C++: static array inside class having unknown size

I wasn't familiar with this. I searched it on google but didn't find my answer. So, posting my question. Just tried following program:

#include <iostream>
class test
{
    static char a[];
    static int b[];
};
int main()
{
    test t;
}

It compiles fine without any warnings on MSVS 2010 & g++ 4.8.1. It also compiles fine in C++14 compiler. (See live demo here.) So, where does C++ standard says about this? If I remove static keyword from the declaration of char array in test class, compiler gives an error ISO C++ forbids zero size array when I use -pedantic-errors command line option in g++ & /Zaoption in MSVS 2010 compiler it says error C2133: 'test::a' : unknown size. So, my questions are:

1) what is the use of unknown size static array?

2) How do I later specify size of them & access that array elements? I am really getting confused.

3) Why removing static keyword leads to compilation errors?

It would be better if someone would explain it using simple example.

Thanks.

Solution

The compiler doesn't care about the size. It's just a declaration for the static field, so telling it you have an array is enough. Size doesn't matter at this point.

You only have a declaration for the static field at this point. You never use those arrays and the compiler is permissive... it won't complain.

If you want to use them however, you'll need a definition. If you omit the size there, you'll get similar error messages you saw before.

There isn't anything special going on.