Re: static array declaration in flyweight pattern

Bart van Ingen Schenau <>
Mon, 01 Jun 2009 08:31:26 +0200
<> wrote:

On May 31, 9:36 pm, Marcel M?ller <>
wrote: wrote:

The line Icon *FlyweightFactory::_icons[]; seems a strange
definition of an array. In the class Icon, this array is declared
as having dimension 5 but the definition doesn't specify the size.

This is allowed, since the compiler already knows the size.

The definition of the array also appears to be uninitialized.


particular, I would be interested to know if the static array
declaration ensures that all the int-pointer members of the array
are null pointers. They are either null pointers or uninitialized
pointers but which?

I would probably have written Icon *FlyweightFactory::_icons[] =
{0, 0, 0, 0, 0};

While this is possible it introduces an implicit dependency on
MAX_ICONS. If MAX_ICONS is less than 5 you will get a compilation
error. Since POD types are automatically initialized to zero in case
of an incomplete initializer list
Icon *FlyweightFactory::_icons[] = {0};
would do the job as well.

but it's possible that the code does this anyway.

Not that I have noticed.

Do the int-pointers
initialize to 0 anyway even if this is not made explicit as I did?

No, their values are undefined. But since there is another variable
_numIcons which seems to store the valid part of _icons, this does
not result in undefined behavior.

As the array has static storage duration, the array is guaranteed to be
zero-initialised. This means that all array elements will be initialised
to a null-pointer.


Thanks, Marcel.

I have two questions. I realise that POD types are automatically
initialized to 0 in the case of an uninitialized list but does this
apply here?

Yes, it does apply.

I incorrectly said that the array holds members of type
int*. Is this a POD type? I didn't think so.
However, the array actually holds members of type Icon* where Icon is
another class. So I don't think the initialization trick you mention
holds, or does it?

As int* and Icon* are both plain pointers, the certainly are POD types.
And the trick with specifying ferwer initialisers than there are array
elements works regardless of the element type of the array.

I suppose my questions are:
1) Does the initialization trick Type LargeArray[] = {0}; work
when Type is the int* type?
2) Does the same initialization trick work when Type is the
FairlyComplexClass* type?

Yes to both.

Many thanks again for your help?

Paul Epstein

Bart v Ingen Schenau
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