Can you call a member of member of template parameter?

Question

I seem to be doing something wrong, but I'm not sure what. Here's the smallest example of what I'm trying to do:

#include <iostream>

using std::cout;

class CallMe {
 public:
  void Maybe() {
    cout << "A";
  }
};

class TemplateValue {
 public:
  static CallMe call_me;
};

template<typename T>
void CallMemberMember() {
  T::call_me.Maybe();
}

int main(int argc, char *argv[]) {
  CallMemberMember<TemplateValue>();
}

When I try to build this, I get a link error:

$ clang++ --std=c++11 repro_link_error.cc
Undefined symbols for architecture x86_64:
  "TemplateValue::call_me", referenced from:
      void CallMemberMember<TemplateValue>() in repro_link_error-9BE9gw.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

What am I doing wrong?

Answer 1

You need to provide definition for static member call_me:

CallMe TemplateValue::call_me;
int main(int argc, char *argv[]) {
  CallMemberMember<TemplateValue>();
}

Answer 2

This has nothing to do with templates: You need to provide the definition for the static class member:

class TemplateValue {
 public:
  static CallMe call_me;
};

CallMe TemplateValue::call_me;

(You should learn to distinguish compiler errors from linker errors. Your code was compiling fine, it was just not complete.)