1. Classes in C++
And comparison to Java
CS-1030
Dr. Mark L. Hornick

2. Classes – declaration & definition
Just like C++ global functions and global variables, C++ classes must be declared before they are used
C++ classes must (generally) be implemented in two parts
Declaration – in a header file
Definition – in a .cpp file
CS-1030
Dr. Mark L. Hornick

3. A C++ Class Declaration (in .h)
class keyword
Name of the class
class Employee
{ private:
string name;
int id;
public:
Employee(string nameArg, int idArg ); string getName();
int getID(); };
Attributes cannot be initialized
Don’t forget the semi-colons!
CS-1030
Dr. Mark L. Hornick

4. Class Definition Notes
public
Visible to everyone
private
Visible only within the class
Used to protect class objects
protected
Visible to classes which derive from the base class
In Java, protected attributes and methods are also visible to other classes in the same package as well
CS-1030
Dr. Mark L. Hornick

5. Defining a Class Member Function (in .cpp)
function’s full name
return type
function’s scope
int Employee::getID () {
return id; }
Employee::Employee( string nameArg, int idArg ) {
name = nameArg; id = idArg; }
CS-1030
Dr. Mark L. Hornick

6. Class Constructors Job is to initialize the object
Called automatically, just like in Java
Here is where attributes are initialized
Same name as the class
Just like Java
No return type (not even void)
Java constructors have the void type
Can be overloaded
CS-1030
Dr. Mark L. Hornick

7. Parameter names optional in declaration!
Constructor Example
Declaration in .h file:
class Employee { // note no “public”
public:
Employee(string nameArg, int idArg);
…};
Definition in .cpp file:
Employee::Employee(string name, int id)
{ // initialization code here }
Parameter names optional in declaration!
No return value
CS-1030
Dr. Mark L. Hornick

8. Creating an instance of a C++ class
#include “employee.h”
int main( int argc, char* argv[] ){
// note difference vs. Java:
Employee pat( “Pat”, 123 );
Employee tim( “tim”, 456 );
int id1 = pat.getID();
// objects are destroyed here… }
CS-1030
Dr. Mark L. Hornick

9. C++ Class Destructor No such method in Java
Cleans up an object at the end of lifetime
Cleans up the data members prior to their own destruction
Closes things (like files), etc.
Called automatically
Special name – similar to constructor
Clock::~Clock() // note the ~
If you don’t supply a destructor, the compiler will!
Compiler-supplied destructor does nothing!
CS-1030
Dr. Mark L. Hornick

10. No arguments or return value
Destructor Example
In .h file
class Employee {…
public:
~Employee();
…};
In .cpp file
Employee::~Employee()
{} // Do nothing
No arguments or return value
CS-1030
Dr. Mark L. Hornick