A static global variable in a DLL

[John E]

Am I doing something stupid here?? The following code is in a DLL:-

// In a header file while building the DLL (i.e. assume that
// LIBTEMPORAL_API handles import or export, as appropriate)

namespace Temporal {

	typedef int64_t superclock_t;

	static superclock_t _superclock_ticks_per_second = 282240000;

	LIBTEMPORAL_API void set_superclock_ticks_per_second (superclock_t sc);

}


// In one of the DLL's source files
void
Temporal::set_superclock_ticks_per_second (Temporal::superclock_t sc)
{
	_superclock_ticks_per_second = sc; // <--- Never changes the value of _superclock_ticks_per_second  !!!!
}

It's almost as if _superclock_ticks_per_second is being treated as const (or are static globals in a DLL automatically const ??

[2kaud]

Why is _superclock_ticks_per_second static? What is trying to be achieved?

[John E]

I don't know - it must've been a limitation in some older version of MSVC. Or maybe it was originally a class member and needed to have the same value for all instances of the class

I'm just surprised that it isn't declared as const and yet it doesn't seem to be changeable

[Edit...] I guess this might be significant... I tried the following search in Google:-

+"C++" +"global variables" +"static"

and the first entry says this:-

A global static variable is one that can only be accessed in the file where it is created. This variable is said to have file scope. In C, the
preprocessor directive #define was used to create a variable with a constant value. This still works in C++, but problems could arise.

So if I'm reading that correctly, if the header file needs to get #included by some other library (rather than libtemporal in this case) the word static tells the compiler that the other library won't neeed access to _superclock_ticks_per_second - but the downside is that _superclock_ticks_per_second will effectively be const

[2kaud]

static at a file level says that the variable can't be used outside of the file in which it resides ie. it can't be used by another compilation unit. I'm suspicious that if you have multiple files all using the same header file then you have multiple instances of the variable - each specific to its own file. So each file is changing it's version only.

If you want one copy of _superclock_ticks_per_second then consider a singleton.