forward declaration and Cast incomplete type C++

Question

I have issues about forward declaration .

namespace downloader {
class IHttpThreadCallback ;
class MemoryHttpRequest ;

}

when I cast

auto responseHttpRequest = dynamic_cast<downloader::MemoryHttpRequest*>(m_callback);

It show warning incomplete type . How should I try , please suggest to me .

beside that I try to include class , but it not work and I think it not good idea . Thank you very much

Answer 1

It show warning incomplete type . How should I try , please suggest to me.

To use dynamic_cast , the type must be complete. Solution: include the definition.

beside that I try to include class , but it not work and I think it not good idea .

Including the class definition is not only a good idea, but mandatory if you need to use dynamic_cast . In this case, using a forward declaration is not a solution to your problem.

in my situation , the class which I need is defined in .cpp file

In that case, you cannot downcast into that type - unless you move the class definition into a header that you include.

Answer 2

dynamic_cast uses the vtable to interrogate and navigate the class hierarchy. it also needs to know the class content/layout in order to calculate offsets. that is why the compiler needs to know the class definition. static_cast needs to have a relationship between the classes.

if you are sure about the return value and are happy to avoid the runtime/type check then you might consider using a reinterpret_cast.

otherwise you will need to include the definition.

I put an example together

#include <iostream>
#include <vector>
#include <iostream>

namespace n {
  class A;
  class B;

  n::A* f();
}

std::ostream& operator<<(std::ostream& os, n::A& a);
std::ostream& operator<<(std::ostream& os, n::B& b);

int main()
{
  n::A* a(n::f());
  n::B* b=reinterpret_cast<n::B*>(n::f());

  std::cerr << "a: " << *a << std::endl;
  std::cerr << "b: " << *b << std::endl;
}

namespace n {
  class A
  {};

  class B: public A
  {};

  n::A* f() {
    return new A();
  }  
}

std::ostream& operator<<(std::ostream& os, n::A& a) {
  os << "in A";
  return os;
}

std::ostream& operator<<(std::ostream& os, n::B& b) {
  os << "in B";
  return os;
}