How to use static const field of a base class as size of an array in functions of a derived class?

Question

I have a hyerarchy of classes. Methods of these classes may create temporary static arrays of the same size. I want to set size as static const field of the base class.

I put declaration of the field in a heared file and initialized it in a source file. This works without issues when compiled using GCC 4.3 but fails with VS compiler.

Base.h

class Base
{
public:
    virtual void function();

protected:
    static const int size;
};

Base.cpp

#include "Base.h"

const int Base::size = 128;

void Base::function()
{
    int array[size];
}

Derived.h

#include "Base.h"

class Derived : public Base
{
    void function();
};

Derived.cpp

#include "Derived.h"

void Derived::function()
{
    int array[size];
}

Main.cpp

#include "Derived.h"

int main()
{
    Base* object = new Derived();
    object->function();
    return 0;
}

I expected that size would be initialized in Base.cpp and considered as const in Derived.cpp. But it works only with GCC compiler. Visual Studio shows the following error messages:

error C2131: expression did not evaluate to a constant

note: failure was caused by non-constant arguments or reference to a non-constant symbol

note: see usage of 'size'

I don't think it should work, since when compile Derived::function, size is still unknown (any maybe even impossible to determine in compile time). (although I'm not sure what the standard say) — apple apple
– apple apple, Commented Jan 27, 2019 at 13:19
does this have something to do with variable length array? — apple apple
– apple apple, Commented Jan 27, 2019 at 13:22
GCC has a VLA extension which is why it works. You should make size constexpr and initialize it in Base declaration. — Holt
– Holt, Commented Jan 27, 2019 at 13:29

eerorika · Accepted Answer · 2019-01-27 13:39:48Z

3

A constant variable can only be used in a constant expression after its initialisation has been encountered (and is a constant initialiser). Base::size hasn't been initialised in Derived.cpp, so it cannot be used in a constant expression (such as length of the array): The program is ill-formed.

How to use static const field of a base class as size of an array in functions of a derived class?

Initialise the size within the class declaration, and declare it constexpr.

edited Jan 27, 2019 at 13:39

answered Jan 27, 2019 at 13:34

eerorika

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Comments

Lourens Dijkstra · Accepted Answer · 2019-01-27 13:50:27Z

1

As pointed out in the comments, a GCC extension makes this compile successfully. The problem is easily fixable by removing

const int Base::size = 128;

And change the Base::size (in .h) to

static constexpr int Base::size = 128;

This makes sure Base::size can be evaluated at compile-time. If you want to be have more influence on the value of size, templates can be used:

template <int N>
class Base {
    protected: static constexpr int size = N;
};

called through

Base<10>::size; // returns 10
Base<128>::size; // returns 128

using my_base = Base<128>;
my_base::size; // returns 128

edited Jan 27, 2019 at 13:50

answered Jan 27, 2019 at 13:42

Lourens Dijkstra

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