Search

Language Selector

8.4 — Access functions and encapsulation

Why make member variables private?

In the previous lesson, we mentioned that class member variables are typically made private. Developers who are learning about object-oriented programming often have a hard time understanding why you’d want to do this. To answer that question, let’s start with an analogy.

In modern life, we have access to many electronic devices. Your TV has a remote control that you can use to turn the TV on/off. You drive a car to work. You take a picture on your digital camera. All three of these things use a common pattern: They provide a simple interface for you to use (a button, a steering wheel, etc…) to perform an action. However, how these devices actually operate is hidden away from you. When you press the button on your remote control, you don’t need to know what it’s doing to communicate with your TV. When you press the gas pedal on your car, you don’t need to know how the combustion engine makes the wheels turn. When you take a picture, you don’t need to know how the sensors gather light into a pixellated image. This separation of interface and implementation is extremely useful because it allows us to use objects without understanding how they work. This vastly reduces the complexity of using these objects, and increases the number of objects we’re capable of interacting with.

For similar reasons, the separation of implementation and interface is useful in programming.

Encapsulation

In object-oriented programming, Encapsulation (also called information hiding) is the process of keeping the details about how an object is implemented hidden away from users of the object. Instead, users of the object access the object through a public interface. In this way, users are able to use the object without having to understand how it is implemented.

In C++, we implement encapsulation via access specifiers. Typically, all member variables of the class are made private (hiding the implementation details), and most member functions are made public (exposing an interface for the user). Although requiring users of the class to use the public interface may seem more burdensome than providing public access to the member variables directly, doing so actually provides a large number of useful benefits that help encourage class re-usability and maintainability.

Note: The word encapsulation is also sometimes used to refer to the packaging of data and functions that work on that data together. We prefer to just call that object-oriented programming.

Benefit: encapsulated classes are easier to use and reduce the complexity of your programs

With a fully encapsulated class, you only need to know what member functions are publicly available to use the class, what arguments they take, and what values they return. It doesn’t matter how the class was implemented internally. For example, a class holding a list of names could have been implemented using a dynamic array of C-style strings, std::array, std::vector, std::map, std::list, or one of many other data structures. In order to the use the class, you don’t need to know (or care) which. This dramatically reduces the complexity of your programs, and also reduces mistakes. More than any other reason, this is the key advantage of encapsulation.

All of the classes in the C++ standard library are encapsulated. Imagine how much more complicated C++ would be if you had to understand how std::string, std::vector, or std::cout were implemented in order to use them!

Benefit: encapsulated classes help protect your data and prevent misuse

Global variables are dangerous because you don’t have strict control over who has access to the global variable, or how they use it. Classes with public members suffer from the same problem, just on a smaller scale.

For example, let’s say we were writing a string class. We might start out like this:

These two variables have an intrinsic connection: m_length should always equal the length of the string held by m_string. If m_length were public, anybody could change the length of the string without changing m_string (or vice-versa). This would put the class into an inconsistent state, which could cause all sorts of bizarre problems. By making both m_length and m_string private, users are forced to use whatever public member functions are available to work with the class (and those member functions can ensure that m_length and m_string are always set appropriately).

We can also help protect the user from mistakes in using our class. Consider a class with an public array member variable:

If users can access the array directly, they could subscript the array with an invalid index, producing unexpected results:

However, if we make the array private, we can force the user to use a function that validates that the index is valid first:

In this way, we’ve protected the integrity of our program. As a side note, the at() function of std::array and std::vector do something very similar!

Benefit: encapsulated classes are easier to change

Consider this simple example:

While this program works fine, what would happen if we decided to rename m_value1, or change its type? We’d break not only this program, but likely most of programs that use class Something as well!

Encapsulation gives us the ability to change how are classes are implemented without breaking all of the programs that use them as well.

Here is the encapsulated version of this class that uses functions to access m_value1:

Now, let’s change the class’s implementation:

Note that because we did not alter the prototypes of any functions in our class’s public interface, our program that uses the class continues to work without any changes.

Similarly, if gnomes snuck into your house at night and replaced the internals of your TV remote with a different (but compatible) technology, you probably wouldn’t even notice!

Benefit: encapsulated classes are easier to debug

And finally, encapsulation helps you debug the program when something goes wrong. Often when a program does not work correctly, it is because one of our member variables has an incorrect value. If everyone is able to access the variable directly, tracking down which piece of code modified the variable can be difficult (it could be any of them, and you’ll need to breakpoint them all to figure out which). However, if everybody has to call the same public function to modify a value, then you can simply breakpoint that function and watch as each caller changes the value until you see where it goes wrong.

Access functions

Depending on the class, it can be appropriate (in the context of what the class does) for us to be able to directly get or set the value of a private member variable.

An access function is a short public function whose job is to retrieve or change the value of a private member variable. For example, in a String class, you might see something like this:

getLength() is an access function that simply returns the value of m_length.

Access functions typically come in two flavors: getters and setters. Getters are functions that return the value of a private member variable. Setters are functions that set the value of a private member variable.

Here’s an example class that has getters and setters for all of its members:

In this class, there’s no problem with allowing the user to directly get or set any of the member variables, so a full set of getters and setters is provided. In the MyString example above, no setter was provided for variable m_length because we don’t want the user to be able to set the length directly (length should only be set whenever the string is changed).

Rule: Only provide access functions when it makes sense for the user to be able to get or set a value directly.

Summary

As you can see, encapsulation provides a lot of benefits for just a little bit of extra effort. The primary benefit is that encapsulation allows us to use a class without having to know how it was implemented. This makes it a lot easier to use classes we’re not familiar with.

8.5 -- Constructors
Index
8.3 -- Public vs private access specifiers

31 comments to 8.4 — Access functions and encapsulation

  • Them damn gnomes!! They get everywhere!

    • Pintsize

      I also laughed about the gnome thing… 😛

      Anyway… Great explanation. It really shows the benefits of working with encapsulation. The Debugging argument is great.

  • Ngugi Thomas

    You are my best teachers

  • Marko

    Alex just want to say your website is better then most books there and i would dare to say that i will stick to this site now rather then even masterpieces like Stephen’s Prata Primer

    The way you explain things with examples and actually sticking to important things without writting novels is something that just works great with my mind.

  • yogal

    Wow, great explanation of encapsulation, and WHY one would use getters/setters instead of directly accessing members!

    One good argument for using encapsulation might be validation. When you have setters set-up you can validate what is passed to members and take appropriate action. Although members have to be of certain type, which provides low-level validation, sometimes you need a more complex requirement!

    Take care and thanks for such a wonderful tutorial!

  • Balu Nair

    Hi Alex,

    Your article rocks buddy! This article is such a precious gem.

    Thanks!!!

  • AsianBorat

    This tutorial is incredible. This article didn’t just explain encapsulation. It gave me a clear example, which FINALLY made me understand why encapsulation is so necessary.

  • yogendra

    great tutorial…..
    It gave me clear idea of what inheritance is and also
    the most basic difference between c and c++ is the
    code maintainability achieved through inheritance

  • DavidE

    Hey, I have a tip for those who like to consolidate their code. Instead of having both setValue() and getValue() methods, I do this:

    getset.h:

    main.cpp

    • Alex

      This strikes me as being more complicated than just having discrete getters and setters. In addition, I’m not sure how you’d tell whether a function supported only GET or SET.

  • Ramseena M.S

    Now too I didn’t understand the real usage of declaring class members private

  • Tonyv

    Great site, though a little less impressed by some of the arguements for access functions. Like, “what happens if one changes the name of a member…” well, what if you change the name of the access function? The debug arguement is better, but a bit arbitrary. Why not use the same argument to put redundant functions all over the place? Best was the string/length example, though only relevent when one has this sort of situation.

    A final grump: c++ is an intrinsicly hard to read language because so often what something means depends on the context. An amusing exercise would be to list all the things “*p” could mean. So while I appreciate the efforts made to improve the “readability” of c++, I read this often enough to think the author(s) consider c++ readability a feature rather than a curse. There are certainly ways to write Chinese to make it more readable, but that does not mean it is a language designed to be easily read.

    • Alex

      > well, what if you change the name of the access function?

      If you change the prototype of any function in your public interface, you risk breaking any programs using the class.

      Fortunately, it’s more common to change an implementation than it is to change the name of a function. In the Date class, you could represent the date in any number of ways, but the concept of getting a day, month, or year is still valid regardless of the specific implementation.

      A problem that is more likely to occur is when you extend your class in the future and your public functions need additional arguments that current programs aren’t supplying. Fortunately, default parameters can often be of help here.

  • WONDERFUL TUTORIAL BY MR.ALEX…

  • Rohan

    I have a problem in the following program…..(I am using turbo C++ i.e old version)

    output:Enter number: 10
    You entered: 10
    Now number changed to: -10

    Can you explain the above program? Especially why did we use ABC in passing reference to object?

    • alan

      /*
      Rohan

      February 1, 2015 at 7:57 am  · Reply

      Well you wouldn’t normally use a getter to actually prompt for data input, it
      is really to return a value to the user and not to output it to the screen.
      But except for a minor difference between compilers that function does work.

      Mainly you asked about why use ABC to pass a reference, its confusing because you call the function
      in main() as obj1.putdata() so it shouldn’t have compiled without an argument of type class ABC, though when modified as below
      it does work.
      In essence when you define a function with parameters you have to state the data type e.g. int, double, char etc.
      A class is a new data type defined by you.
      If you want a function to receive a class object by reference then you have to tell the function what to expect, hence
      "ABC &obj" just the same as if want a function to receive say an int by reference you would tell it "int &nNum"

      Hopefully that explains it, however I don’t think its what you really want to do, I’ve added
      another function "change_me()" which I think is what you were trying to do.

      Of course since you defined "int data" as public there is no need for a function anyway, as per my last example.
      */
      #include <iostream>

      using namespace std;

      class ABC
      {
      public:int data;
             void getdata()
             {
                  cout << "Enter the number 10 ";
                 cin >> data; // VS 2013 doesn’t accept cout <>, but you missed the prompt anyway
                 cout << "You Entered:" << data;
             }
             void putdata(ABC &obj)
             {
                 obj.data = -obj.data;
                 cout << "Now number is changed to:" << obj.data;
             }
              void change_me()
              {
                  data = data++ ;
                  cout << "Number is now: " << data ;
              }

      };
      void main()
      {
          ABC obj1;
          obj1.getdata();
          obj1.putdata(obj1); // added obj1 as the argument
          obj1.change_me();
          obj1.data = obj1.data++;
          cout << "Number is now: " << obj1.data;
      }
      /*
      output:Enter number : 10
          You entered : 10
          Now number changed to : -10
          Number is now: -9
          Number is now: -8

      */

  • Nikhil Mittal

    Alex, your tutorials are awesome! I am loving these. Thanx!

  • sandip patidar

    Very beautiful way to explain C++.
    Alex,
    You really did excellent Job. Keep it up!!!

  • Methos

    I’ve been using these lessons for a while and have done quite a bit of Get/Set functions in the project I’m working on, but have run into a snag. I’m trying to generate a maze to explore and as part of that, I have the following:

    (there’s more to that)

    And a series of functions like the following

    I say like, because I’ve already checked for typos and such (and corrected one unrelated error). Unfortunately, for some reason I cannot determine, SetIsActive() (which looks like that but with the correct variables) does nothing and SetIsPassable() is setting every bool member except m_bIsQuestBoss and m_bMonsterMoved to true. Which then for a variety of reasons stops the maze from being generated.

    I’m certain this is the issue as I’ve put cout statements in the maze generating code so I can monitor these values.

    Anyway I was wondering if you could think of anything that could cause this sort of weirdness. I changed IDE to code blocks in case it was a compiler problem (which maybe I have wrong as it wanted to look for more than one compiler when I installed it), but the error persists.

    • Methos

      Oy, nevermind.

      Apparently half of my Get() functions which were copypasted initially weren’t corrected to return the proper variable. Which I would have caught four days ago if I had searched for "passable" on the header file rather than other places I thought the Set function might have appeared.

    • Alex

      I don’t think I have enough information to effectively determine what this issue is. The only thing I see that’s weird is that your set function returns *this. Normally setters return void. Is your maze a 2d array of MazeTiles?

      • Methos

        The error was in my Getters, a bunch of them were returning m_bIsPassable instead of the variable I meant them to because I managed to copypaste them and change the function names and not the variables.

        The set function returns *this so I can chain a bunch of them on one line where appropriate, so for example in my maze creating function, I have the following

        I then start building the maze from where ever the entrance got put, the above code makes sure that tile is actually in the interior.

        Yes, my Maze is an array of MazeTiles of size Width * Length and sits inside of an object I call MazeData (Place is a MazeData object, pMap is a Maze object pointer) which has member variables for width, length, what direction the player is facing, how many monsters and traps are left, etc.

  • Naga Pushkal

    Hi Alex,

    Can you please explain what is the difference between Abstraction and Encapsulation in OOP? I have gone through many sites to find the answer but none of them explained it clearly.

    • Alex

      Sure. Encapsulation generically refers to the process of enclosing one or more items inside a container (such as a function, array, or class). In C++, it specifically refers to the mechanism for restricting access to some of an object’s components (data or functions). A class encapsulates its members, using access specifiers to control access. You work with the object through the public interface. This interface generally requires no knowledge of how the object is implemented internally to use.

      Some people also define encapsulation as the binding of data and behaviors together -- but we just call that object-oriented programming.

      Abstraction, on the other hand, generically means reducing the complexity of something by providing a simplified interface that leaves out complexity that may be too much to deal with or irrelevant. For example, a remote control provides an abstraction for the physical circuitry required to send a message to the TV to change the channel.

      In C++, abstraction also is typically used to refer to virtual functions or interface (pure virtual) classes. These allow you to create interfaces for something without regard to how they’re implemented.

  • Matt

    Found another typo: "getLength() is an access function that simply returns the value of m_nLength."

    In the example, it returns the value of m_length.

  • Matt

    Alex. I’m following you for a few months now and Learncpp is my first resource when in need to revise anything, all in all good job!

    Anyway to the point. As a rule of thumb, does it make sense to have private getters/setters for managing internal class workings as well?

    • Alex

      Not usually. Getters and setters are typically used to allow external classes to get and set data in the class. Since internal members can do that directly, there’s usually no need.

      The exception case would be if getting or setting a variable was non-trivial (e.g. if setting a value could cause dynamic reallocation to occur, for example). In that case, it might make sense to write a “set” function of some kind to handle all of that logic (whether you expose this function as public or private depends on your needs).

Leave a Comment

Put C++ code inside [code][/code] tags to use the syntax highlighter

  

  

  

nineteen + 8 =