Implementing variadic method in C++ issue

Question

I'm trying to implement a variadic method but when I try the following code it doesn't compile:

Configurations.h

#ifndef Test_Configurations_h
#define Test_Configurations_h

#include <vector>

class Configuration
{
public:
    Configuration(){}
    int someData;
};


class Configurations
{
public:
    void addConfiguration (Configuration config);

    template<typename... Args>
    void addConfiguration (Configuration first, Args... args);

private:
    std::vector<Configuration> _configs;

};

#endif

Configurations.cpp

#include "Configurations.h"

void Configurations::addConfiguration (Configuration config)
{
    // do something
}

template<typename... Args>
void Configurations::addConfiguration (Configuration first, Args... args)
{
    // Do something
}

main.cpp

#include <iostream>
#include "Configurations.h"


int main(int argc, const char * argv[]) {

    Configuration c1;
    Configuration c2;

    Configurations configurations;

    configurations.addConfiguration(c1, c2);

    return 0;
}

When I try to call the method

addConfiguration (config1, config2)

The compiler give me the following error:

Undefined symbols for architecture x86_64:
  "void Configurations::addConfiguration<Configuration>(Configuration, Configuration)", referenced from:
      _main in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I think that I'm not declaring properly the method, but I don't know where is the error.

Thanks in advance!

You have a link error, do you implement variadic `addConfiguration` in cpp and use it in other translation unit ? — Jarod42, Oct 30 '14 at 15:17
Nope, the point is that if I use an easy example it just works, but when I want to add my own class i have this error — Joan P.S, Oct 30 '14 at 15:19
I think the body of the template function should be `addConfiguration(first); addConfiguration(args);` otherwise all it will do is add the last configuration passed, and ignore all of the others. — cdhowie, Oct 30 '14 at 15:20
I'm guessing that the "easy example" wasn't a template, and that the implementation is in a source file, not a header. A template must be usually be implemented in a header, since the definition must be available in every translation unit that needs to instantiate it. — Mike Seymour, Oct 30 '14 at 15:25
@MikeSeymour Is there a way to accept argument packs *without* templates? — cdhowie, Oct 30 '14 at 15:37
@cdhowie: Of course not. If the "easy example" was still variadic, then that part of my guess is wrong. — Mike Seymour, Oct 30 '14 at 15:40
these type of variadic functions are purely compile time phenomenon (as opposed to `va_arg`). you need templates. — Karoly Horvath, Oct 30 '14 at 15:43
I just edited the question with the full code to reproduce the error — Joan P.S, Oct 30 '14 at 15:55
How can this be variadic, when the ... is not at the end of the parameter list? — harper, Oct 30 '14 at 16:12
do you mean this? addConfiguration (Configuration first, Args... args) — Joan P.S, Oct 30 '14 at 16:14

score 0 · Answer 1 · answered Oct 30 '14 at 17:29

The problem is as Mike said: you're declaring the function template in a header but defining it in a source file. Both the declaration and the implementation need to stay in the same translation unit. I've closed this question as a duplicate with a more comprehensive answer.

Implementing variadic method in C++ issue

1 Answers1