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8.5 — Constructors

When all members of a class (or struct) are public, we can initialize the class (or struct) directly using an initialization list or uniform initialization (in C++11):

However, as soon as we make any member variables private, we’re no longer able to initialize classes in this way. It does make sense: if you can’t directly access a variable (because it’s private), you shouldn’t be able to directly initialize it.

So then how do we initialize a class with private member variables? The answer is through constructors.

Constructors

A constructor is a special kind of class member function that is automatically called when an object of that class is instantiated. Constructors are typically used to initialize member variables of the class to appropriate default or user-provided values, or to do any setup steps necessary for the class to be used (e.g. open a file or database).

Unlike normal member functions, constructors have specific rules for how they must be named:

  1. Constructors must have the same name as the class (with the same capitalization)
  2. Constructors have no return type (not even void)

Default constructors

A constructor that takes no parameters (or has parameters that all have default values) is called a default constructor. The default constructor is called if no user-provided initialization values are provided.

Here is an example of a class that has a default constructor:

This class was designed to hold a fractional value as an integer numerator and denominator. We have defined a default constructor named Fraction (the same as the class).

Because we’re instantiating an object of type Fraction with no arguments, the default constructor will be called immediately after memory is allocated for the object, and our object will be initialized.

This program produces the result:

0/1

Note that our numerator and denominator were initialized with the values we set in our default constructor! This is such a useful feature that almost every class includes a default constructor. Without a default constructor, the numerator and denominator would have garbage values until we explicitly assigned them reasonable values (remember: fundamental variables aren’t initialized by default).

Direct and uniform initialization using constructors with parameters

While the default constructor is great for ensuring our classes are initialized with reasonable default values, often times we want instances of our class to have specific values that we provide. Fortunately, constructors can also be declared with parameters. Here is an example of a constructor that takes two integer parameters that are used to initialize the numerator and denominator:

Note that we now have two constructors: a default constructor that will be called in the default case, and a second constructor that takes two parameters. These two constructors can coexist peacefully in the same class due to function overloading. In fact, you can define as many constructors as you want, so long as each has a unique signature (number and type of parameters).

So how do we use this constructor with parameters? It’s simple! We just use the direct initialization form of initialization:

This particular fraction will be initialized to the fraction 5/3!

In C++11, we can also use uniform initialization:

Note that we have given the second parameter of the constructor with parameters a default value, so the following is also legal:

Default values for constructors work exactly the same way as with any other function, so in the above case where we call six(6), the Fraction(int, int) function is called with the second parameter defaulted to value 1.

Rule: Use direct or uniform initialization with your classes

Copy initialization using equals with classes

Much like with fundamental variables, it’s also possible to initialize classes using copy initialization:

However, we recommend you avoid this form of initialization with classes, as it may be less efficient. Although direct initialization, uniform initialization, and copy initialization all work identically with fundamental types, copy-initialization does not work the same with classes (though the end-result is often the same). We’ll explore the differences in more detail in a future chapter.

Rule: Do not copy initialize your classes

Reducing your constructors

In the above two-constructor declaration of the Fraction class, the default constructor is actually somewhat redundant. We could simplify this class as follows:

Although this constructor is still a default constructor, it has now been defined in a way that it can accept one or two user-provided values as well.

When implementing your constructors, consider how you might keep the number of constructors down through smart defaulting of values.

A reminder about default parameters

The rules around defining and calling functions that have default parameters (described in lesson 7.7 -- Default parameters) apply to constructors too. To recap, when defining a function with default parameters, all default parameters must follow any non-default parameters.

This may produce unexpected results for classes that have multiple default parameters of different types. Consider:

With s4, we’ve attempted to construct a Something by providing only a double. This won’t compile, as the rules for how arguments match with default parameters won’t allow us to skip a non-rightmost parameter (in this case, the leftmost int parameter).

If we want to be able to construct a Something with only a double, we’ll need to add a second (non-default) constructor:

An implicitly generated default constructor

If your class has no other constructors, C++ will automatically generate a public default constructor for you. This is sometimes called an implicit constructor (or implicitly generated constructor).

Consider the following class:

This class has no constructor. Therefore, the compiler will generate a constructor that behaves identically to the following:

This particular implicit constructor allows us to create a Date object with no parameters, but doesn’t initialize any of the members (because all of the members are fundamental types, and those don’t get initialized upon creation).

Although you can’t see the implicitly generated constructor, you can prove it exists:

The above code compiles, because date object will use the implicit constructor (which is public).

If your class has any other constructors, the implicitly generated constructor will not be provided. For example:

Generally speaking, it’s a good idea to always provide at least one constructor in your class. This explicitly allows you to control how objects of your class are allowed to be created, and will prevent your class from potentially breaking later when you add other constructors.

Rule: Provide at least one constructor for your class, even if it’s an empty default constructor.

Classes containing classes

A class may contain other classes as member variables. By default, when the outer class is constructed, the member variables will have their default constructors called. This happens before the body of the constructor executes.

This can be demonstrated thusly:

This prints:

A
B

When variable b is constructed, the B() constructor is called. Before the body of the constructor executes, m_a is initialized, calling the class A default constructor. This prints “A”. Then control returns back to the B constructor, and the body of the B constructor executes.

This makes sense when you think about it, as the B() constructor may want to use variable m_a -- so m_a had better be initialized first!

In the next lesson, we’ll talk about how to initialize these class member variables.

Constructor notes

Many new programmers are confused about whether constructors create the objects or not. They do not (the code the compiler creates does that).

Constructors actually serve two purposes. The primary purpose is to initialize objects that have just been created. The secondary purpose is to determine whether creation of an object is allowed. That is, an object of a class can only be created if a matching constructor can be found. This means that a class without any public constructors can’t be created!

Although the main purpose of a constructor is to initialize an object, whether the constructor actually does an initialization is up to the programmer. It’s perfectly valid to have a constructor that does no initialization at all (the constructor still serves the purpose of allowing the object to be created, as per the above).

Finally, constructors are only intended to be used for initialization when the object is created. You should not try to call a constructor to re-initialize an existing object. While it may compile, the results will not be what you intended (instead, the compiler will create a temporary object and then discard it).

Quiz time

1) Write a class named Ball. Ball should have two private member variables with default values: m_color (“black”) and m_radius (10.0). Ball should provide constructors to set only m_color, set only m_radius, set both, or set neither value. For this quiz question, do not use default parameters for your constructors. Also write a function to print out the color and radius of the ball.

The following sample program should compile:

and produce the result:

color: black, radius: 10
color: blue, radius: 10
color: black, radius: 20
color: blue, radius: 20

Show Solution

1b) Update your answer to the previous question to use constructors with default parameters. Use as few constructors as possible.

Show Solution

2) What happens if you don’t declare a default constructor?

Show Solution

8.5a -- Constructor member initializer lists
Index
8.4 -- Access functions and encapsulation

282 comments to 8.5 — Constructors

  • Deepanshu

    Hi Alex,
    You wrote in this article, "Before the body of the constructor executes, m_a is initialized, calling the class A default constructor". I am unable to understand it. How the m_a will get initialized before the body of constructor B()? Please explain.

    • Alex

      When you instantiate a class, three things happen: First, the appropriate constructor for the class is called. Then, there's an initialization stage, where all initializations happen. Then the body of the constructor executes.

      Now, consider what would happen if your code has this:

      In this case, because you've supplied no parameters to the constructor, variable a (of type A) will be initialized using the A class default constructor.

      Similarly, because we haven't specified anything for the initialization portion of our constructor, member variable m_a (of type A) will use the A class default constructor to initialize.

      In the very next lesson, we'll show you how to initialize your variables yourself rather than just relying on the default constructor.

      Make sense?

  • Gapo

    Hey Alex could you please explain this code from your quiz, I don't quite understand it.

    • Alex

      This function is a constructor for class Ball, which means it can be called to construct an object of type Ball. It also makes use of default parameters, which will be used if you don't pass in a corresponding argument.

      This function will handle instantiating a Ball object in 3 different ways:

      Because this constructor works with no parameters, it's considered a default constructor.

      Note that this constructor will not allow you to only pass in a radius (and use the default color).

  • Hi Alex,

    Another question about the line

    I simplify the code just to have one constructor. I had information about the memory address when the m_color and MyColor object is created. The Address of both object are different. So m_color is, I think a copy of the MyColor object. According of what you explain on previous comments about the const, if the object was passed by reference, I think the address of both objects should be the same. My question is why the address of m_color and MyColor are different?

    Output:
    MyColor is blue, MyColor addres = 0x7ffd2efcced0
    color: blue, radius: 10
    Class Ball m_color address = 0x7ffd2efccec0
    MyColor is red, MyColor addres = 0x7ffd2efcced0
    color: blue, radius: 10
    Class Ball m_color address = 0x7ffd2efccec0

    • Alex

      They have different addresses because MyColor and m_color are different variables. MyColor is a local variable living in function main(), and m_color is a separate variable that lives inside variable blue.

      • Hi Alex,

        I finally understand my mistake by reading again the comments of this thread and the section "7.3-Passing arguments by reference".

        By using

        the variable color is not copied to the constructor but it is used to copy the variable color to the variable m_color. This is why the address of both variable are different. Am I correct?

  • tawanda

    hie , thank you for this article. i am receiving an error:
    19    2 [Error] 'Invoice::Invoice(std::string, std::string, int, int)' is private
    and

    96    71[Error] within this context

    here is my code

  • In the section "Classes without default constructors" above, you talk about how the implicitly defined default constructor for "Date" class would result in the member variables of "date" object holding garbage values. However, from some programming that I've been doing, it looks like if the member variables are actually Class objects themselves, then this implicitly defined default constructor does not assign them garbage values but calls their own default constructors. Is this observation technically correct and generally valid?

    • Alex

      Yes, if a member variable is a class, then the constructor for that member variable will be called as part of the construction of the outer class. I'll update the lesson to make this clear.

  • Rahul

    Hey Alex,
    Why you use assert() in the example under heading"Direct and uniform initialization using constructors with parameters"?
    What is the purpose of using this function?

  • shahadat hussain

    please help me i am very problem
    what is constructore or
    use why

  • Christos

    Thank you Alex.

    I tried to initialize something that wasn't existing yet. When I started reading the next chapter, I realized how obvious it was.

  • Christos

    Hi Alex,

    Thank you for your great job. You have made the most complete and comprehensible guide.

    May I ask you, why when I use "{ }" inside the constructors to set the members, vb2015 return me the error "C2064: term does not evaluate to a function taking 1 arguments"?

    #include "stdafx.h"
    #include <iostream>
    #include <string>
    #include <cassert>

    using namespace std;

    class Ball
    {
        string m_color;
        double m_radius;

    public:
        Ball()
        {
            m_color = "black";
            m_radius = 10.0;
        }

        Ball(const string &color)
        {
            m_color{ color };
            m_radius{ 10.0 };
        }

        Ball(double radius)
        {
            m_color{ "black" };
            m_radius{ radius };
        }

        Ball(const string &color, double radius)
        {
            m_color{ color };
            m_radius = { radius };
        }

        void print()
        {
            cout << m_color << " / " << m_radius << endl;
        }
    };

    int main()
    {
        Ball def;
        def.print();

        Ball blue("blue");
        blue.print();

        Ball twenty(20.0);
        twenty.print();

        Ball blueTwenty("blue", 20.0);
        blueTwenty.print();

        return 0;
    }

    • Alex

      This is invalid. You can not initialize members inside the constructor body, you can only assign. Assignment can only be done via operator=.

      If you want to initialize members, it must be done before the constructor body, via a constructor member initialization list (covered next lesson).

  • Will

    Is it a good practice to always provide a default "garbage" constructor, such that we can use std::array to store objects of this class?

    • Alex

      Generally, you will always want your classes to have at least one public constructor (otherwise, how will you construct them). Whether you allow that constructor to be a "default" constructor or not is really up to you and what makes sense for the class. For classes that just hold data values, you'll almost definitely want a default constructor. For classes that require the user to provide some input in order to do a job (e.g. pass the class a filename to work with) there's no default value that really makes sense, so in that case, no, you wouldn't want a default constructor.

      If you want to create an array of a class that doesn't have a default constructors, there are other ways to do that (such as having the array hold a pointer to the class object rather than the actual class object).

      • Will

        Can you illustrate a way to create an array of a class that doesn't have a default constructor other than using pointers? I know we can use uniform initialisation. But considering the following code:

        There are two problems with it. First, the code become very ugly if the length of the array is very long. Second, there is clearly some relationship between each element. Is there a way to use a loop to construct the array so that we can save some effort as well as improve maintainability?

  • Lam Gia Khang

    I have some confuse about this line of code:

    Why we have the ampersand "&" before parameter color here? Because when I try it, the program still ran okay without "&". So I don't know what is it used for??

  • Matthieu B.

    The code contained in the following code frame causes a compiler error since "default" is a reserved keyword:

  • Connor

    Hello again Alex. I was fooling around with classes and I've encountered an error when using two unique class constructors:

    My compiler (VS '15) keeps giving me an error in that there is more than one default constructor. For some reason these aren't existing in Harmony. WHAT AM I DOING WRONG??????

    • Alex

      A class can only have one default constructor, and you've declared two. When you instantiate object point1, is the compiler supposed to call the constructor with no parameters (in which case, m_x = 0 and m_y = 0) or the constructor that has all defaulted parameters (in which case, m_x = 10 and m_y = 10)? It can't tell, so it's giving you an error.

      • Connor

        Isn't

        a default constructor taking no parameters and

        a constructor taking two parameters? I was under the impression that NUMBER 1 was a constructor taking no parameters and NUMBER 2 was a constructor with parameters? you had said in the topic
        "Direct and uniform initialization using constructors with parameters"
        that we can have two constructors living in harmony, a default case and one taking two parameters...but when I try to define them as above I get the error (however, copying and pasting your example works just fine). Can you please elaborate on how the compiler is interpreting these constructors? Why isn't the one with parameters simply overloaded?

        Again - thank you for the time and effort you've put into these tutorials.

        • Alex

          Yes, multiple constructors can live in harmony. However, a class can only have ONE default constructor, and both of your constructors qualify as default constructors (because they can both be called with no parameters).

          So when you do this:

          The compiler doesn't know which constructor to call. Both are valid resolutions.

  • Rob G.

    Hi,

    I am trying to force the program to use a default value in the initialization parameters of object date_2 while I provide 2. Is this possible, and if so how do I do it?

  • Alex

    I still don't understand what you're looking for. 🙁 Is this it?

  • Jim

    Alex,

    I was able to figure out how to compile and run all the constructor Faction examples you showed in this lesson, Fraction six{6}; etc.. But I can't figure out how to use directly initialization with int x {5};.  This has to be simple, but I've tried to do this a lot of ways without any luck.

    Can you please give me some idea of how to do this.  Thanks

    • Alex

      • Jim

        Alex,
        I guess I didn't make my post clear. I did it exactly as you showed only with int x {5} and no y.  How does the program know what to use x for? I would have assumed it should replace the numerator value but I'm probably wrong. I even tried numerator = x, used m_numerator = x, vice versa and same other ways. But nothing worked. Can you show it used in an example program please?

  • subh samal

    Hi Alex,

    In question 1b (Quiz time), we can use a default constructor as below to handle all cases except Ball twenty(20.0);(my block of code is not exactly same to 1b.)
        
    Ball (string color2 = "black", double radius2 = 10.0)   //this is able to handle no parameter, only color, both color and radius.
         {
             color = color2;
             radius = radius2;
         }
    So my question is why it is not able to act as a constructor for the object with only radius parameter (eg: Ball twenty(20.0)) ? I can ask more elaborately in case my question is not understood!

    • Alex

      Unfortunately, C++ does not have the capability to "skip" arguments in a function call. I wish you could do something like this:

      I'm sure the C++ designers had a good reason for disallowing this, but if so, I don't know what it is.

      • Darren

        Hazarding speculation, could be to do with: type checking; order the arguments are passed to functions; the number of arguments supplied; dealing with the 'this' pointer for member functions. In other-words, not a Scooby. (Scooby-do rhymes with "clue"). But yes it would be nice to be able to use this syntax. (BTW "Ball ball" - has someone been watching the 'Goldbergs' by any chance?).

  • Jim

    Alex,
    I see so by defining a class we are essentially creating an object with that classes name like Fraction used in this lesson. In that class we defined 2 private parameters. After which we used constructors Fraction with or without parameters, only we now define the parameters as public  So constructors are similar to a function since they both can have arguments passed to them. We then use get and return to supply the input and output.

    You did say there a many classes predefined in C++, however how do we know what they are? What does the #include <cassert> header cover.

    • Alex

      Constructors are not similar to functions, they ARE functions (just ones with a special purpose).

      The only way I'm aware of to see all the classes in C++ is to look at reference material. Much of what's in the standard library are classes (e.g. std::cin and std::cout are objects of class ostream).

      The cassert header includes the assert function.

  • Shiva

    Alex, are constructors ever made private? If yes, can you give an example scenario?

    • Alex

      Yes, they are occasionally used. For example, if you make a default constructor private, then you can't instantiate your class externally. This can be useful when you want more control over if or how many class objects are instantiated. Private constructors are mostly used for implementing Singletons and Factories. Consider:

  • Connor

    Hello Alex,
    for quiz Q1, when I define

    the << between "Color: " and m_color gives an error: no opererator "<<" matches these operands.
    What am I doing wrong?

    • Alex

      I'm not sure, I don't see anything obviously wrong.

      Some things to check:
      1) Erase the entire line and re-type it to ensure you don't have any invisible characters or fancy quotes
      2) Make sure you've included the string header
      3) Make sure m_color is a variable of some type that can be output via <<, like std::string or char[].

      • Connor

        OMG, you're right - I forgot to

        I was pulling my hair out for hours trying to solve this one...don't quit my day job I suppose.

        • Darren

          Very good example of the process of learning. Now if you seen that error again you'll know to check for missing header file includes as a first port of call.

  • Vegeta

    Hey, in quiz 1b you created these two constructors:

    While I created these two (I used char pointer):

    So basically in my main function the user can write both:

    Is it better? It looks convenient. Or is that bad practice?

    • Alex

      I've actually never seen anybody ask this before. I think it's generally a good idea to enforce a specific parameter ordering. A lack of consistency tends to lead to potential errors, especially in the future as you look to extend the code (what happens when you add a third parameter to the constructor?).

      Also, your example doesn't quite work: if you define a ball with no parameters, the compiler won't be able to determine which of the two constructors to call.

      • jo

        Dont you think that an advantage of the unsorted arguments is, if you ask a user "Enter color and/or radius of a ball!" it doesnt matter in which order the user provides the input?

        You can build the constructor like this and to ensure define works.

        • Alex

          > if you ask a user "Enter color and/or radius of a ball!" it doesnt matter in which order the user provides the input?

          There are a number of challenges with this kind of approach:
          1) The more ways there are for a user to do something, the more way there are for something to go wrong. It's better to be more explicit about what you expect the user to do (enter the color first, and the radius second, or vice-versa, but pick one).
          2) Because radius and color are different types, they're not interchangable. So you're going to have to parse the user input to determine whether they entered (color, radius) or (radius, color). If you're doing that, you now have a deterministic ordering, so you might as well call the constructor the same way every time.
          3) Having constructors that allow either ordering leads to less extensibility later. When you add a 3rd parameter are you going to write 7 more constructors to have every permutation?
          4) If two of the parameters are the same type, this doesn't work, because you can't disambiguate which is which.

  • Nachiket

    I have just started learning C++. I have one question in very first code on this page. What is the difference between following two lines?

    Thanks.

  • George

    Hi!

    Can you tell me how class string uses constructor in this way: std::string()
    Overloaded parenthesis operator or something else?

    I also found somewhere something like this: XClass foo = XClass();

    Thanks in advance!

  • Aaron

    Thanks again for the free textbook online!

    For the constructors for the Ball class, it seems to be adequate to provide only two of them: one with the 2 parameters as optional (with default values), and another with only the 2nd parameter as required. For example:

    If the user provides no arguments, both arguments, or a string argument, the first constructor is called. With a type double argument, the second is used. Anything wrong with this?

    • Alex

      Not at all! I've updated the quiz question to have the reader do things both ways (without and with default parameters), because I think it's instructive. Thanks for the thought!

  • Ishtmeet Singh

    Hi Alex,
    In your first quiz question:

    ,
    why did you write const string instead of using string only? And why are we passing string by reference instead of value?

    • Alex

      Because it's better to pass non-fundamental variables by const reference than by value. That avoids making an unnecessary copy when the argument is passed to the parameter.

  • Brett

    Shouldn't it be:

    instead of:

    for the examples on this page?

  • Lokesh

    In quiz-2 solution, you have said:
    "The compiler creates an empty one for you. If there are no other constructors, this created constructor is public. Otherwise it is private."
    I think you should omit the part "Otherwise it is private" as it creates confusion. If there are other constructors(whether it is default or non-default), the compiler would not create any constructor. Hence, you would have to define your own default constructor. So, the answer should be something like:
    "If there are no other constructors defined, the compiler will create an empty default constructor. This automatically created constructor would be public.
    However, if there are other constructors that take parameters, we would have to define our own default constructor and explicitly make it public using the public access specifier."
    See, that's why I commented earlier that the concept is a bit twisty.

  • Lokesh

    In the default constructor code example, shouldn't the pre-processor directive such as #include be at the top of code file by convention?

  • Kiran C K

    Hi Alex,

    I was writing a code that just instantiates an object with default constructor and the member function does the other works. here is my code:

    Here are my questions:
    1. I have declared a default constructor under

    . But if it is not defined, it shows error as

    when creating an object. I am wondering that if the class members are private, would the default constructors would also be private. If that is the case, what would be the access if a class has both private and public members?

    2. Without defining the above public default constructor, if I created an object as

    , it shows the error

    .
    And I get the similar error for all the three member functions.

    and

    are doing the same thing. Then why is the compiler shows two different error messages?

    I am using Code::Blocks IDE GCC compiler

    • Alex

      1) If you don't provide _any_ constructors, C++ will provide a public default constructor for you. In this case, since you have a private non-default constructor, if you remove the public default constructor, C++ won't provide one for you. This means your class has no default constructors, which means it can't be instantiated directly.

      2) I think the compiler may be getting confused and thinking that pass is a function, and that Booking pass() is a forward declaration rather than a variable definition.

  • Vaibhav

    Hi alex,
    The first example for default constructor that you gave- while calling Fraction default in the int main my compiler is showing error ,whereas instead of using the word default if i use anything else it works..What is happening wrong!?
    The compiler shows error as improper use of typedef 'Fraction'

    • Alex

      default is a keyword in C++11, so your compiler is probably getting confused. I've updated the example to give the variable a different name.

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