The class name is already a namespace, so there's no reason to give the impl such a long name:
class LongDescriptiveClassName
{
public:
// Stuff...
private:
struct Impl;
// shared_ptr is also an option if that's
// the semantics you want.
std::unique_ptr<Impl> Data;
};
// and off in the implementation, we have...
struct LongDescriptiveClassName::Impl
{
void PrivateMethod1();
void PrivateMethod2();
// and so on...
};
void LongDescriptiveClassName::Impl::PrivateMethod1()
{
// Stuff...
}
It works just fine.
Incidentally, your code is not an example of the pimpl idiom. The "p" in "pimpl" means "pointer", and this is important. A reference means that the object does not own its implementation.
It's not necessarily wrong; there are sometimes good reasons to wrap a reference in a class, but it's not the pimpl idiom.