Re: A question related to virtual function

From:

Salt_Peter <pj_hern@yahoo.com>

Newsgroups:

comp.lang.c++

Date:

18 May 2007 00:14:47 -0700

Message-ID:

<1179472487.575867.65640@y80g2000hsf.googlegroups.com>

On May 18, 1:06 am, babu <nasif4...@gmail.com> wrote:

class CPrintString
{
        void printName(Person* p)
        {
                printf("person");
        }

        void printName(Student * s)
        {
                printf("student");
        }

};

Here, Person is the base class of Student. Now if we write -

Person * st = new Student();
CPrintString * pr =new CPrintString();
pr->printName(st);

The output is "person". But we need "student" to be printed. Why such
thing occurs?

Now What is the solution so that the correct function is called at
runtime based on the parameter object type, so that "student" can be
printed in the previous calling sequence? Is there any solution
without changing class CPrintString? If exists please give.

Thanks
Babu

CPrintString should not be a class unless its purpose is akin to a
printer.
Make it a function instead.
If its purpose was to be a printer, its purpose in life is not to
detect *what* type of Person is being passed to it. Let the Person
object determine that.
The printer doesn't care.

#include <iostream>
#include <string>

class Person
{
  std::string m_s;
public:
  Person()
    : m_s("person") { }
  Person(const std::string s)
    : m_s(s) { }
  std::string getString() const
  {
    return m_s;
  }
};

class Student : public Person
{
public:
  Student()
    : Person("student") { }
};

struct CPrintString
{
  void printName(const Person& r_p)
  {
    std::cout << r_p.getString();
    std::cout << std::endl;
  }
};

int main()
{
  Student student;
  CPrintString printer;
  printer.printName(student);
}

This way, if you add another type of Person or Student to your
project, the printer is unaffected. Note: pointers replaced by const
references.