Why should the “PIMPL” idiom be used? [duplicate]

Answer 1

I don't know if this is a difference worth mentioning but...

Would it be possible to have the implementation in its own namespace and have a public wrapper / library namespace for the code the user sees:

catlib::Cat::Purr(){ cat_->Purr(); }
cat::Cat::Purr(){
   printf("purrrrrr");
}

This way all library code can make use of the cat namespace and as the need to expose a class to the user arises a wrapper could be created in the catlib namespace.