将基类的指针强制转换为子类的指针-C ++

Question

This post is a working example for this Question . However, there are some points that confuse me. That is why I am posting it as another question.

Basically, the problem is how to cast a base class to its subclasses. People have suggested using dynamic memory and new ing a fresh instance of the subclass.

I cam up with the following solution, and now wonder how this works. I create an object of the base class (Employee). Set its Id. Then I cast its pointer to a pointer of the subclass and set more members.

// Example program
#include <iostream>
#include <string>
#include <list>
#include <algorithm>
using namespace std;

class Employee {
 public:
  int getId() { return id;}
   void setId( int id) {this->id = id;}
 protected:
  int id;

};

class PartTimeEmployee : public Employee {

};

class FullTimeEmployee : public Employee {
 public:
  int getGrade() {return grade;}
  void setGrade(int grade) {this->grade = grade;}
 private:
  int grade;
};

class Organization {
 public:    
  void addEmployee(Employee* e) { empList.push_back(e); }
  Employee* getEmployee(int id) { 
      for (std::list<Employee*>::iterator it=empList.begin(); it!=empList.end(); ++it) {
          if((*it)->getId() == id) {return (*it);}
      }
      return NULL; 
  }
 private:
  std::list<Employee*> empList;
};

int main()
{
  Employee e1;
  e1.setId(5);

  FullTimeEmployee *pFt1 = (FullTimeEmployee*) &e1; 
  pFt1->setGrade(1);                

  Organization org1;
  org1.addEmployee(pFt1);
  FullTimeEmployee* pFt2 = (FullTimeEmployee*) org1.getEmployee(5);
  cout << pFt2->getId() << endl;
  cout << pFt2->getGrade() << endl;
}

Why does it work? I see it as follows:

Employee: |--id--|... FullTimeEmployee: |--id--|--grade--|...

1- Is this what happens in the background? When I cast to the FullTimeemployee, C++ copies all pre-existing members of the Employee class plus giving you more space based on the size of the new type.

2- Is it safe to use this approach? Should I prefer dynamic memory allocation, eg FullTimeEmployee *pFT1 = new FullTimeEmployee(); over this? If yes why? why can't I just go with pointer casting?

UPDATE: What makes this example different from casting to void* ? You just play with memory addresses. What can go wrong as long as you know how much memory you need?

Answer 1

It only seems to work, because e1 is not an instance of any of the child classes, leading to undefined behavior .

To make it work, you need to use dynamic memory allocations:

Emplyoee *e1 = new FullTimeEmployee;

Now it works with the casting, because e1 actually points to a FullTimeEmployee object.

But you should really use static_cast , like

FullTimeEmployee *pFt1 = static_cast<FullTimeEmployee*>(e1);
pFt1->setGrade(1);

Or just

static_cast<FullTimeEmployee*>(e1)->setGrade(1);

Also you should read about downcasting , which is what you are doing.