templates and static objects (undefined reference & required from)

Question

I'm implementing some ideas using templates and static members. Although the "real" code produces another error, this is the one which i still have on a toy example code

#include <string>
#include <iostream>

template<int dim>
class Class1 {
  public:
     Class1() {};
    ~Class1() {};
     void foo() {
        std::cout<<"foo-1"<<std::endl;
     }
   protected:
     std::string name;
};

template<int dim>
class Class2 : public Class1<dim>
{
  public:
     Class2(const int & a, const int &b) : 
        number( Class2<dim>::id_generator++ ) 
     {
        Class1<dim>::name = "My-name"; 
        foo(); // (1)
      };

 void foo() {
    Class1<dim>::foo();
    std::cout<<"foo-2"<<std::endl;
 }

 private:
    const unsigned int number;
    static unsigned int id_generator;
};

 int main() 
 {
    int a = 1, b=2;
    Class2<2> class2(a,b);   // (2)
 }

with the linker error:

 undefined reference to `Class2<2>::id_generator'

rea-life example produces 2 errors

 (1) required from 'Class2<dim>::Class2(int, int) [with int dim = 3]'
 (2) required from here.

that real-life errors tell me absolutely nothing at all! :( I hope if the toy-problem is solved, the real-life one will be gone too, but if anyone has any ideas on the "real-life" errors (those 2 lines) in the context of the structure, pls, let me know.

possible duplicate of [C++: undefined reference to static class member](http://stackoverflow.com/questions/272900/c-undefined-reference-to-static-class-member) — Mat, Mar 30 '13 at 14:03
What do you mean it tells you nothing at all? It tells you that you have an undefined reference, and then it tells you the places where a reference was required. That's everything you need! — Lightness Races in Orbit, Mar 30 '13 at 14:24
@LightnessRacesinOrbit i ment the second error, which actually stays in the real-life code, but the code compiles — Denis, Mar 30 '13 at 14:29
@Denis: Undefined reference is a _linker_ error so, yes, your code compiles — Lightness Races in Orbit, Mar 30 '13 at 14:38
@LightnessRacesinOrbit the errors i mean are two 'required from...' written in the description, NOT `undefined reference`. — Denis, Mar 30 '13 at 15:36
@Denis: Those are not errors. Those are two lines that go with the error, telling you more about it. — Lightness Races in Orbit, Mar 30 '13 at 15:55
@LightnessRacesinOrbit interesting thing is that i have those lines even without any errors at all. — Denis, Mar 30 '13 at 18:23
@Denis: You're misreading something, because that does not happen. — Lightness Races in Orbit, Mar 30 '13 at 19:07

score 1 · Accepted Answer · answered Mar 30 '13 at 14:03

1

You forgot to add a definition for your static data member id_generator. Add this at global namespace level:

template<int dim>
unsigned int Class2<dim>::id_generator = 0;

With this addition, you can see your code correctly compiling and linking here.

answered Mar 30 '13 at 14:03

Andy Prowl

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score 1 · Answer 2 · answered Mar 30 '13 at 14:05

Well, as the error message says, there is no definition of the static data member. If this was an ordinary class, you'd put the definition in a source file:

// header:
class C {
    static int i;
};

// source:
int C::i = 3;

A template is a pattern; the compiler uses it to generate code. So what you want to end up with when the compiler instantiates the template is something like the preceding code. But template code goes in headers, not source files, so you write it like this:

// header:
template <class T>
class C {
    static int i;
};

template <class T>
int C<T>::i = 3;

thanks for your answer, it's a pity i can mark only single one as a correct. — Denis, Mar 30 '13 at 15:37

templates and static objects (undefined reference & required from)

2 Answers2