Question: I hope the problem is clear. Is there an alternative way not to get undefined reference error without implementing template functions in header files? Thanks in advance.

Answer: 
Gollum (https://github.com/github/gollum) is a Git-powered wiki written by GitHub in Sinatra. You can both push and pull from command line or use the included web interface to edit content.

Unfortunately it seems like GitHub have some what abandon development/maintenance of it, so it has some rough edges. It's also very basic, so it does not include features such as authentication[1].

I'm gonna use Gollum with gollum-site (https://github.com/dreverri/gollum-site), a static file generator for Gollum, and just use Gollum as the admin backend.

1: A basic solution to authentication can be found at <https://github.com/github/gollum/issues/107#issuecomment-2608061>

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There is also Regulate, <https://github.com/quickleft/regulate>:

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> Rails 3 engine that provides a Git backed CMS that allows for an admin to define editable regions in a page view.
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> 
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Maybe you could combine a git wrapper for editing, etc:

<https://github.com/schacon/ruby-git>

with your own rendering code based on a simple file based storage system.

I'm not so familiar with Ruby though, so can't help much on the ruby rendering side of things.

This comment you make "most of my site will contain advanced dynamic functionality" looks like you are going to need to roll your own solution based on your unique requirements.