2-3-1) Different C++ Pointers
The Four Types of Pointers in C++
This section is a summary of the four types of pointers found in C++. They are: pointers to ordinary memory (heap and stack space), pointers to functions, pointers to data members of objects and pointers to methods of objects.

Pointers to Data
This examples illustrates pointers to "ordinary" memory and pointers to data members of objects.

#include <iostream.h>

class Student
{
    friend ostream & operator<<(ostream & out, const Student & s);
public:
    int age;
    int studentNumber;
    char * name;
    char * address;

    Student(char * theirName, char * theirAddress, int theirAge, int theirStNum) :
      name(theirName), address(theirAddress), age(theirAge), studentNumber(theirStNum) {}
};

ostream & operator<<(ostream & out, const Student & s) {
    out << s.name;
    return out;
}

//function that accesses member data of object through a member pointer
void displayIntField(const Student & s, int Student::* mp) {
    cout << "IntValue = " << s.*mp << "\n";
}

void main( )
{
Student lou("Lou", "45 Elgin St.", 25, 12345);
Student sue("Sue", "145 Sussex Drive.", 23, 22222);
int x = 5;
int * ip = &x;

cout << "the int: " << *ip << "  The student: "  << lou << "\n";

int Student::* stdNumPtr = &Student::studentNumber;

displayIntField(lou, &Student::age);
displayIntField(lou, stdNumPtr);
displayIntField(sue, &Student::age);
displayIntField(sue, stdNumPtr);

//You cannot mix the pointer types
//int * p = &Student::age;  ERROR
//int Student::* stdp2 = &x; ERROR


}

/*
the int: 5  The student: Lou
IntValue = 25
IntValue = 12345
IntValue = 23
IntValue = 22222

*/

Pointers to Functions
You can take the address of a function. The invoking code then does not know what function it is invoking.
This is a very powerful feature. It is the basis for how dynamic binding (virtual functions) are implemented.

#include <iostream.h>
#include <string.h>

void sayAgain(const char * str, int n){
 for(int i=0; i<n; i++) cout << str << "\n"; }

void sayTimes(const char * str, int n){
 cout << str << " times " << n << "\n"; }

void saySpace(const char * str, int n){
 for(unsigned  i=0; i <  strlen(str); i++) {
  cout << str[i];
  for(int j=0; j<n; j++) cout << " ";
 }
 cout << "\n";
 
}

void doit(const char * string, int times, void (*f) (const char *, int)) {
  f(string, times);
}

void main(){

 doit("Hey", 3, &sayAgain);
 doit("Hello", 5, &sayTimes);
 doit("Bye", 5, &saySpace);

}
 

Output:
Hey
Hey
Hey
Hello times 5
B     y     e

Method Pointers

One can take the address of a method and then apply that method to an object.

#include <iostream.h>

class Student
{
    friend ostream & operator<<(ostream & out, const Student & s);
public:
    int age;
    int studentNumber;
    char * name;
    char * address;

    char * getName(void) {return name;}
    char * getAddress(void) {return address;}

    Student(char * theirName, char * theirAddress, int theirAge, int theirStNum) :
      name(theirName), address(theirAddress), age(theirAge), studentNumber(theirStNum) {}
};

ostream & operator<<(ostream & out, const Student & s) {
    out << s.name;
    return out;
}

//function that calls member methods through a method pointer
void display(const Student & s, char * (Student::* mp) (void) ){
    cout << "result = " << (s.*mp)() << "\n";
}

void main( )
{
Student lou("Lou", "45 Elgin St.", 25, 12345);
Student sue("Sue", "145 Sussex Drive.", 23, 22222);
int x = 5;
int * ip = &x;

cout << "the int: " << *ip << "  The student: "  << lou << "\n";

char * (Student::* memberPtr) (void) = &Student::getAddress;

display(lou, &Student::getName);
display(lou, memberPtr);
display(sue, &Student::getName);
display(sue, memberPtr);

}

/*
the int: 5  The student: Lou
result = Lou
result = 45 Elgin St.
result = Sue
result = 145 Sussex Drive.
*/

Method Pointers and Function Pointers cannot be mixed.

char * (fp)(void);
char * (Person:: mp)(void);

char * foo() {return "George";}

fp = &foo;  //OK
fp = foo;   //OK
fp = &Person::getName; //ERROR

mp = &Person::getName; //OK
mp = Person::getName; //ERROR
mp = &foo; //ERROR