内存池简单实现 C++

// 内存池技术.cpp: 定义控制台应用程序的入口点。
//

#include "stdafx.h"

#include<stdio.h>
#include<iostream>
#include<Windows.h>
using namespace std;

#define CRITICAL_ALIGN 8
#define CRITICAL_COUNT 16       //分16级
#define CRITICAL_VALUE 128      //128字节

#define FREELIST_INDEX(n) ((n -1) / CRITICAL_ALIGN)
#define ROUND_UP(n) ((n / 8 + 1) * 8)

//
union obj {
	obj* free_list_link;
	char client_data[1];
};

obj* freeList[CRITICAL_COUNT] = { NULL };


char* start_free;   //内存池首
char* end_free;     //内存池尾
size_t heap_size;

//从内存池分配内存(向freeList中进行分配)
char* chunk_alloc(size_t size, int& nobjs)
{
	char* result;
	size_t total_bytes = size * nobjs;
	size_t bytes_left = end_free - start_free;
	if (bytes_left >= total_bytes) {
		result = start_free;
		start_free += total_bytes;
		return result;
	}
	else if(bytes_left >= size) {
		nobjs = bytes_left / size;
		total_bytes = nobjs * size;
		result = start_free;
		start_free += total_bytes;
		return result;
	}
	else {
		size_t bytes_to_get = 2 * total_bytes + ROUND_UP(heap_size >> 4);
		if (bytes_left >= CRITICAL_ALIGN) {
			obj** my_free_list = freeList + FREELIST_INDEX(bytes_left);
			((obj*)start_free)->free_list_link = *my_free_list;
			*my_free_list = (obj*)start_free;
		}
		start_free = (char*)malloc(bytes_to_get);
		if (start_free == 0) {
			int i;
			obj** my_free_list;
			obj* p;
			for (i = size;i < CRITICAL_VALUE;i += CRITICAL_ALIGN) {
				my_free_list = freeList + FREELIST_INDEX(i);
				p = *my_free_list;
				if (p != NULL) {
					start_free = (char*)p;
					end_free = start_free + i;
					*my_free_list = p->free_list_link;
					return chunk_alloc(size, nobjs);
				}
			}
		}
		heap_size += bytes_to_get;
		end_free = start_free + bytes_to_get;
		return (chunk_alloc(size, nobjs));
	}
}

void * refill(size_t n) 
{
	int nobjs = 20;//定为20
	char* chunk = chunk_alloc(n, nobjs);
	memset((void*)chunk, 0x00, n*nobjs);//内存清零
	if (nobjs == 1) return (void*)chunk;//分配到一个直接给用户

	obj** my_free_list = freeList + FREELIST_INDEX(n);
	obj* tmp_free = (obj*)chunk;
	*my_free_list = tmp_free;
	for (size_t i = 1; i < nobjs; i++) {
		tmp_free->free_list_link = (obj*)(chunk + n);
		tmp_free = tmp_free->free_list_link;
	}
	obj* result = *my_free_list;
	*my_free_list = result->free_list_link;
	return (void*)result;
}

void* allocate(size_t n)
{
	obj** my_free_list;
	obj* result;
	if (n > CRITICAL_VALUE) {
		return malloc(n);
	}
	else {
		//交给内存池进行分配
		my_free_list = freeList + FREELIST_INDEX(n);
		result = *my_free_list;
		if (result == NULL) {
		   //如果是空的，那么就从内存池分配内存
			return refill(ROUND_UP(n));
		}
		*my_free_list = result->free_list_link;
		return (void*)result;
	}
} 

int main()
{
	void* link = allocate(13);
	
    return 0;
}