A first class

Overview

Teaching: 10 min
Exercises: 5 min

Questions

• What is a class?

• How do you create a class?

• How do you do use a class?

Objectives

• Create our first class to store some integers.

Previously we created an array of integers. It is very handy to be able to dynamically allocate memory of a given size that could potentially be determined at run time. However, as we just saw this introduces having to manage the dynamically allocated memory with new/new[] and delete/delete[]. The main way C++ tries to organize the management of these sorts of things is to use classes. So lets try that out and create a new Vector class which will allow us to more easily manage allocation of dynamic arrays of integers.

Start by coping our memory.cpp program from last episode.

$ cp memory.cpp classes.cpp
$ nano classes.cpp

Then modify it to look like the code below.

#include <iostream>

class Vector{
public:
  int size;
  int* data;
};

int main(){
  Vector a;
  a.size=10;
  a.data=new int[a.size];
  a.data[9]=15;
  std::cout<<"a.size="<<a.size<<"\n";
  std::cout<<"a.data[9]="<<a.data[9]<<"\n";
  delete[] a.data;
}

Here we have put the variable size and the array a renamed to data into our new Vector class. We have also used the . operator to access the classes public members.

$ g++ classes.cpp -o classes
$ ./classes

a.size=10
a.data[9]=15

Class keyword

class is a keyword like, struct is a key word. It is used to declare a new class and is followed by the class name, an open { and close } as is the case with a struct. Inside the {} is the definition of the class, e.g. the member variables like size above.

Class access modifiers

public is a class access modifier. It describes how the members of the class after the access modifier are to be treated.

• public: indicates that the following members can be accessed from anywhere.
• private: indicates that the following members can be accessed only from within the class (this will make more sense when we talk about member functions).
• protected: indicates that the following members can only be accessed by the class and any child classes (child classes will make more sense when we talk about inheritance).

Using classes

You can declare new objects as that type of class. Think data type (e.g. int, a struct you created, etc.) and the object as the variable you create. You can access members of a class (in the case of classes this is both member functions and member variables) with the . operator as you would with C structs, note the line a.size=10; above.

Access modifiers

 1 #include <iostream>
 2 
 3 class A{
 4 private:
 5   int foo;
 6 public:
 7   int bar;
 8 };
 9 
10 int main(){
11  A a;
12  a.foo=1;
13  a.bar=2;
14  std::cout<<"foo="<<a.foo<<" bar="<<a.bar<<"\n";
15 }

Given the above access_modifiers.cpp file what happens when the above is compiled and run?

1. The following compiler error is generated:

   access_modifiers.cpp: In function ‘int main()’:
   access_modifiers.cpp:12:5: error: ‘int A::foo’ is private within this context
      a.foo=1;
        ^~~
   access_modifiers.cpp:5:7: note: declared private here
      int foo;
          ^~~
   access_modifiers.cpp:14:24: error: ‘int A::foo’ is private within this context
      std::cout<<"foo="<<a.foo<<" bar="<<a.bar<<"\n";
                           ^~~
   access_modifiers.cpp:5:7: note: declared private here
      int foo;
          ^~~
   

2. The following output is generated:

   foo=1 bar=2
   

3. The following compiler error is generated:

   access_modifiers.cpp: In function ‘int main()’:
   access_modifiers.cpp:12:5: error: ‘int A::foo’ is private within this context
      a.foo=1;
        ^~~
   access_modifiers.cpp:5:7: note: declared private here
      int foo;
          ^~~
   access_modifiers.cpp:13:5: error: ‘int A::bar’ is private within this context
      a.bar=2;
        ^~~
   access_modifiers.cpp:7:7: note: declared private here
      int bar;
          ^~~
   access_modifiers.cpp:14:24: error: ‘int A::foo’ is private within this context
      std::cout<<"foo="<<a.foo<<" bar="<<a.bar<<"\n";
                           ^~~
   access_modifiers.cpp:5:7: note: declared private here
      int foo;
          ^~~
   access_modifiers.cpp:14:40: error: ‘int A::bar’ is private within this context
      std::cout<<"foo="<<a.foo<<" bar="<<a.bar<<"\n";
                                           ^~~
   access_modifiers.cpp:7:7: note: declared private here
      int bar;
          ^~~
   

Solution

1. YES: correct, foo has been declared a private member, however bar is public and so is fine to access outside of the class.

2. NO: if foo was declared as public this is what we would have gotten. Unfortunately foo is private and so can not be accessed from outside the class.

3. NO: while foo is indeed private, bar has been declared as public so it is accessible from outside the class.

Key Points

• Use the class keyword to create a new class.

• Class access modifiers control what parts of the program can access the classes members.

• Class members can be accessed with the . operator.