在单独的.cpp文件中专门化功能模板

Question

I've written a template for a very simple print function, and I've put it in a library of custom functions that all deal with console IO.

I've written a second library for a specific project, that is a child of the original. It specializes one of the templates.

I'm running in to an error which (I suspect) is being caused by the call in main.cpp occurring before the declaration of the specialized template. The error contains this line:

 In instantiation of 'static void baseIO::print(S) [with s = std::vector<int>]'

Which means its calling baseIO::print() instead of specialIO::print()

I tried making specialIO::print() a regular function instead of a template, and declaring it as normal in the header, but that denied main.cpp access to the base template.

Is there a way to make my specialization usable in main.cpp WITHOUT having to declare implement it there?

//in main.cpp
#include <vector>
#include "specialIO.h"

main(){
    std::vector<int> myVector;
    specialIO::print(myVector);
    specialIO::print("hello world");
    return 1;
}

.

//in baseIO_templates.cpp - templates are outside of the baseIO.cpp file because of linker errors
template<typename S>     //primary template
void baseIO::print(S str){
    std::cout << str;
}

//baseIO.h
class baseIO{
public:
    template<typename S> //primary template
    static void print(S str);
}
#include "baseIO_templates.cpp"

.

//specialIO.cpp
template<>               //specialized template
void static specialIO::print(vector<int> myVector){
    for(int i : myVector){
        baseIO::print(i)
    }
}

//specialIO.h
class uberIO : public baseIO {
    //empty
}

Answer 1

All template code must be available to the compiler when the code is called. So if you have declared template<T> void SomeFunction(T x); - then the compiler will need to know the definition of SomeFunction when you call it with std::string , or float or MyStruct . Since there is no way the compiler can figure out how to find the right implementation (or generate code for the right implementation) if it didn't know the type until now...

Therefore, you can't put template instantiations in a .cpp file - it is possible to put the specialization in a .cpp file if you declare them in a header-file, so:

class specialIO : public baseIO {
 public:
   template<>void static print<std::vector>(std::vector<int> v);
};

But if you leave it empty, the compiler won't even know there is such a print function, so can't call it - it will try to pass the std::vector into the regular baseIO::print<T> , which won't work because it's not capable of doing that.