CODEWITHSUNDEEP
c++copy-constructorvariadic-functions
Destructor
Destructor

Reputation: 14438

What is the meaning of ellipses in a copy constructor?

Consider the following program:

#include <iostream>
struct Test
{
    int a;
    Test() : a(3)
    { }
    Test(const Test& t...)
    {
        std::cout<<"Copy constructor called\n";
        a=t.a;
    }
    int get_a()
    {
        return a;
    }
    ~Test()
    {
        std::cout<<"Destructor is called\n";
    }
};
int main()
{
    Test t;
    Test* t1=new Test(t);
    std::cout<<t.get_a()<<'\n';
    std::cout<<t1->get_a()<<'\n';
    delete t1;
}

Closely observe the three dots in parameter of copy constructor I was really surprised when I tried this program. What is the use of it? What does it mean?

What the language specification says about this?

I know that three dots are used to represent variable length arguments in variadic functions like printf() and scanf() etc and also variadic macros introduced by C99. In C++, if I am not wrong, they are used in variadic templates.
Is this code well formed? Is this variadic copy constructor that can take any number of arguments?

It compiles & runs fine on g++ 4.8.1 & MSVS 2010.

Upvotes: 15

Views: 1099

Answers (2)

Shafik Yaghmour
Shafik Yaghmour

Reputation: 158599

The draft standard in section 8.3.5 [dcl.fct] says , ... is synonymous with ... unless ... is part of abstract-declarator (emphasis mine):

[...]If the parameter-declaration-clause terminates with an ellipsis or a function parameter pack (14.5.3), the number of arguments shall be equal to or greater than the number of parameters that do not have a default argument and are not function parameter packs. Where syntactically correct and where “...” is not part of an abstract-declarator, “, ...” is synonymous with “...”.[...]

So it is a variadic function and as far as I can tell without additional arguments this is also a valid copy constructor, from section 12.8 [class.copy]:

A non-template constructor for class X is a copy constructor if its first parameter is of type X&, const X&, volatile X& or const volatile X&, and either there are no other parameters or else all other parameters have default arguments (8.3.6).

and this note says that ellipses are not parameters:

void g(int = 0, ...); // OK, ellipsis is not a parameter so it can follow
                      // a parameter with a default argument

which is backed up by the normative text above which says:

If the parameter-declaration-clause terminates with an ellipsis[...]

Note, since it was asked an abstract-declarator is a declarator without an identifier.

Upvotes: 5

edmz
edmz

Reputation: 8492

What is the use of it? What does it mean?

Yes, it introduces a variadic function.

In C++ If I am not wrong they are used in variadic templates.

The syntax and semantics are different. It's a "C-style" variadic function, not a variadic template function. A copy constructor, moreover, cannot be a template function.

A non-template constructor for class X is a copy constructor if its first parameter is of type X&, const X&, volatile X& or const volatile X&, and either there are no other parameters or else all other parameters have default arguments (8.3.6).

Per §12.8.2 in the final draft (emphasis mine)

Is this code well formed? Is this variadic copy constructor that can take any number of arguments?

If the ellipsis contains parameters, it is no more a copy constructor but a simple constructor. If there aren't, then it is a valid copy constructor.

 X(const X&, int = 1, double = 5); // copy-ctor
 X(const X&, int = 1, double); // constructor

Upvotes: 5

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