How to initialize static members without repeating their type

Question

Let's say we have the following class:

class A {
    static SomeLongType b;
};

Now we have to initialize it in the appropriate cpp file. I can think of the following ways:

SomeLongType A::b{}; // repetition of SomeLongType
decltype(A::b) A::b{}; // A::b written two times

Both seem to be kind of cumbersome to me. Is there a better way?

If the type is of an integer or enumeration type, it can be initialized in the class declaration (in C++11). Otherwise, no, you have to initialize it in the implementation (much like member functions that are not implemented in the class declaration). — Zac Howland, Jan 24 '14 at 18:20
Try to use template class with static variable type as parameter. — PiotrNycz, Jan 24 '14 at 18:22
It'd be nice to use `auto`, but see http://stackoverflow.com/questions/14285198/why-doesnt-the-c11-auto-keyword-work-for-static-members?rq=1 — ecatmur, Jan 24 '14 at 18:24
@herohuyongtao Nope, b can be private. But `static` variables need an definition. — harper, Jan 24 '14 at 18:29
Since you are using `decltype` can we assume C++11? I've edited the tags. — John Dibling, Jan 24 '14 at 18:46

score 3 · Answer 1 · answered Jan 24 '14 at 18:38

3

The perfect solution would be to use C++11 auto. But as ecatmur commented, thats not allowed by the language.

Why not just define a simple macro?

#define DEFINE(x) decltype(x) x{}

struct A
{
    static SomeLongType b;
};

DEFINE( A::b );

I really hate C macros, but they are usefull in certain cases.

answered Jan 24 '14 at 18:38

Manu343726

And for bonus points, you can make it a [variadic macro](http://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html) to take care of more complicated constructors. Not sure how portable that syntax is, though... – Xavier Holt Jan 24 '14 at 18:58
2

@XavierHolt: Variadic macros are allowed in c++11. – Alexandre C. Jan 24 '14 at 19:49
2

@XavierHolt or just define the macro as `#define DEFINE(x) decltype(x) x`, using it as `DEFINE(A::b);`, or `DEFINE(A::b){ some params here };`, or `DEFINE(A::b) = something;`. I think its more clear than `#define DEFINE(X , params...)` `DEFINE(A::b , p1 , p2 , p3, etc )`, and its less tricky and more flexible. – Manu343726 Jan 25 '14 at 10:53

1 Answers1