initializing static (shared) array?

[mzimmers]

Hi -

I have a class whose objects should share an array of coefficients. Normally, I'd make the shared element(s) static, but...I initialize their value from reading a file. Any ideas how I can:

1) initialize this array
2) keep only one copy among all objects within the class

Thanks.

[hamsterman]

Making something static doesn't make it constant. I don't see what problems you could be having with initializing a static array at run time..

[mzimmers]

I know it's not constant; that's not necessary.

I was hoping to initialize the array when the constructor was invoked for the first time, but I don't see a clean way to do that.

[hamsterman]

Just write a static function Initialize() and call it in main.
If you want to do this from the constructor, you'll need to know whether the array has already been initialized. Either have a static boolean "initialized" and check it in the ctor or use a dynamic array and check if the pointer is null. Though there is no need to bind initialization of static members to a ctor.

[webJose]

Is this a C-style array? You could:

#include <iostream>

using std::cout;
using std::endl;

class MyClass
{
public:
	static const double Coefficients[];
};

const double MyClass::Coefficients[] = { 1, 2, 3 };

#ifndef _countof
#define _countof(x) (sizeof(x) / sizeof(x[0]))
#endif

int main()
{
    for(int i = 0; i < _countof(MyClass::Coefficients); i++) cout << MyClass::Coefficients[i] << endl;
}

Line 12 will be run before main() by the CRT.

[mzimmers]

That's a clever approach, Jose, and I'm going to remember it. I think, though, that Hamsterman is probably right; I was trying to get too "clever" for my own good. I'll just include something in main() that calls the function a bunch of times.

Thanks, guys.

[mzimmers]

Well, it turns out that I get a build-time error when I try to make member data static. Here's a snippet from the declaration:

private:
	static	long	coeffI[NBR_CELLS];

I reference the static variable twice, as arguments to non-member functions.

The error message I get is:

Undefined symbols:
"DemodShaper::coeffI", referenced from:
__ZN11DemodShaper6coeffIE$non_lazy_ptr in DemodShaper.o
(maybe you meant: __ZN11DemodShaper6coeffIE$non_lazy_ptr)
ld: symbol(s) not found

Any ideas what I'm doing wrong here? Thanks.

[webJose]

Show code.

[mzimmers]

OK, here's the first time it's referenced:

long	DemodShaper::initCoeff()
{
	string s;
	s = "coeffI.txt";
	getCoeff (s, NBR_CELLS, coeffI);
	s = "coeffQ.txt";
	getCoeff (s, NBR_CELLS, coeffQ);
	return 0;
}

And here's the second (different function):

			cellArrayI.at(i).cycle(cellSampleInputI,		// I.
								coeffI[i],
								cellSumInputI,
								feedbackInputI,
								clockEnable,
								resetFlag);

[webJose]

The use of the array seems OK. Show the definition of NBR_CELLS, please.

Also make sure you are not trying to access the array from outside of the DemodShaper class.

[mzimmers]

(in an include file)

const long NBR_CELLS=32;

And, as I said, both function calls are non-member functions. But, since I'm just passing an address, it shouldn't matter, right?

[webJose]

It matters because the identifier "DemodShaper::coeffI" is only known inside DemodShaper. If you want direct access to the array from outside the class, the array needs to be public.

[mzimmers]

But, to the called routines, it's just a pointer to a long. I'm not trying to actually reference the private element, just read from and write to its address. Isn't that OK, or am I going brain-dead?

[webJose]

void PrintValue(const long someArr[], int index)
{
    //It is only here where you "just get an address".  Not in the function calls.
    cout << "I can access the array:  " << someArr[index];
}

class SomeClass
{
private:
    static const long Arr[];

public:
    void Foo();
};

const SomeClass::Arr[] = { 1, 2, 3, 4, 5 };

void SomeClass::Foo()
{
    PrintValue(SomeClass::Arr, 3); //Correct:  This is a method inside the class.
}

void OutsideFoo()
{
    PrintValue(SomeClass::Arr, 2); // Error:  An outside function such as OutsideFoo() cannot access SomeClass::Arr.
}

[webJose]

BTW, in a header file you should have:

extern const long NBR_CELLS;

And in a single CPP file you would have what you currently have now in your header file.

[mzimmers]

Maybe I wasn't clear above: the second example I posted is from within a member function. Accordingly, it has access to the private data. Even though it's passing it to a non-member function, shouldn't it still be OK?

[mzimmers]

If I declare the constant as external in my header file, I can't use it to bound the arrays I define in the class.

[webJose]

I see. But since the arrays are private, so should its length, I would think. Make NBR_CELLS a private member of DemodShaper? Just a suggestion. Whatever works for you.

[mzimmers]

But, this isn't causing my problem, is it? I still need to figure out how to get rid of this build error.

[mzimmers]

So, to summarize...if I remove the static qualifier from the class definition, everything is OK. But, when I add "static" I get the build error mentioned above. Anyone have an idea about what's going on?

EDIT:

Found it! I'd forgotten to define the arrays outside of the class. (I'd forgotten about that aspect of static members.)

Thanks for the help, guys.