Trailing Array Idiom

What is Trailing Array Idiom ?

P.S : Googling this term gives The vectors are implemented using th开发者_StackOverflowe trailing array idiom, thus they are not resizeable without changing the address of the vector object itself.

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If you mean the trailing array idiom mentioned in the GCC source code (where your quote comes from), it seems to refer to the old C trick to implement a dynamic array:

typedef struct {
    /* header */
    size_t nelems;

    /* actual array */
    int a[1];
} IntVector;

where an array would be created with

IntVector *make_intvector(size_t n)
{
    IntVector *v = malloc(sizeof(IntVector) + sizeof(int) * (n-1));
    if (v != NULL)
        v->nelems = n;
    return v;
}

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It seems to refer to arrays in structs, which may have a variable array-size. See:

http://blogs.msdn.com/b/oldnewthing/archive/2004/08/26/220873.aspx and http://sourceware.org/gdb/current/onlinedocs/gdbint/Support-Libraries.html

Another tip, if you google for an expression put the expression in "" like "trailing array" this will give you more specific results. Google knows about trailing arrays.

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I think what is meant is:

struct foo {
  ... some data members, maybe the length of bar ...
  char bar[]; /* last member of foo, char is just an example */
};

It is used by allocating with malloc(sizeof(struct foo)+LEN), where LEN is the desired length of bar. This way only one malloc is needed. The [] can only be used with the last struct member.

And, as fas as I understand the GCC doc, struct foo can also only be (reasonably) used as last member of another struct, because the storage size is not fixed -- or as pointer.