions, etc , But no matter what type of quantity the argument is , When you make a function call , They all have to have a certain value , To pass these values to the formal parameters

Formal parameters are formal parameters
Formal parameters refer to the variables in brackets after the function name , Because formal parameters are only instantiated when the function is called （ Allocate memory units ）, So it's called formal parameter .
After the formal parameters are called, they are destroyed by themselves , So formal parameters are only valid in functions .

for instance

void exchange_num(int* pa, int* pb)
{
    
	int tmp = *pa;
	*pa = *pb;
	*pb = tmp;
}


int main()
{
    
	int a = 10;
	int b = 20;
	
	printf(" Exchange before : a = %d,b = %d\n", a, b);
	exchange_num(&a,&b);
	printf(" After exchanging ：a = %d,b = %d", a, b);
}

Or this code , The actual parameter &a,&b, The formal parameter is pa,pb.

Nested calls and chained access to functions

Nested calls

As the name suggests, nested calls to functions are , Functions can be combined arbitrarily , You can call each other according to your own needs . Let's give you an example

#include<stdio.h>
int main()
{
    
	int len = strlen("abcdef");
	printf("%d\n", len);
}

As shown in the figure, we want to print the string "abcdef" The length of , We use strlen() Library function to find the length of the string, and then use len Variable reception , Finally using printf() Library functions to print , Is it too much trouble to write like this , We can use nested calls of functions , Direct will strlen() Print the value of the library function .

#include<stdio.h>
int main()
{
    
	printf("%d\n", strlen("abcdef"));
}

As shown in the figure, the two printing results are the same , This is what we call nested function calls , Be careful Functions can be called nested , But you can't nest definitions .

Chained access

Chain access is Take the return value of one function as an argument to another function , Let's use examples to let you understand what chain access is

int main()
{
    
	printf("%d", printf("%d", printf("%d", 43)));
	return 0;
}

Let's analyze what the code outputs , We found that , We'll be back printf() The return value of the library function is the previous printf() Arguments to library functions , Then we need to know what his return value is , Last time we shared how to learn library functions , Now we can apply what we have learned

We're on the website www.cplusplus.com Inquire about printf() This library function , See the description of his return value type , He returns the number of characters printed on the screen . So we output for the first time 43, The second output is the number of characters printed on the screen 2, The third output 1, So the final result is

I don't know. After our explanation, everyone is right Nested calls and Chained access Whether our understanding has deepened .