Question

In C++ I have an array of doubles which need initialising programmatically, at run time, just once, for the whole class to share. They could be both static and constant. How best can I initialise them? I have gone off using static with the prodigious popularity of parrallel processors presently pervading. So must I set a flag to run once or is there some static const magic which will initialise them as a variable local to a function (ok) or class (ok too)?

    double sumOfWeights = 0.0;
    double fracObs = 0.0;
    for (int i = 0; i < NUMTRACES; i++) {
        double weightAtI = SQUARED(1 - SQUARED(MAXTRACEWRTMIDTRACE * (MIDTRACE - i)
                                        / double(MIDTRACE)));
        sumOfWeights += weightAtI;
        fracObs += obsArray[i] * weightAtI;
    }
    return fracObs / sumOfWeights;

In the code above I’d like to make weightAtI a lookup with each double already divided by sumOfWeights so I can retrieve them without iterating through NUMTRACES.

_EDIT_
It’s okay, that’s what constructors are for 🙂
Just hoping to tackle my static, const and initialization gremlins into the bargain. Thanks Seth

_EDIT_
Not sure it is quite the effect I wanted though. The constructor runs on each instance, even if the members are static, no? No. Lemme see…

_EDIT_
I think the most efficient solution, is to guard the initializer loop with a static flag, in the constructor. Being a POD flag I’m sure it should behave appropriately, I’m just not quite sure what that is at this stage.

_EDIT_
Ahh, got it:

class X
{
public:
    static int i;
};
int X::i = 0; // definition outside class declaration

_EDIT_
Unfortunately, when it comes to my code,

    static const int MIDTRACE = 3;
    static const int NUMTRACES = 2 * MIDTRACE + 1;
    static double WEIGHTATI[NUMTRACES];

I get linker errors:

meobj.obj : error LNK2020: unresolved token (0A00001C) “private: static double * mens:meclass::PIMPL::WEIGHTATI” (?WEIGHTATI@PIMPL@meclass@mens@@$$Q0PANA)
meobj.obj : error LNK2001: unresolved external symbol “private: static double * mens:meclass::PIMPL::WEIGHTATI” (?WEIGHTATI@PIMPL@meclass@mens@@$$Q0PANA)

due to my constructor:

meclass::PIMPL() {
    if (!doneStaticInit) {
        double sumOfWeights = 0.0;
        for (int i = 0; i < NUMTRACES; i++) {
            WEIGHTATI[i] = SQUARED(1 - SQUARED(MAXTRACEWRTMIDTRACE * (MIDTRACE - i) / double(MIDTRACE)));
            sumOfWeights += WEIGHTATI[i];
        }
        for (int i = 0; i < NUMTRACES; i++) WEIGHTATI[i] /= sumOfWeights;
        doneStaticInit = true;
    }
}

Answer 1

You can put a static bool flag in your constructor. The flag will only be initialized to false the first time it is called. After that it will remain true.

// foo.h

class Foo {
  static const int MIDTRACE = 3; // static const, no definition needed
  static const int NUMTRACES = 2 * MIDTRACE + 1; // static const, no definition needed
  static double WEIGHTATI[NUMTRACES]; // not const, so need definition outside of class
public:
  Foo() {
    static bool array_initialized = false;
    if( !array_initialized ) {
      // Initialize array
      array_initialized = true;
    }
  }
  // Other members
};

In a source file, not header file:

// foo.cpp
include "foo.h"
double Foo::WEIGHTATI[NUMTRACES];