How do I initialize this static class variable in my template class?

Question

I have the following code (it's on ideone.com):

template<class T>
class CMemoryPool
{
public:
    CMemoryPool(int param1)
        : stuff(param1)
    {}

private:
    T stuff;
};

template<class T>
class CList
{
public:
    struct Entry
    {
        T data;
    };

    static CMemoryPool<Entry> s_pool;
};

template<class T>
CList<T>::CMemoryPool<CList<T>::Entry>::s_pool(1);

int main()
{
    CList<int> list;
}

I can't seem to get the initialization of s_pool outside of the class to compile. Can anyone help me figure out how to make this work? Note I'm using C++03 only.

This has been asked many times: - stackoverflow.com/questions/185844/… - stackoverflow.com/questions/8461546/… — Jeef
– Jeef, Commented Mar 28, 2014 at 13:34
this is for a template class, your links to the simple scenarios do not answer my question — void.pointer
– void.pointer, Commented Mar 28, 2014 at 13:36
Pretty sure you need a typename for CList<T>::Entry, but unfortunately this also didn't fix it :( ... — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Mar 28, 2014 at 13:43

Matthieu M. · Accepted Answer · 2014-03-28 13:55:02Z

2

I think that you forgot how initializing a static data member works in general:

struct Data { int i; };

struct Holder { static Data StaticMember; };

Data Holder::StaticMember = { 1 };
^    ^~~~~~~~~~~~~~~~~~~~ static member qualified name
\~~~ static member type

If you look at your declaration, it is striking that you forgot one of the two above:

// Only a single element: there should be a type and a name
template<class T>
CList<T>::template CMemoryPool<typename CList<T>::Entry>::s_pool(1);

// Two elements
template<class T>
CMemoryPool<typename CList<T>::Entry> CList<T>::s_pool(1);
^                                     ^~~~~~~~~~~~~~~~ name
 \~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ type

Once corrected it just works

answered Mar 28, 2014 at 13:55

Matthieu M.

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2 Comments

void.pointer Over a year ago

The code itself is fairly obfuscated so I missed an basic and fundamental piece.

Matthieu M. Over a year ago

Yes, that's the problem with the spaces popping within types: it looks right, but in fact isn't. What clued me in is that in your declaration somehow CMemoryPool ended up as a nested type inside CList which seemed fishy and I had to step back to realize what was truly happening.

jsantander · Accepted Answer · 2014-03-28 13:53:25Z

1

EDIT: I was under the impression that:

You must explicitly give value to the static value for each instantiation of the template:
CList<int>::CMemoryPool<CList<int>::Entry>::s_pool(1);
must be somewhere in your *.C files...

Alternatively, use a static local variable in a method of the table used to get the value.

But after playing a bit, this seems to compile in ideone

template<class T>
CMemoryPool<typename CList<T>::Entry> CList<T>::s_pool(1);

I still recommend @FredOverflow solution as it protects your from static initialization problems

edited Mar 28, 2014 at 13:53

answered Mar 28, 2014 at 13:33

jsantander

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3 Comments

void.pointer Over a year ago

Most other template statics can be initialized with type T, so why do I need to do it explicitly for each type in this case?

void.pointer Over a year ago

Ah I feel stupid now! The static initialization statement was missing its type! Great job!

jsantander Over a year ago

@RobertDailey ... still... I'm not sure how this would play when this template is properly defined on its *.h file and included in multiple places... anyone knows?

fredoverflow · Accepted Answer · 2014-03-28 13:37:45Z

1

Static data members inside class templates are somewhat of a pain to initialize. I suggest a factory function instead. Then you don't need to worry about defining the variable somewhere else.

Just rewrite the line

static CMemoryPool<Entry> s_pool;

to

static CMemoryPool<Entry>& s_pool()
{
    static CMemoryPool<Entry> foobar;
    return foobar;
}

And then use s_pool() instead s_pool everywhere. You get lazy initialization as a benefit.

answered Mar 28, 2014 at 13:37

fredoverflow

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4 Comments

void.pointer Over a year ago

Doesn't answer my question

jsantander Over a year ago

@FredOverflow probably you're missing static CMemoryPool<Entry> foobar(1); that would replace the original CList<T>::CMemoryPool<CList<T>::Entry>::s_pool(1);

πάντα ῥεῖ Over a year ago

@RobertDailey I've been playing around with your code a bit, and couldn't get it syntactically right. I'd say Freds answer well answers your question, or let's better say: solves your problem elegantly.

void.pointer Over a year ago

I'm not looking for an elegant solution, this is an experiment for me (learning exercise). I found this problem and can't solve it, would be nice to see the solution. I understand there are alternative ways to do the same thing, but that's not what I'm asking for.

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