Different Between Static Mutext and Not Static Mut

问题:

I have a code in .cpp

namespapce A
{
    namespace
    {
        static CMutex initMutex;
    }

    void init()
    {
        //code here
    }

    void uninit()
    {
        //code here
    }
}

What is the different if I remove the static in the mutex and if there is a static? And what is the use of the static?

Thanks!

回答1:

If mutex is static and if it would have been in the header and that header included in 2 cpp files(2 translational units), the lock applied by the code in first file will not be seen by the second file which is dangerous. This is because the 2 units has separate static of the mutex. In that case a global mutex is preferable.

If this is C++ then use RAII mechanism to manage mutex lock and unlock. THis is c++, where is the class? Encapsulate things into a class.

RAII example (basic one, things can be encapsulated into class): http://en.wikipedia.org/wiki/Resource_Acquisition_Is_Initialization

回答2:

You are Kind of mixing up C and C++. The keyword static in C has the intention to narrow the scope of a variable down to the translation unit. You could define it globally in the translation unit, but it was not visible to other translation-units. Bjarne Stroustrup recommends to use anonymous namespaces in C++ instead of using static like in C.

From this post it says

The C++ Standard reads in section 7.3.1.1 Unnamed namespaces, paragraph 2:

The use of the static keyword is deprecated when declaring objects 
in a namespace scope, the unnamed-namespace provides a superior alternative.

Static only applies to names of objects, functions, and anonymous unions, not to type declarations.

回答3:

static merely does two things:

• makes a variable to exist for the entire life of a program (but this is global level, so anything here exist for the whole program life!)

• makes a variable visible only in the translation unit it is declared (but this apply to whatever is in an anonymous namespace).

So, in fact, in this particular context, static does nothing.