如何在模板类中初始化此静态类变量？

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.

Answer 1

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

Answer 2

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

Answer 3

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.