Different Between Static Mutext and Not Static Mutex

Question

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!

I think this is a duplicate of: http://stackoverflow.com/questions/11623451/static-vs-non-static-variables-in-namespace — maditya, Apr 05 '13 at 08:10
Yes, its anonymous namespace so that other code can't use it. — domlao, Apr 05 '13 at 08:10
`static` changes the linkage of the variable. It's for you to decide whether or not you want static linkage (e.g. is the code in a header or in a source file). — Kerrek SB, Apr 05 '13 at 08:10
So what is the use of that static in the code? So I just created initMutex object? — domlao, Apr 05 '13 at 08:22
If it is in the header and included in 2 cpp files. 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 — Abhijit-K, Apr 05 '13 at 08:27

Abhijit-K · Answer 1 · 2013-04-05T08:36:29.537

5

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

edited Apr 05 '13 at 08:36

answered Apr 05 '13 at 08:31

Abhijit-K

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"THis is c++, where is the class? Encapsulate things into a class." C++ doesn't says to cram everything into class, namespaces and free standing functions are fine by themselves, if used correctly. – Abhinav Gauniyal Sep 22 '16 at 10:25

score 2 · Answer 2 · edited May 23 '17 at 12:17

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.

score 2 · Answer 3 · answered Apr 05 '13 at 08:41

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.

Different Between Static Mutext and Not Static Mutex

3 Answers3

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