看到一段代碼，學習一下：
?1?void*?mymemcpy(?void*?dest,?const?void*?src,?size_t?count?)
?2?{
?3?????char*?d?=?(char*)dest;
?4?????const?char*?s?=?(const?char*)src;
?5?????int?n?=?count>>3;
?6?????switch(?count?&?7?)
?7?????{
?8??????????? case?0:??
?9?????????do?{??
10?????????????*d++?=?*s++;
11?????????????case?7:????? *d++?=?*s++;
12?????????????case?6:????????*d++?=?*s++;
13?????????????case?5:????????*d++?=?*s++;
14?????????????case?4:????????*d++?=?*s++;
15?????????????case?3:????????*d++?=?*s++;
16?????????????case?2:????????*d++?=?*s++;
17?????????????case?1:????????*d++?=?*s++;
18?????????}?while?(--n?>?0);
19?????}
20?????return?dest;
21?}
實際上也可以這么寫
?1?void*?mymemcpy(?void*?dest,?const?void*?src,?size_t?count?)?
?2?{?
?3?????char*?d?=?(char*)dest;?
?4?????const?char*?s?=?(const?char*)src;?
?5???//??int?n?=?(count?+?7)?/?8;?//?count?>?0?assumed?
?6?????int?n?=?count?>>?3;?
?7?????switch(?count?&?7?)?
?8?????{?
?9???????????????do?{??*d++?=?*s++;?
10?????case?7:????????*d++?=?*s++;?
11?????case?6:????????*d++?=?*s++;?
12?????case?5:????????*d++?=?*s++;?
13?????case?4:????????*d++?=?*s++;?
14?????case?3:????????*d++?=?*s++;?
15?????case?2:????????*d++?=?*s++;?
16?????case?1:????????*d++?=?*s++;?
17?????case?0??????????}?//while?(--n?>?0);?
18??????????????????while?(n--?>?0)?
19?????}?
20?
21?????return?dest;?
22?}?
23?
24?
這么寫更容易理解
實際上這里也可以是4個字節(jié)來復制，而不是8。其實這個看喜好。