Posted on 2010-02-23 17:09
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C++
c/c++ 柔性数组
2009-12-09 09:56
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结构体变长的妙用——0个元素的数组
有时我们需要产生一个结构体，实现了一种可变长度的结构。如何来实现呢？
看这个结构体的定义：
typedef struct st_type
{
int nCnt;
int item[0];
}type_a;
（有些编译器会报错无法编译可以改成：）
typedef struct st_type
{
int nCnt;
int item[];
}type_a;
这样我们就可以定义一个可变长的结构，用sizeof(type_a)得到的只有4，就是sizeof(nCnt)=sizeof(int)那个0个元素的数组没有占用空间，而后我们可以进行变长操作了。
C语言版：
type_a *p = (type_a*)malloc(sizeof(type_a)+100*sizeof(int));
C++语言版:
type_a *p = (type_a*)new char[sizeof(type_a)+100*sizeof(int)];
这样我们就产生了一个长为100的type_a类型的东西用p->item[n]就能简单地访问可变长元素，原理十分简单，分配了比sizeof(type_a)多的内存后int item[];就有了其意义了，它指向的是int nCnt;后面的内容，是没有内存需要的，而在分配时多分配的内存就可以由其来操控，是个十分好用的技巧。
而释放同样简单：
C语言版：
free(p);
C++语言版：
delete []p;
其实这个叫灵活数组成员(fleible array member)C89不支持这种东西，C99把它作为一种特例加入了标准。但是，C99所支持的是incomplete type，而不是zero array，形同int item[0];这种形式是非法的，C99支持的形式是形同int item[];只不过有些编译器把int item[0];作为非标准扩展来支持，而且在C99发布之前已经有了这种非标准扩展了，C99发布之后，有些编译器把两者合而为一。
下面是C99中的相关内容：
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里使用两者之一都能通过编译并且完成操作，而会产生warning C4200: nonstandard extension
used : zero-sized array in struct/union的警告消息。
而在DEVCPP里两者同样可以使用，并且不会有警告消息。
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