still no way to initialize private static field in .h files?

Question

I have Commons.h file that contains "general" structures and classes declarations. Among others it contains such class declaration (only static int nInstrument is important, you can ignore the rest):

class Instrument
{
public:

    int Id() {
        return _id;
    }

    int GateId() {
        return _gateId;
    }

    const std::string& const ClassCode() const {
        return _classCode;
    }

    const std::string& const Ticker() const {
        return _ticker;
    }

    Instrument(int gateId_, std::string classCode_, std::string ticker_)
    {
        _gateId = gateId_;
        _classCode = classCode_;
        _ticker = ticker_;
        _id = nInstrument;
        std::cout << "New Instrument created << " << _ticker << " id = " << _id << std::endl;
        ++nInstrument;
    }

private:
    static int nInstrument;
    int _id;
    int _gateId;
    std::string _classCode;
    std::string _ticker;
};

I need to initialize static int nInstrument; with 0. It's easy If I have Commons.cpp file as answered in this question. But I don't ave Commons.cpp file and I don't want to create it just to put one line int Instrument::nInstrument = 0;

I want to double check that C++11 or future standart still doesn't have such feature? So I have to add cpp just to put one initialization line?

c++11 has "constexpr" keyword which can be used to initialize static const members inline — bobah, Jan 08 '14 at 13:15
It should also be possible to put the initialization into any compilation unit that uses the header. But creating the cpp is easier to understand for others — user1781290, Jan 08 '14 at 13:16
You can put that line in any file; it doesn't have to be Commons.cpp. — molbdnilo, Jan 08 '14 at 13:17
@javapowered Aren't all static members initialized to zero by default? — Marian, Jan 08 '14 at 13:22
The value has to be stored somewhere, which is why you need to define it in a c(pp) file. Putting them in a header would cause them to be defined in every cpp that includes that header. The only exception are templates, which have to be fully defined in a header, you could make a workaround using them. — riv, Jan 08 '14 at 13:23
@bobah but it looks like OP wants to be able to modify the static at runtime. — juanchopanza, Jan 08 '14 at 13:23
you can use `#ifdef`s in your header to make it work (and small changes to your build script), but I'd advise against this — user1781290, Jan 08 '14 at 13:24
I just want something like `static int nInstrument = 0;`. I can do it in `C#` for example. BTW Not sure if static int is initialized by default to `0` — Oleg Vazhnev, Jan 08 '14 at 13:25

score 2 · Answer 1 · answered Jan 08 '14 at 13:25

2

If want want to have a static variable into a header only, you may use the following in your class:

static int& nInstrument() { static int s_i = 0; return s_i; }

answered Jan 08 '14 at 13:25

Jarod42

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@riv not with my compiler (VS2012 and VS2013) – Martin Schlott Jan 08 '14 at 13:34
I just tried `static int test() { static int val = 0; return val++; }` with VS2010 and it printed 0 twice when called from different source files. Was something changed regarding this behavior? – riv Jan 08 '14 at 13:35
@riv give me a second, I check that with 2012. – Martin Schlott Jan 08 '14 at 13:38
Oh sorry I missed the part that said "in your class". That indeed works in VS2010. – riv Jan 08 '14 at 13:40
@riv yep. I wrote a short sample. It even works when you use it as a function with the inline keyword. – Martin Schlott Jan 08 '14 at 13:51

score 1 · Accepted Answer · edited May 23 '17 at 10:25

1

The C++11 has this kind of feature, but the most common compiler do not support it (yet). I tested that with XCode and VS2013. I got another solution, based on a question I already asked yesterday.

you have to form the attribute into a method:

class Instrument
{
 // your stuff

static int getInstrument()
{
   static int nInstrument=1234;
   return nInstrument;
}

};

This is header only. You can even extend that, look at the answers at: How to change a behavior for all instances of a class in a header only class

edited May 23 '17 at 10:25

Community

1
1

answered Jan 08 '14 at 13:25

Martin Schlott

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@Jarod42 Yep, but I wanted to stick to the question. I added a link where that is described. – Martin Schlott Jan 08 '14 at 13:29

score 0 · Answer 3 · answered Jan 08 '14 at 13:32

You can make a workaround using templates (which allow instantiating static members directly in a header file):

class Instrument
{
  template<class T>
  struct Counter { static int value; };
  int _id;
public:
  Instrument()
    : _id(Counter<int>::value++)
  {}
  int id() { return _id; }
};
int Test::Counter<int>::value = 1;

still no way to initialize private static field in .h files?

3 Answers3