You are probably suffering from missing a valid instantiation. If you put your template definition in a separate .cpp file, when the compiler compiles that file it may not know which instantiations you need. Conversely, at the call sites which would instantiate the correct version of the template function, if the definition of the function body isn’t available the compiler won’t have the information to instantiate the required specializations.
You have two options. Put the function body for the function template in the header file.
e.g. in the header file:
template <typename T>
inline T* find_name(std::vector<T*> v, std::string name)
{
// ...
}
or explicitly instantiate the template in the .cpp where you’ve defined the template.
e.g. in the source file (will probably require #includeing the file that defines Item):
template <typename T>
T* find_name(std::vector<T*> v, std::string name)
{
// ...
}
template Item* find_name<Item>(std::vector<Item*> v, std::string name);