templated virtual functions??

Hi
Let say that I wanted to do a filesystem browser. The filesystem is a
tree structure, so my idea was to make an general tree browser that
could browse anything that implemented the ITreeStructure interface.
Now I want to make a generic container class that is a tree and can
hold any data type.
The declarations for this look like this:

template<typename T>
class Tree : public ITreeStructure
{
     typedef unsigned int size_type;
     typedef std::vector<Tree>::iterator iterator;
     typedef const std::vector<Tree>::iterator const_iterator;

     bool is_leaf() = 0;
     ITreeStructure& parent() = 0;
     iterator children_begin() = 0;
     iterator children_end() = 0;
     T get_value(void) = 0;
};

class TreeBrowser : public IWindowControl
{
public:
    TreeBrowser();
    TreeBrowser(ITreeStructure& tree);
    void BrowseTree(ITreeStructure& tree);

private:
    ITreeStructure* m_treep;
};

Now the question is how to define the ITreeStructure interface?
I would like to define it like this
class ITreeStructure
{
public:
    virtual bool is_leaf() = 0;
    virtual ITreeStructure& parent() = 0;
    virtual std::vector<ITreeStructure>::iterator children_begin() = 0;
    virtual std::vector<ITreeStructure>::iterator children_end() = 0;
    template<typename T> virtual T get_value(void) = 0;
};

The problem is the get_value method since templated virtual functions
are not allowed.
I could have parameterized the whole interface like this
template<typename T>
class ITreeStructure
{
public:
    virtual bool is_leaf() = 0;
    virtual ITreeStructure& parent() = 0;
    virtual std::vector<ITreeStructure>::iterator children_begin() = 0;
    virtual std::vector<ITreeStructure>::iterator children_end() = 0;
    virtual T get_value(void) = 0;
};
But that defeats the whole purpose of having an interface, since I
want to decouple the TreeBrowser from any implementation of a tree
structure. If the whole interface is parameterized so must the
TreeBrowser class be.

So my questions are:
1. Why isn't templated virtual functions allowed in C++?
2. Is there an reasonably simple way around this? Or can I change my
design some way and have both decoupling and still be able to have an
parameterized implementation of a tree structure?

Thanks in advance
/JK

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