The value returned by the copy constructor

Question

In most of the c++ code that i have studied, there is no return statement in the copy constructor function, yet this one must return an object. I want to understand why?

What about default constructors? Do they have a return value? — Luchian Grigore, Aug 08 '13 at 10:03
@WildThing Copy assignment is not the same as copy construction. [E.g. see here](http://www.learncpp.com/cpp-tutorial/911-the-copy-constructor-and-overloading-the-assignment-operator/). — BoBTFish, Aug 08 '13 at 10:06
I've coded C++ for 25 years and taken things for granted with regard to this. Never thought of why constructors don't return themselves so to say. Think it is a good question. Everybody with C++ knowledge knows that a constructor does not return a value, but what would be wrong with some kind of return syntax-wise. I think that is interesting and therefore I think it is a good question. — , Aug 08 '13 at 10:15
*"In most of the c++ code that I have studied, there is no return statement in the copy constructor function"* - Not *"most"*, all (well, except if it was invalid code of course). — Christian Rau, Aug 08 '13 at 11:37

score 3 · Accepted Answer · answered Aug 08 '13 at 10:05

3

The constructors doesn't return a value, they are simply called as part of the object construction, and the actual "returning an object" is the job of the compiler and its generated code.

For example, lets say you have a class Foo, then when declaring a variable of that class

Foo myFoo;

the compiler creates the object for you, and calls the appropriate constructor

answered Aug 08 '13 at 10:05

Some programmer dude

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what about the = operator? – WildThing Aug 08 '13 at 10:07
1

@WildThing that is not a constructor. – juanchopanza Aug 08 '13 at 10:08
@WildThing It's the same thing. When used in initialization of a new variable the compiler still creates the object for you and then calls the copy constructor. When used for assignment the object already exists since before. – Some programmer dude Aug 08 '13 at 10:09
@JoachimPileborg should i return *this when defining the = operator function? – WildThing Aug 08 '13 at 10:11
@JoachimPileborg what if the =operator function is declared void? – WildThing Aug 08 '13 at 10:15
2

@WildThing While you *can* declare `operator=` returning `void`, it's not really useful if you want to allow chaining of assignments. And `operator=` is *copy assignment* and not object creation. – Some programmer dude Aug 08 '13 at 10:17

score 2 · Answer 2 · answered Aug 08 '13 at 10:11

The default constructor doesn't return a value. It is simply called at the object construction.

From the standard :

12.1 Constructors [class.ctor]

A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X, a constructor having no parameters is implicitly declared as defaulted (8.4). An implicitly-declared default constructor is an inline public member of its class.

[....]

No return type (not even void) shall be specified for a constructor. A return statement in the body of a constructor shall not specify a return value. The address of a constructor shall not be taken.

Here is an example of use :

class Foo
{
public:
    Foo() {} // User defined default constructor
};
Foo myFoo;

The value returned by the copy constructor

2 Answers2