CODEWITHSUNDEEP
c++templates
poy
poy

Reputation: 10537

c++ undefined reference using templates

So I have the following code:

template<class T>
class A
{
public:

  class B
  {
  public:
    virtual void Destroy(T obj);
  };

  B &_b;

  A(B b) : _b(b)
  {
  }

  void Go(T obj)
  {
    _b.Destroy(obj);
  }
};

class X : public A<int>::B
{
public:
  void Destroy(int x)
  {
    //do something
  }
};

int main()
{
  X x;
  A<int> a(x);
  a.Go(5);
  return 0;
}

But I get a compile error:

undefined reference to 'A<int>::B::Destroy(int)'

I've seen issues before when doing templates in separate .hpp and .cpp files... but this is all in one file.

Thanks.

Upvotes: 0

Views: 104

Answers (3)

Adam Zalcman
Adam Zalcman

Reputation: 27233

You haven't provided implementation for Destroy() in B, e.g.:

template<class T>
void A<T>::B::Destroy(T a) {
  // do something
}

Note that you also have other compilation errors: missing semicolon after the body of class B and missing argument in call to Go().

Upvotes: 0

Kerrek SB
Kerrek SB

Reputation: 477494

Add a definition for virtual void Destroy(T obj);:

  • Either in place:

    class B
{
public:
    virtual void Destroy(T obj) { /* here! */ }
    // ==== ALTERNATIVELY: ====
    virtual void Destroy(T obj) = 0;  // pure-virtual
};

  • Or afterwards:

    template <typename T>
void A<T>::B::Destroy(T obj) { /* here. */ }

In any event, the constructor for A should take a reference:

A(B & b) : _b(b) { }
//^^^^^

Upvotes: 1

Luchian Grigore
Luchian Grigore

Reputation: 258638

  • virtual void Destroy(T obj); isn't implemented. Implement it or mark it as virtual pure = 0

  • There's no ; after the definition of X

  • a.Go(); is wrong, Go should take a parameter.

Upvotes: 2

Related Questions