How to call the function in derived class to process handle_post?(Function call missing argument list to create pointer)

Question

in my base class, this is a public non-static member function:

void BaseClass::SetPorcessMethod()
{
     //do something
    Listener.support(methods::POST, this->HandlePost);
    //do something
}

In the function above, Listener is a static member variable. Function HandlePost is a pure virtual function which is implemented by all derived classes. I want to use this pointer to call different HandlePost from different derived class. Like:

class BaseClass
{
 public:
   static http_listener listener;
   void SetPorcessMethod();
   virtual void HandlePost((http_request request) = 0;
   /*listener is init in constructor , not display here*/
}
 class Derived2:public BaseClass
{
  void HandlePost(http_request request);
}
class Derived1:public BaseClass
{
  void HandlePost(http_request request);
}

Derived1 instance1;
instance1.SetPorcessMethod();
Derived2 instance2;
instance2.SetPorcessMethod();

However, it displaysBaseClass::HandlePost:function call missing argument list use &BaseClass::HandlePost:function to create a pointer to member.I know this is because Your code attempts to pass a pointer-to-member-function, which cannot be converted to a pointer-to-function. This is because a pointer-to-member-function can only be called on an object, so it wouldn't know what object to use. Function call missing argument list to create pointer But what should I do so that I can call the function from derived class withsupport()?

Answer 1

You are trying to pass a member function to support. In order to call such a function the caller need the member function arguments and the pointer to the instance to call the function on.

However support expects a std::function<void(http_request)>, i.e. without the instance pointer. So you have to wrap the call into another callable which does not need to have the BaseClass instance pointer passed. You can do that with a lambda (or std::bind if you prefer):

Listener.support( methods::POST,
    [this](http_request request){return HandlePost(request);} );

Listener.support( methods::POST,
    std::bind(&BaseClass::HandlePost, this, std::placeholders::_1) );

#include<functional> for the latter variant.