Question

These are my files:

——–[ c.hpp ]——–

#ifndef _C
#define _C

#include<iostream>
class C
{
public:
    template<class CARTYPE> void call(CARTYPE& c);
};

#endif

——–[ c.cpp ]——–

#include "c.hpp"

template<class CARTYPE> void C::call(CARTYPE& c)
{
    //make use of c somewhere here
    std::cout<<"Car"<<std::endl;
}

——–[ v.cpp ]——–

class Merc
{};

——–[ main.cpp ]——–

#include "c.hpp"
#include "v.cpp"
//#include "c.cpp"

int main()
{
    Merc m; 
    C someCar;
    someCar.call(m);

}//main

I’m able to generate “.o” files for all the above files, with the command g++ -c main.cpp and g++ -c c.cpp and so on.
But when I try linking the “.o” files with g++ -o car c.o main.o v.o
I get this error:

main.o: In function `main':
main.cpp:(.text+0x17): undefined reference to `void C::call<Merc>(Merc&)'
collect2: ld returned 1 exit status

The error goes away when I uncomment the line #include “c.cpp” in main.cpp, but I feel it may be bad practice to include the cpp file this way. Am I doing this wrong? Is there a better way to cater to templated declarations while creating separate object files and linking them?
p.s: I’m using the templated function in a much more complex class structure. What is shown here is just a small example for the sake of showing you the kind of error I’m facing.

Answer 1

A way to solve this problem is to

a. remove ‘#include “c.hpp”‘ from c.cpp AND

b. include ‘c.cpp’ at the end of ‘c.hpp’ (strange sounding ‘#include “c.pp”‘)

This way the template definitions are availabe to each translation unit that includes ‘c.hpp’ without explicitly doing so in each .cpp file. This is called the ‘inclusion model’