C++ standard - how "array of unknown bound of T" is treated

I've got confused by the fact that this code works:

struct S
{
  char c[];
};
S s;

According to C++ s开发者_C百科tandard, chapter 8.3.4:

"If the constant expression is omitted, the type of the identifier of D is “derived-declarator-type-list array of unknown bound of T”, an incomplete object type."

But I cannot figure out how "incomplete object type" becomes complete.

Thanks for help!

You've said the code you posted will compile in VS10. Turn off language extensions, and then it won't. Project>Properties>C/C++>Language>Disable Language Extensions = Yes. This is compiling because you are using a MS-specific extension to the C++ language.

In short, according to the standard, your code should not compile.

It seems like this language feature is invented in order to allow the array to be initialized later in the source file. If I make c non-static, then at least on Visual Studio 2010 it fails to compile, saying that the length of c has been fixed at 0, and member redeclaration is not allowed.

// header file
struct S
{
    static char c[];
    static size_t len;
};
extern S s;

// source file
char S::c[] = "haha";
size_t S::len = (sizeof(S::c) / sizeof(S::c[0])) - 1;