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C++
c/c++ 柔性數組
2009-12-09 09:56
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結構體變長的妙用——0個元素的數組
有時我們需要產生一個結構體，實現了一種可變長度的結構。如何來實現呢？
看這個結構體的定義：
typedef?struct?st_type
{
??? int?nCnt;
???
int?item[0];
}type_a;
（有些編譯器會報錯無法編譯可以改成：）
typedef?struct?st_type
{
???
int?nCnt;
???
int?item[];
}type_a;
這樣我們就可以定義一個可變長的結構，用sizeof(type_a)得到的只有4，就是sizeof(nCnt)=sizeof(int)那個0個元素的數組沒有占用空間，而后我們可以進行變長操作了。
C語言版：
???
type_a?*p?=?(type_a*)malloc(sizeof(type_a)+100*sizeof(int));
C++語言版:
???
type_a?*p?=?(type_a*)new?char[sizeof(type_a)+100*sizeof(int)];
這樣我們就產生了一個長為100的type_a類型的東西用p->item[n]就能簡單地訪問可變長元素，原理十分簡單，分配了比sizeof(type_a)多的內存后int?item[];就有了其意義了，它指向的是int?nCnt;后面的內容，是沒有內存需要的，而在分配時多分配的內存就可以由其來操控，是個十分好用的技巧。
而釋放同樣簡單：
C語言版：
???
free(p);
C++語言版：
???
delete?[]p;
其實這個叫靈活數組成員(fleible?array?member)C89不支持這種東西，C99把它作為一種特例加入了標準。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[];只不過有些編譯器把int?item[0];作為非標準擴展來支持，而且在C99發布之前已經有了這種非標準擴展了，C99發布之后，有些編譯器把兩者合而為一。
下面是C99中的相關內容：
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編譯并且完成操作，而會產生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，并且不會有警告消息。
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