Global Static Pointer Issue

Question

This is a rather a problem from a convoluted situation. I have a static pointer sitting in one of my header files. Which is being included everywhere, this is why I put it as a static pointer. So that I can quickly initilize it in my main function so that other files can use it.

The problem is this, even after I initialize it and put stuff into it. Other files only find it NULL. It is like every file that includes the header with the static pointer makes a copy of it for itself and even when others initialize it, each file has their own separate copy. Negating ofcourse, the purpose of having a global variable.

How can I cope up with this?. Maybe I am understanding a static variable wrong, or maybe is it because its a pointer?

Should i be declaring it as: &variable = 5; or just as variable = 5; or &variable = (int)5?

Answer 1

It is like every file that includes the header with the static pointer makes a copy of it for itself and even when others initialize it, each file has their own separate copy.

That's what static means when applied to a variable at namespace scope: the variable is given internal linkage, making it "local" to a given translation unit (source file).

If you have a static variable at namespace scope in a header file and you include that header file in multiple .cpp files, there will be multiple instances of that variable: one for each of the .cpp files that include the header file.

If you want a global variable that is shared across multiple source files, you need to make it extern. Declare the variable as extern in the header file, then define the extern variable in exactly one of your .cpp files.

Answer 2

Defining the global pointer static results in the C++ compiler creating a separate pointer for every translation unit, the definition is included in. I'm pretty sure it's not what you want and it's the cause of your problems.

You should declare it extern in a header and define once in one cpp file.

I can imagine the confusion came from the way, static keyword works inside a class or a struct definition. Indeed, it works differently there and it creates one variable, common for all instances of the class or struct.

Btw. In case you actually want a variable to be visible only in one translation unit the recommended way is to put ii into an anonymous namespace. static works too, but it's use is discouraged as deprecated.

Answer 3

Yes, you are understanding the static keyword wrong. Static keyword at namespace and file scope declares a variable that is only visible inside the translation unit (basically a single .cpp file plus all of its includes) where it is declared. By declaring a static variable in header that you use in multiple .cpp files you are effectively defining multiple variables.

If you want a global pointer then declare it extern without static in the single header and define it without extern or static in a single .cpp file. E.g.:

In foo.h:

extern int * p;

and in main.cpp:

int * p = something;