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9.11 — The copy constructor

Recapping the types of initialization

Since we’re going to talk a lot about initialization in the next few lessons, let’s first recap the types of initialization that C++ supports: direct initialization, uniform initialization or copy initialization.

Here’s examples of all of those, using our Fraction class:

We can do a direct initialization:

In C++11, we can do a uniform initialization:

And finally, we can do a copy initialization:

With direct and uniform initialization, the object being created is directly initialized. However, copy initialization is a little more complicated. We’ll explore copy initialization in more detail in the next lesson. But in order to do that effectively, we need to take a short detour.

The copy constructor

Now consider the following program:

If you compile this program, you’ll see that it compiles just fine, and produces the result:

Let’s take a closer look at how this program works.

The initialization of variable fiveThirds is just a standard direct initialization that calls the Fraction(int, int) constructor. No surprises there. But what about the next line? The initialization of variable fCopy is also clearly a direct initialization, and you know that constructor functions are used to initialize classes. So what constructor is this line calling?

The answer is that this line is calling Fraction’s copy constructor. A copy constructor is a special type of constructor used to create a new object as a copy of an existing object. And much like a default constructor, if you do not provide a copy constructor for your classes, C++ will create a public copy constructor for you. Because the compiler does not know much about your class, by default, the created copy constructor utilizes a method of initialization called memberwise initialization. Memberwise initialization simply means that each member of the copy is initialized directly from the member of the class being copied. In the above example, fCopy.m_numerator would be initialized from fiveThirds.m_numerator, etc…

Just like we can explicitly define a default constructor, we can also explicitly define a copy constructor. The copy constructor looks just like you’d expect it to:

When this program is run, you get:

Copy constructor called
5/3

The copy constructor we defined in the example above uses memberwise initialization, and is functionally equivalent to the one we’d get by default, except we’ve added an output statement to prove the copy constructor is being called.

Unlike with default constructors (where you should always provide your own default constructor), it’s fine to use the default copy constructor if it meets your needs.

One interesting note: You’ve already seen a few examples of overloaded operator<<, where we're able to access the private members of parameter f1 because the function is a friend of the Fraction class. Similarly, member functions of a class can access the private members of parameters of the same class type. Since our Fraction copy constructor takes a parameter of the class type (to make a copy of), we're able to access the members of parameter fraction directly, even though it's not the implicit object. Preventing copies

We can prevent copies of our classes from being made by making the copy constructor private:

Now when we try to compile our program, we’ll get a compile error since fCopy needs to use the copy constructor, but can not see it since the copy constructor has been declared as private.

The copy constructor may be elided

Now consider the following example:

Consider how this program works. First, we direct initialize an anonymous Fraction object, using the Fraction(int, int) constructor. Then we use that anonymous Fraction object as an initializer for Fraction fiveThirds. Since the anonymous object is a Fraction, as is fiveThirds, this should call the copy constructor, right?

Run this and compile it for yourself. You’d probably expect to get this result (and you may):

copy constructor called
5/3

But in actuality, you’re more likely (but not guaranteed) to get this result:

5/3

Why didn’t our copy constructor get called?

Note that initializing an anonymous object and then using that object to direct initialize our defined object takes two steps (one to create the anonymous object, one to call the copy constructor). However, the end result is essentially identical to just doing a direct initialization, which only takes one step.

For this reason, in such cases, the compiler is allowed to opt out of calling the copy constructor and just do a direct initialization instead. This process is called eliding.

So although you wrote:

The compiler may change this to:

which only requires one constructor call (to Fraction(int, int)). Note that in cases where eliding is used, any statements in the body of the copy constructor are not executed, even if they would have produced side effects (like printing to the screen)!

Finally, note that if you make the copy constructor private, any initialization that would use the copy constructor will cause a compile error, even if the copy constructor is elided!

9.12 -- Copy initialization
Index
9.10 -- Overloading typecasts

31 comments to 9.11 — The copy constructor

  • Sanjeev

    Hi,
    For the class A can I have copy constructor like A(const A*) instead of A(const A&). What will be the difference. Can anybody please explain?

    • HYA

      Hi Sanjeev
      In that case you have to write any object intialization like this A a, aa1 = &a; … which is a bit confusing …but you can implement both copy contructors as

    • HYA

  • subbaiahtp

    Please let me know the B’s copy constructor implementation?

    • subrat

  • saurabh

    Here is answer to why dont we use pass by pointer in copy constructor so see following scenario

    See above example , if we take pointer type object in copy constructor
    two flaws are here
    1) We can assign NULL value to simple object of that class
    2) see this line ABC d2 = d1;
    here d2 is ABC type while d1 is ABC* type means as per rule we are violating basic rules by allowing to assign simple type with pointer type.
    If there is any pointer type member variable in side ABC class means DEEP Copy scenario like int * m_iVal; then it will crash out during calling copy constructor by passing NULL object, so stop such mistake at design time we do use const reference object of same class type in copy constructor as a parameter.Hoping this will clear you.

  • rohitnitp

    how it come while defining copy constructor you used object of the same class to access private data member which belongs to that object only..since now this object as it is already created how can we access private data directly to anywhere else even if it copy constructor of the same class..it will violate the idea of data encapsulation and data hiding..i tried to access m_nCents via GetLength() at that point but compiler has given me error..why is it so,please explain..

  • Marko

    Hi Alex,

    I want to thank you for this brilliant tutorial.
    It is helping me a lot, I have c/java background and this is all what I needed.
    It is going into my head like it already was there, don’t really have a sense that I am learning something new, I am reading this easier than a note I would have to write to myself.  
    Unfortunatly I dont have a money, cause I am still studying :(, but if I get a job, I will deffinitly remembre to this, and pay my debt to society (you :)).

    ty::ItIsProbablyObviouslyThatIAmNotNativeEnglishWriter(LOVE YOU)

  • Siva

    why copy constructor returns this pointer while overloaded assignment operator returns. Even though both updates in this pointer. Anyone please explain this.

    Thanks in Advance,
    Siva

    • Alex

      The copy constructor doesn’t return anything (because it doesn’t need to).

      The assignment operator returns *this so you can chain multiple assignments together (a = b = c = d)

  • Ankit

    Hi Alex,

    I am working on project and I got a half written code from my previous mate. He has declared copy constructors as private. Is there any specific use or advantage of using copy constructor as private? I am not able to get why someone will declare a copy contructor as private??

    Regards
    Ankit Dimri

    • Alex

      Making the copy constructor private ensures that a copy can’t be made of the object. This can be desirable in some cases, such as when you want to ensure only one of an object exists (e.g. a log file).

  • Mohammed

    Dear Alex would you check this code and plz tell me the answer of the following question;
    When i use the overrided assignment operator iam returning by value, so i think what should happen when the instruction pointer reaches return engdist(feet,inches)
    1-  2 argument constructor is called to create a temp object;
    2-  Copy constructor is called to create a copy which will be returned;

    However what actually happens is that the 2 argument is called but the copy constructor is not called, does this mean that the temp object is the one which returned to e3

    • Mohammed

      Guys if some one knows the answer please say it cuz it is driving me crazy
      and i have an interview after 1 week and one of the topics is OOP in C++

    • Alex

      I presume you’re talking about what happens when we execute “e4=e3”. In this case, your overloaded assignment operator is called. Because the assignment operator is returning by value, I would expect the copy constructor to be called at that point. However, compilers are allowed to optimize away copy constructors (even if they have other side effects!), and I think that what may be happening here.

  • jonas

    hi Alex, may you explain the dangerousness of self assignment in more details.(….In these cases, the assignment operator doesn’t need to do anything (and if the class uses dynamic memory, it can be dangerous if it does))

  • jonas

    may result in memory leak?

  • ritz301

    Excellent tutorial! (y)

  • ganesh

    How to implement in parameterized and copy constructor????

  • Darren

    You can pay good money to have your dangling pointer dereferenced.

  • Anddo

    Typo

    should be

  • Matt

    In the third full code example, which implements a copy constructor in the class definition, you left in a comment(in main) which I think should be updated to reflect the change. You wrote:
    "Fraction fCopy(fiveThirds); // Direct initialize -- with what constructor?"

  • Soterite

    Hi Alex!

    Maybe it’s just me and I simply cannot grasp something, but shouldn’t the copy constructor in your second example look rather like this?:

    It seems to me, that the argument of a copy constructor is an original object. The copy constructor is special, because it is called automatically by a compiler in some situations, but in other aspects it behaves like an ordinary one. In fact this code works:

    Please rectify, if I’m wrong with something.

    Last but not least thank you VERY much for your work. Outstanding job!

    • Alex

      If you don’t provide a copy constructor, the compiler will provide one for you. That’s why the second example compiles without one.

      Your copy constructor above is essentially the same as mine, except mine initializes the class data and yours assigns it. Your won’t work with const members.

      Did I miss the point of your question?

      • Soterite

        I’m sorry, I’m not a native speaker and maybe I didn’t express myself clearly enough. It’s all about naming of the argument to the copy constructor. You named it a "copy" and I simply don’t understand why:

        I believe it should be called "original".

        As I understand it, the copy constructor is given an original object (via const reference) and "produces" (even if it doesn’t return) a copy of that object. So it operates just like any other constructor, in particular it can be called explicitly with const reference.

        I find it confusing, that you name this argument a "copy", but maybe it is I, who misses some quirks of this constructor 🙂

        Kind regards

        • Alex

          Ah, I named it copy because it’s the object being passed in to make a copy of. But I can see where your confusion is, since it isn’t the actual copy, the implicit class is. I’ll change the name.

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