How to define a template function of a template class outside the header?

Question

I have the following code:

test.h

struct A {void funA() { cout<<"Hello"; } };

template<typename T>
struct B: public T {
  template<typename X>
  void funB(X x) {
    this->funA();
    cout<<" world, I'm " <<typeid(x).name()<<"\n";
  }
};
template class B<A>;

main.cpp

int main() {
  B<A> o;      
  o.funB('a');
  o.funB(1);
}

Yields:

Hello world, I'm c

Hello world, I'm i

No Problem!

But I'm unable to move the definition of the function 'funB' to test.cpp.

I tried the following and failed:

test.h

...
  template<typename X>
  void funB(X x);
...

test.cpp

...
template<typename T>
template<typename X>
void B<T>::funB(X x) {
  this->funA();
  cout<<" world, I'm " <<typeid(x).name()<<"\n";
}
...

Yields:

undefined reference to 'void B::funB(char)'

undefined reference to 'void B::funB(int)'

Basically you don't. Template code needs to live in the header file. — NathanOliver, Dec 04 '19 at 16:07
I don't mind explicit instantiation - As mentioned in one of the answers specified in the original(not mine) question. — Nujufas, Dec 04 '19 at 16:10
Why would you want to do that? It's a lot of work for basically zero gain. — NathanOliver, Dec 04 '19 at 16:12
I don't mind the extra work. If it can be done I need to know how it can be done — Nujufas, Dec 04 '19 at 16:14
[This questions](https://stackoverflow.com/questions/45596922/explicit-instantiation-of-member-function-template-of-class-template) shows you how you would have to explicitly instantiate the templates. — NathanOliver, Dec 04 '19 at 16:15
As you see its a question - and at the bottom it says 'but it doesn't work'. — Nujufas, Dec 04 '19 at 16:18
Look at the *I need something like: which becomes a little verbose if I start having a lot of types of T and U* part. The last bit is the shortcut the OP tried since it can get very verbose. — NathanOliver, Dec 04 '19 at 16:19

How to define a template function of a template class outside the header?

0 Answers0