Copy constructors; Best practices (and private)

Quote:

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Originally Posted by C++ Primer, Lippman and others

It is usually best to define either implicitly or explicitly the default and copy constructors. The default constructor is synthesized only if there are no other constructors. If the copy constructor is defined, then the default constructor must be defined as well.

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("must" was made bold by me)

I have trouble understanding the last phrase. Especially because while studying through this chapter, I've built a class that doesn't implement a default constructor. Maybe that "must" should be read "should". However, I cannot understand why I should (must?) define the default constructor.


Also, on a related note, is there any instance in which it may make sense to create a private copy constructor but still define it? I cannot fanthom for the life of me, a situation in which I will want only my member functions and friends to make copies.

It must be defined if you want it to be available.

>> is there any instance in which it may make sense to create a private copy constructor but still define it?
I don't know of any examples offhand, but you explained the potential situation yourself. If you only want member functions (perhaps static) and friends to be able to create copies.

In such a case the defult constructor will not be created for you.
Thus you must either create one, or create another non-copy constructor to allow you to construct the object.

If an object only has a copy constructor it can never be used.

I really don't understand why word "must" is there.
Here's an example:

Code:

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class Test
{
    int x;
public:
    Test(const Test& t)
    {
        x = t.x;
    }
    Test(int c)
    {
        x = c;
    }
   
    void show()
    {
        cout << x << endl;
    }   
};

int main ( )
{
    Test a(10);
    Test b(a);
    b.show();

    return 0;

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In this example copy constructor is defined and other constructor that takes int as argument, but default constructor is not defined and it's OK.

That's only because you didn't try to write Test c; but you could have wrote

Code:

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Test::Test(int c = 0) {
    x = c;
}

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Now Test has a constructor for when it has a parameter and when it doesn't.

Quote:

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Originally Posted by citizen

That's only because you didn't try to write Test c; but you could have wrote

Code:

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Test::Test(int c = 0) {
    x = c;
}

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Now Test has a constructor for when it has a parameter and when it doesn't.

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Of course I didn't try to make object with Test c; because it would be a compiler error since once you define one constructor, all others you need to define manually includeing default. i want to prove with my code that if you define copy constructor you don't need to define DEFAULT constructor as well.

Yes, that was my confusion too. There is no need to define a default constructor as long as some other valid constructor is defined. In other words, what must be defined is some valid constructor. It doesn't necessarily need to be the default constuctor.

Thanks all. King Mir nailed it.

Quote:

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Originally Posted by Mario F.

Yes, that was my confusion too. There is no need to define a default constructor as long as some other valid constructor is defined. In other words, what must be defined is some valid constructor. It doesn't necessarily need to be the default constuctor.

Thanks all. King Mir nailed it.

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yes, he did. but what I think others aren't getting is that if you define a copy constructor you must define another valid constructor.

Code:

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class Test
{
public:
  Test(const Test &rhs) {}
};

int main()
{
    Test t(?????); // what goes here? chicken and egg!!
}

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Code:

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int main() {
    Test foo;
    Test bar1 = foo; // calls copy constructor
    Test bar2(foo); // calls copy constructor
}

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Quote:

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Originally Posted by Desolation

Code:

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int main() {
    Test foo;
    Test bar1 = foo; // calls copy constructor
    Test bar2(foo); // calls copy constructor
}

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if there's a point there, I've missed it. :D

Nevermind, I thought your question in the code was an actual question.

I'm guessing the quote was from a context discussing whether a default constructor is provided by the compiler or not and as I said before, the "must" means "must, if you want one". I didn't see that quote in the 3rd version of the book. I assume you are using the 4th edition?

Well, yes. I should probably have stated this is taken from the 4th edition for those who wished to take a read at the book.

And yes, you were quiet right. But I believe you can now understand why it still left me in the dark. The quote is outside any context. It is presented in the book as just a note on good practices at the end of the section dealing with copy-constructors.

However, truth be told I had a duh feeling after King Mir reply. Maybe a little more thinking and I wouldn't have the need to post the question :)

If it really is without context, then that should probably be noted in an errata if they have one for that edition yet. Otherwise you might want to report it to the publisher so that it can be re-worded. Without proper context it is just wrong.

If you don't define a default constructor, and the compiler can't generate one because you've provided a copy-constructor, then good luck trying to use the class in a std::map, for example.

Basically, you might not need to use the default constructor, but something else might need it! Not doing so makes your class unnecessarily limited in terms of reuse.

thanks iMalc. That has been discussed to exhaustion.