Customize my_ Strcpy and library strcpy [analog implementation of string related functions]

It's hard to study , The result was cool  

️ reminder ：

You children's shoes must pay attention to your body on the road of inner rolling , Don't sit in front of the computer all day , Multi motion .

Catalog

One 、 Using library functions strcpy Realize string copy

Two 、 Customize my_strcpy Realize string copy

3、 ... and 、my_strcpy Copy string optimized version

 1. The first one is

️ 2. The second kind

Four 、my_strcpy Copy string depth optimization  

1. Use assert() Assertion

️2. add to const

️ 3.const Modify a pointer

️ 4. Cool skin boy Online

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One 、 Using library functions strcpy Realize string copy

#include<stdio.h>

#include<string.h>

int main()
{
	char arr1[20] = "xxxxxxxxxxxxxxxx";
	char arr2[20] = "hello";
	strcpy(arr1, arr2);//arr1 Destination space start address ,arr2 Source space start address 
	printf("%s\n", arr1);
    return 0;
}

Current code print hello It's over , because strcpy Functions not only put hello Copy it , Together with the following \0 Also copied together , The end of string flag is \0, It will only print out hello 了 .

Two 、 Customize my_strcpy Realize string copy

The array name is the address of the first element , That is to say char The address of

use my_strcpy take src Point to copy to dest The space it points to

When put h After copying , To copy other content, make dest and src Point to the next content

#include<stdio.h>

#include<string.h>

void my_strcpy(char* dest, char* src)//dest Means the destination ,src Means the source 
{
	while (*src != '\0')
	{
		*dest = *src;
		dest++;
		src++;
	}
	*dest = *src;
}

int main()
{
	char arr1[20] = "xxxxxxxxxxxxxxxx";
	char arr2[20] = "hello";
	my_strcpy(arr1, arr2);//arr1 Destination space start address ,arr2 Source space start address 
	printf("%s\n", arr1);
	return 0;
}

There is something you don't understand about pointers , You can read my previous articles

【 Clear explanation C Language 】 Detailed explanation of the initial stage of the pointer _ Read the algorithm blog on the toilet -CSDN Blog C Elementary language 、 Primary pointer 、 programing language https://blog.csdn.net/m0_63033419/article/details/124002865

3、 ... and 、my_strcpy Copy string optimized version

 1. The first one is

*dest++ = *src++ yes hello A copy of the

*dest = *src yes \0 A copy of the

️ 2. The second kind

        First the h Assign to *dest,h Of ASCII Value is not equal to 0. Later, we will e、l、l、o Assign to *dest, Their ASCII Values are not equal to 0, But will \0 Assign to *dest When ,\0 Of ASCII The value is 0, Expression is false , So the cycle is over .

Four 、my_strcpy Copy string depth optimization  

 

1. Use assert() Assertion

If arr1 perhaps arr2 yes Null pointer （NULL）, Means dest perhaps src There is no point anywhere , At this time, if you dereference again, there will be a problem .

  terms of settlement ：assert()// Assertion

        Our program should be able to Check arr1 and arr2 Is it a null pointer , If so, prompt , If not, run .assert() and if() Very similar , It's just assert() The expression in parentheses It's true , Nothing happens , For false , Just a hint of error .

for example ：

assert(src != NULL)

in addition assert Use the header file

#include<assert.h> 

️

Be careful ：assert() For real, nothing happens , If it is false, it will prompt an error , This sum if The statement is just the opposite , Be careful not to confuse when using .

#include<stdio.h>

#include<string.h>

#include<assert.h>

void my_strcpy(char* dest, char* src)//dest Means the destination ,src Means the source 
{
	assert(src != NULL);// Assertion 
	while (*dest++ = *src++)
	{

	}
}

int main()
{
	char arr1[20] = "xxxxxxxxxxxxxxxx";
	char arr2[20] = "hello";
	my_strcpy(arr1, NULL);//arr1 Destination space start address ,arr2 Source space start address 
	printf("%s\n", arr1);
	return 0;
}

 arr1 and arr2 It could be NULL, So assert both

The code is as follows ：

#include<stdio.h>

#include<string.h>

#include<assert.h>

void my_strcpy(char* dest, char* src)//dest Means the destination ,src Means the source 
{
	assert(dest != NULL);// Assertion dest
	assert(src != NULL);// Assertion src
	while (*dest++ = *src++)
	{

	}
}

int main()
{
	char arr1[20] = "xxxxxxxxxxxxxxxx";
	char arr2[20] = "hello";
	my_strcpy(arr1, arr2);//arr1 Destination space start address ,arr2 Source space start address 
	printf("%s\n", arr1);
	return 0;
}

️2. add to const

If The starting address of the source space and the starting address of the target space are reversed , It has been written. *src++ = *dest++, The string copy is reversed , take arr1 String in （ a pile x） copy to arr2 In the middle , The program will crash , because arr2 can't let go arr1 The content of

When we reverse the source space start address and the target space start address , How can we find it in time ？

terms of settlement ：const

Add a... Before the address of the source space const

for example ：

void my_strcpy(char* dest, const char* src)

        When we add const after , Then compile the code , take *dest Copy to *src, The program will report an error ; And even if it's written backwards , Programs can also run , add const After that, the program can't even run , There will be no running errors .

  Then add one from const Is a good protection

️

Be careful ： If *dest and *src No reverse , Even with const The program will not report a mistake

The code is as follows ：

#include<stdio.h>

#include<string.h>

#include<assert.h>

void my_strcpy(char* dest, const char* src)//dest Means the destination ,src Means the source 
{
	assert(dest != NULL);
	assert(src != NULL);// Assertion 
	while (*dest++ = *src++)
	{

	}
}

int main()
{
	char arr1[20] = "xxxxxxxxxxxxxxxx";
	char arr2[] = "hello";
	my_strcpy(arr1, arr2);//arr1 Destination space start address ,arr2 Source space start address 
	printf("%s\n", arr1);
	return 0;
}

️ 3.const Modify a pointer

 

  Now? requirement num The value of cannot be changed , To be in num On the left Add a const, That's what it says ：

const int num = 10;

const The function of is to modify variables , This variable is called a constant variable , Has constant properties , Cannot be modified , But it's essentially a variable .

 

When we run the code, we will find , Even with const,num The value of has been changed .

 

        Join in const My original intention is not to change num Value , But now it has changed . although num By const modification ,num The value of cannot be changed , but num He gave his address to p. Through to p Dereference will still num The value of is changed . For example, we have to enter the classroom , Normally, you can only enter through the door , But now the door is locked , We can also go in through the window .

 

We add const The purpose of this is very clear , That is to make num The value of cannot be changed . You can still access the address here num The value of the change , It seems very clever , But it doesn't meet our requirements . Then what can I do to lock the window ？

terms of settlement ： take p Add one to the left of const

That's what it says ：

const int* p = #

Run the code and you'll find , The program will report an error . It's equivalent to locking the windows here .

  The code is as follows ：

#include<stdio.h>

int main()
{
	const int num = 10;
	const int* p = &num;
	//const When modifying the pointer 
	//const If you put it in * Left side , Modification is *p, Indicates what the pointer points to , It can't be changed by pointer 
	*p = 20;
	printf("%d\n", num);
	return 0;
}

️

Be careful ： although const Modification is *p, But for my pointer variable p No impact

#include<stdio.h>

int main()
{
	const int num = 10;
	const int* p = &num;
	//const When modifying the pointer 
	//const If you put it in * Left side , Modification is *p, Indicates what the pointer points to , It can't be changed by pointer 
	//*p = 20;
    // But the pointer variable itself can be modified 
	int n = 100;
	p = &n;// Revised p Variable itself 
	printf("%d\n", num);
	return 0;
}

The current code run result is : 10

️ 4. Cool skin boy Online

The above situation is const Put it in * On the left , So let's talk about const Put it in * On the right .

 

    For example, now there is a girl and a boy 1, They are boyfriend and girlfriend . The boy has ten dollars , The girl wants to eat cold skin , Let the boy buy her cold skin , And a bowl of cold skin costs ten yuan , What happens at this time is *p = 0, It's equivalent to the boy spending ten yuan .

 

    In fact, the boy is very stingy , He was thinking that I only had ten dollars , Please eat , I have no money , Then I won't treat you to . So he thought of a way , Is in the * Add a to the left of const. modification *p, This is the time *p = 0 This action can't be completed .

 

  The girl is angry , You can't give up ten dollars , I think the boy 2 Also very handsome , Break up ！ I'll be a boy 2 My girlfriend . What happens at this time is p = &n.

 

    This time the boy 1 panic , You're breaking up with me , That won't work , So the boy thought of a way , That is in * Add... To the right of const.

one day , The boy came up with a bad idea . That's going to be * Add... On both sides of the const