c++ - multiple definition of template specialization when using different objects

Question

When I use a specialized template in different object files, I get a "multiple definition" error when linking. The only solution I found involves using the "inline" function, but it just seems like some workaround. How do I solve that without using the "inline" keyword? If that's not possible, why?

Here is the example code:

paulo@aeris:~/teste/cpp/redef$ cat hello.h 
#ifndef TEMPLATE_H
#define TEMPLATE_H

#include <iostream>

template <class T>
class Hello
{
public:
    void print_hello(T var);
};

template <class T>
void Hello<T>::print_hello(T var)
{
    std::cout << "Hello generic function " << var << "
";
}

template <> //inline
void Hello<int>::print_hello(int var)
{
    std::cout << "Hello specialized function " << var << "
";
}

#endif

---

paulo@aeris:~/teste/cpp/redef$ cat other.h 
#include <iostream>

void other_func();

---

paulo@aeris:~/teste/cpp/redef$ cat other.c 
#include "other.h"

#include "hello.h"

void other_func()
{
    Hello<char> hc;
    Hello<int> hi;

    hc.print_hello('a');
    hi.print_hello(1);
}

---

paulo@aeris:~/teste/cpp/redef$ cat main.c 
#include "hello.h"

#include "other.h"

int main()
{
    Hello<char> hc;
    Hello<int> hi;

    hc.print_hello('a');
    hi.print_hello(1);

    other_func();

    return 0;
}

---

paulo@aeris:~/teste/cpp/redef$ cat Makefile
all:
    g++ -c other.c -o other.o -Wall -Wextra
    g++ main.c other.o -o main -Wall -Wextra

---

Finally:

paulo@aeris:~/teste/cpp/redef$ make
g++ -c other.c -o other.o -Wall -Wextra
g++ main.c other.o -o main -Wall -Wextra
other.o: In function `Hello<int>::print_hello(int)':
other.c:(.text+0x0): multiple definition of `Hello<int>::print_hello(int)'
/tmp/cc0dZS9l.o:main.c:(.text+0x0): first defined here
collect2: ld returned 1 exit status
make: ** [all] Erro 1

If I uncomment the "inline" inside hello.h, the code will compile and run, but that just seems like some kind of "workaround" to me: what if the specialized function is big and used many times? Will I get a big binary? Is there any other way to do this? If yes, how? If not, why?

I tried to look for answers, but all I got was "use inline" without any further explanation.

Thanks

Answer 1

Intuitively, when you fully specialize something, it doesn't depend on a template parameter any more -- so unless you make the specialization inline, you need to put it in a .cpp file instead of a .h or you end up violating the one definition rule as David says. Note that when you partially specialize templates, the partial specializations do still depend on one or more template parameters, so they still go in a .h file.