Posted on 2010-02-23 17:09
S.l.e!ep.￠% 閱讀(634)
評(píng)論(0) 編輯 收藏 引用 所屬分類(lèi):
C++
c/c++ 柔性數(shù)組
2009-12-09 09:56
|
結(jié)構(gòu)體變長(zhǎng)的妙用——0個(gè)元素的數(shù)組
有時(shí)我們需要產(chǎn)生一個(gè)結(jié)構(gòu)體，實(shí)現(xiàn)了一種可變長(zhǎng)度的結(jié)構(gòu)。如何來(lái)實(shí)現(xiàn)呢？
看這個(gè)結(jié)構(gòu)體的定義：
typedef?struct?st_type
{
??? int?nCnt;
???
int?item[0];
}type_a;
（有些編譯器會(huì)報(bào)錯(cuò)無(wú)法編譯可以改成：）
typedef?struct?st_type
{
???
int?nCnt;
???
int?item[];
}type_a;
這樣我們就可以定義一個(gè)可變長(zhǎng)的結(jié)構(gòu)，用sizeof(type_a)得到的只有4，就是sizeof(nCnt)=sizeof(int)那個(gè)0個(gè)元素的數(shù)組沒(méi)有占用空間，而后我們可以進(jìn)行變長(zhǎng)操作了。
C語(yǔ)言版：
???
type_a?*p?=?(type_a*)malloc(sizeof(type_a)+100*sizeof(int));
C++語(yǔ)言版:
???
type_a?*p?=?(type_a*)new?char[sizeof(type_a)+100*sizeof(int)];
這樣我們就產(chǎn)生了一個(gè)長(zhǎng)為100的type_a類(lèi)型的東西用p->item[n]就能簡(jiǎn)單地訪問(wèn)可變長(zhǎng)元素，原理十分簡(jiǎn)單，分配了比sizeof(type_a)多的內(nèi)存后int?item[];就有了其意義了，它指向的是int?nCnt;后面的內(nèi)容，是沒(méi)有內(nèi)存需要的，而在分配時(shí)多分配的內(nèi)存就可以由其來(lái)操控，是個(gè)十分好用的技巧。
而釋放同樣簡(jiǎn)單：
C語(yǔ)言版：
???
free(p);
C++語(yǔ)言版：
???
delete?[]p;
其實(shí)這個(gè)叫靈活數(shù)組成員(fleible?array?member)C89不支持這種東西，C99把它作為一種特例加入了標(biāo)準(zhǔn)。但是，C99所支持的是incomplete?type，而不是zero?array，形同int?item[0];這種形式是非法的，C99支持的形式是形同int?item[];只不過(guò)有些編譯器把int?item[0];作為非標(biāo)準(zhǔn)擴(kuò)展來(lái)支持，而且在C99發(fā)布之前已經(jīng)有了這種非標(biāo)準(zhǔn)擴(kuò)展了，C99發(fā)布之后，有些編譯器把兩者合而為一。
下面是C99中的相關(guān)內(nèi)容：
6.7.2.1 Structure and union specifiers
As a special case, the last element of a structure with more than one named member may have
an incomplete array type; this is called a flexible array member. With two exceptions, the
flexible array member is ignored. First, the size of the structure shall be equal to the offset
of the last element of an otherwise identical structure that replaces the flexible array member
with an array of unspecified length.106) Second, when a . (or ->) operator has a left operand
that is (a pointer to) a structure with a flexible array member and the right operand names that
member, it behaves as if that member were replaced with the longest array (with the same element
type) that would not make the structure larger than the object being accessed; the offset of the
array shall remain that of the flexible array member, even if this would differ from that of the
replacement array. If this array would have no elements, it behaves as if it had one element but
the behavior is undefined if any attempt is made to access that element or to generate a pointer
one past it.
例如在VC++6里使用兩者之一都能通過(guò)編譯并且完成操作，而會(huì)產(chǎn)生warning C4200: nonstandard extension
used : zero-sized array in struct/union的警告消息。
而在DEVCPP里兩者同樣可以使用，并且不會(huì)有警告消息。
|