I have to use an external library, but am getting a "multiple definition error" from following template function and its explicit specialization, if it gets called with a std::string.
template <typename T>
void foo(T& value);
template <>
void foo(std::string& value);
even if I change the 2nd function to
void foo(std::string& value);
the problem is the same.
According to [1] at least the version without a template (the "plain old function") should be prefered over the template version.
Does anybody have a clue, where the problem could be?
You're breaking the one-definition rule.
Unless a function is inline, it can only be defined once. If you mark the function as inline, so long as the definitions match they can be defined as often as desired. Template functions behave as if they were implicitly inline, so you don't get errors with templates.
However, an explicit specialization or non-template function is not implicitly inline and because you're including it in multiple translation units, you get multiple definitions; this breaks the rule. You should mark it as inline:
template <>
inline void foo(std::string& value);
(If you're getting this before link time, you need include guards.)