Include guard style, C++

Question

I have a .h file which contains several class definitions. I'd like to use C++'s include guards in this file; however, I was wondering which way of using the include guards is considered proper/correct?

One guard protecting everything

#ifndef FOO_BAR
#define FOO_BAR

class Foo
{
};

class Bar
{      
};

#endif

or multiple separate guards.

#ifndef FOO
#define FOO

class Foo
{
};

#endif

#ifndef BAR
#define BAR

class Bar
{      
};

#endif

Answer 1

They are include guards, preventing double inclusion of files. So they should be defined once per file, not per class or function or whatever.