I'm not aware of any tool that can automatically create declarations; such a feat would require mind reading, at a bare minimum.

Given a declaration, it should be possible to automatically provide a stub implementation. But there are a lot of assumptions involved even with this: a source file must be created if one doesn't already exist, but this is tricky because implementation can be spread across several source files if desired. Also, implementation within the header file is also legitimate in C++, and depending on compiler such an implementation may have certain advantages over one placed in a separate source file. Then there's the matter of templates; what to do about potential specializationts? Where should they go? What about partial specializations? Should common operations like assignment, copy and initialization be stubbed out, provided with partial implementation, or be left unimplemented? And the list goes on.

C++ is a complex language, and coming up with a solution that would please all users in all cases is essentially impossible, as are the chances of coming up with a solution that would please even a significant fraction of users.

If you have a particular approach that would work well in your particular case, though, you can search for lex-compatible C++ language templates and build your own.