今天重新看了《STL源代碼剖析》,不禁要讚歎STL設計的經典。STL 的空間適配代碼設計的尤爲精闢,不僅考慮到內存碎片的隱患,而且考慮到指針空間的節儉和複用,降低維護鏈表(lists)帶來額外的負擔。我們來看看如下代碼;
先看看STL的結構體,
union obj{
union obj *free_list_link;
char clent_data[1]; /* the client sees this */
};
obj 之所以用union,由於union之故,從其第一個字段觀之,obj可被視爲一個指針,指向相同形式的另一個obj。從其第二個字段觀之,obj可被視爲一個指針,指向實際區域。一物二用的結果是,不會爲了維護鏈表所必須的指針而造成內存的另一種讓費。
看看如下示例代碼,你就發現它的設計精妙之處!
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<iostream>
using namespace std;
union obj{
union obj *free_list_link;
char clent_data[1]; /* the client sees this */
};
int main()
{
obj *op1 = (obj *)malloc(32);
strcpy(op1->clent_data,"hello free!");
obj *op2 = (obj *)malloc(32);
op2->free_list_link = NULL;
obj *op3 = (obj *)malloc(32);
strcpy(op3->clent_data,"hello free l");
printf("----------------------------------------\r\n");
printf("address freelist:%ld\r\n",(long)(op3->free_list_link));
printf("address clentdata:%ld\r\n",(long)&(op3->clent_data));
obj *op4 = (obj *)malloc(32);
op4->free_list_link = op2;
obj *op5 = (obj *)malloc(32);
op5->free_list_link = op4;
printf("----------------------------------------\r\n");
obj *begin = op5;
while(begin){
printf("%ld\r\n",(long)begin);
begin = begin->free_list_link;
}
/* release op3 */
op2->free_list_link = op3;
op3->free_list_link = NULL;
printf("----------------------------------------\r\n");
printf("address freelist:%ld\r\n",(long)(op3->free_list_link));
printf("address clentdata:%ld\r\n",(long)&(op3->clent_data));
printf("----------------------------------------\r\n");
begin = op5;
while(begin){
printf("begin:%ld\r\n",(long)begin);
begin = begin->free_list_link;
}
return 0;
}
輸出如下:
----------------------------------------
address freelist:1819043176
address clentdata:143143000
----------------------------------------
143143080
143143040
143142960
----------------------------------------
address freelist:0
address clentdata:143143000
----------------------------------------
begin:143143080
begin:143143040
begin:143142960
begin:143143000