如何覆盖在C ++函数内部的另一个类中的一个类中定义的虚拟方法

Question

I have a class A which is in a separate file( sayfile1.cpp )

class A{
public:
      virtual int add(){
      int a=5;
      int b=4;
      int c = a+b;
      return c;
      }
};

Now in a different file(say file2.cpp ), i have a function(i have a many other things in this function) inside which i want to create a class inherited from class A and implement the virtual method declared in class A .

void function(Mat param1, Mat param2)
{
  //Some process here..
  ..
  ..
  int c=100;
  class B:public A{
  public:
        virtual int add(){

        return c;
        }
  };

}

Now if i were to call the function int add() , i want the result of c to be 100 and not 9.

Is it possible to do something like this in C++ ??

Thanks in advance

Answer 1

Define member variable:

class B: public A {
    int c_;
public:
    explicit B(int c):c_(c){};
    virtual int add() {
        return c_;
    }
}
B variable((100));

Answer 2

You need to split your file1.cpp into file1.h :

#ifndef FILE1_H
class A {
public:
  virtual int add();
};
#endif

and file1.cpp with it's implementation:

int A::add { /*your code * }

In the other file you include only the header file:

#include "file1.h"

The following is not legal in C++:

void function(Mat param1, Mat param2)
{
  //Some process here..
  ..
  ..
  int c=100;
  class B:public A {
  public:
    virtual int add(){

    return c;
    }
 };

}

Instead, you need something like this:

class B : public A {
public:
    B(int v) : c(v) {}
    virtual int add(){ return c; }
private:
    int c;
};

void function(Mat param1, Mat param2)
{
  //Some process here..
  ..
  ..
  int c=100;
  B o(c);

}