error LNK2005: "double __cdecl str_to_T<double>()" already defined

[ausairman]

I have the following function definition in a header guarded file called "lib_tools.h":

template<typename T>
T str_to_T(const char text_element []) {
	// assume integer
	return atoi(text_element);
}

template<>
double str_to_T<double>(const char text_element []) {
	return atof(text_element);
}

However, when I try to compile I get the following linker errors:

1>main.obj : error LNK2005: "double __cdecl str_to_T<double>(char const * const)" (??$str_to_T@N@@$$FYANQBD@Z) already defined in fun_load_training_data.obj
1>main.obj : error LNK2005: "double __cdecl str_to_T<double>(char const * const)" (??$str_to_T@N@@YANQBD@Z) already defined in fun_load_training_data.obj
1>lib_tools.obj : error LNK2005: "double __cdecl str_to_T<double>(char const * const)" (??$str_to_T@N@@$$FYANQBD@Z) already defined in fun_load_training_data.obj
1>lib_tools.obj : error LNK2005: "double __cdecl str_to_T<double>(char const * const)" (??$str_to_T@N@@YANQBD@Z) already defined in fun_load_training_data.obj
1>struct_horizon_value.obj : error LNK2005: "double __cdecl str_to_T<double>(char const * const)" (??$str_to_T@N@@$$FYANQBD@Z) already defined in fun_load_training_data.obj
1>struct_horizon_value.obj : error LNK2005: "double __cdecl str_to_T<double>(char const * const)" (??$str_to_T@N@@YANQBD@Z) already defined in fun_load_training_data.obj

Now, fun_load_training_data.h/.cpp contain nothing more than a single function declaration (not for str_to_T of course), and a #include of lib_tools.h (where str_to_T is defined). lib_tools.h is properly header guarded (checked it thrice), so what else could be causing this issue? I tried a clean/rebuild but that didn't help...

[Peter87]

Not sure but maybe you have to put the definition of str_to_T<double> in a source file or declare it as inline.

[LB]

Yes, explicitly instantiating a template means is can be multiply defined.

[Disch]

+1 Peter

Once you specialize it, it's no longer a template and is an actual function. Therefore normal function rules apply (inline or have the body in only one source file)

[ausairman]

Hmm interesting, I didn't put them in a source file because they're templates, but inlining them fixed the errors. Do you know why?

[Zhuge]

Inline functions are allowed to be multiply defined because the compiler needs the function body in the source file that is calling it in order to inline it.

[ausairman]

Makes sense, thanks :)