C++ - Separate declaration/definition for template function in template class

Before I answer your question, let me first say: Don't do this. Extend std::string by using free-functions instead, much like the standard library implements many algorithms. Additionally I'd suggest doing it for ranges rather than strings only but that's more subjective.

Also note that std::string avoids implicit conversions to C-strings not to make your life harder, but to protect your code from a wide variety of obscure bugs that can be caused by unexpected implicit conversions. I would think very long and hard about implementing them. Just think about this: It takes you a few extra moments to type.c_str() once when you write the code, and for the rest of eternity anyone who reads your code will immediately know that it's being used as a C-style string and not as a std::string.

To answer your question, just put the code in the header:

//! Converts a value of the given type to a string.
template <class _ValTy> static String ConvertFrom(_ValTy val)
{
    // Code here
}

Finally note that identifiers starting with underscore+capital letter (and many other things starting with _) are reserved for the compiler and thus all bets are off as to your program's functionality.

Syntax of definition of template member functions outside of template is like this:

template <class T> struct A
{
   template <class X> void f();
};

template<class T> template<class X> void A<T>::f()
{
}

So your code is correct.

Would like to note that defining template members in .cpp is not very useful. In this case you shall explicitly instantiate them with all types you need to use with this template. Or do not use them outside this .cpp which doesn't make sense.