[C language] flexible array

Catalog

Flexible array concept ：

characteristic ：

Comparison with pointer dynamic development

  Pointer dynamic development

Flexible array

  Disadvantages of pointer dynamic development

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Flexible array concept ：

A flexible array is a special kind of array

It is also the last member of the structure

in other words , It exists in the position of the last member of the structure

characteristic ：

1. The size of the flexible array in the structure is unknown , stay sizeof Its size is not calculated in

#include<stdio.h>
struct S
{
	int n ;
	int arr[0];// perhaps int arr[];
};
main()
{
	printf("The size of the structure is  %d",sizeof(struct S));
	return 0;
}

2. In the structure , If there is a flexible array , It must be preceded by other members , To apply for space ,, Because his size itself is unknown , No size .

3. A structure containing a flexible array depends on malloc Apply for this space dynamically , This reflects the flexibility of flexible arrays , That is, you can control the size , And the allocated space must be larger than the size of other members .

Comparison with pointer dynamic development

In everyday programming , Sometimes it is necessary to store an array with dynamic length in the structure

General practice , Is to define a pointer member in the structure , This pointer member points to the dynamic memory space where the array is located

  Pointer dynamic development

#include<stdio.h>
struct S
{
	int n;
	int* arr;
};

int main()
{
	struct S* ps = (struct S*)malloc(sizeof(struct S));
	ps->n = 100;
	ps->arr = (int*)malloc(40);
	free(ps->arr);
	ps->arr = NULL;
	free(ps);
	ps = NULL;
	return 0;
}

  The following is implemented through flexible arrays ：

Flexible array

#include<stdio.h>
struct S
{
	int n;
	int arr[0];// Flexible array 
};
int main()
{
	struct S* ps = (struct S*)malloc(sizeof(struct S)+40);
	// Use 
	// Change the space of the application 
	struct S* tmp= (struct S*)realloc(ps, sizeof(struct S)+80);
	if (tmp == NULL)
	{
		return;
	}
	if (tmp != NULL)
	{
		ps = tmp;
	}
	return 0;
}

 

  Disadvantages of pointer dynamic development

1. This method will be developed many times to release the space for dynamic application , It is easy to make mistakes

2. Prone to memory fragmentation

When the pointer dynamically opens up enough , There will be many memory fragments in the middle , Each memory fragment is discontinuous

It will lead to a waste of space

3. Compared to flexible array ,CPU The access speed should be slower

CPU Most of the memory access is after accessing a space , Will access adjacent spaces , Do not access the next contiguous space at one time , This will lead to , A certain degree of deceleration . 

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