C++: error "explicit specialization in non-namespace scope" - c++

Question

template<typename T1, typename T2>
class Bimap {
public:
class Data {
private:
template<typename T> Data& set(T);
template<> Data& set<T1>(typename T1 v) { /*...*/ }
};
};
That gives me the error:
error: explicit specialization in non-namespace scope 'class Bimap<T1, T2>::Data'
I understand what the error is saying. But why I can't I do this? And how can I fix it?

Answer 1

One way forget templates, overload:
Data& set(T1 v) { /*...*/ }
but here is a trick which I use sometimes
you can specialize class template within class:
class {
template<typename T>
struct function_ {
static void apply(T);
};
template<>
struct function_<int> {
...
};
template<typename T>
void function(T t) { return function_<T>::apply(t); }

Answer 2

#Albert
I had a similar problem when I wanted to add a "trim-excess-capacity" to a custom made container. The std::vector swap trick and changing the declaration of the existing container were not valid options. So I've come up with this:
template <class T, bool isPtr> struct DeleteImp
{
static void Trim(T* to, unsigned int count);
};
template <class T> struct DeleteImp<T, false>
{
static void Trim(T* to, unsigned int count) {}
};
template <class T> struct DeleteImp<T, true>
{
static void Trim(T* to, unsigned int count)
{
for(unsigned int i=0; i<count; i++)
delete to[i];
}
};
used by my container like this:
DeleteImp<T, TypeTraits<T>::isPointer>::Trim(buf + length, truelength-length);
You may also want to check out this resource.