another bloody basic question

[jesse]

In java, one constructor can call another constructor through this(...)

for instance

class foo
{
public:
foo(int k) { this(k,false)};
foo(int k, boolean m){...};
}

It seems i can't find similar syntax in c++.
what is the notion for one constructor to call another?

sorry for this bloody basic question.

jesse

[Janusz Szpilewski]

Re: another bloody basic question

jesse wrote:
[color=blue]
> what is the notion for one constructor to call another?
>[/color]

The short answer is you cannot do it directly. Take a look at:

http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.3

I think bookmarking the entire FAQ would be a good idea.

Regards,
Janusz

[Andrey Tarasevich]

Re: another bloody basic question

jesse wrote:
[color=blue]
> In java, one constructor can call another constructor through this(...)
>
> for instance
>
> class foo
> {
> public:
> foo(int k) { this(k,false)};
> foo(int k, boolean m){...};
> }
>
> It seems i can't find similar syntax in c++.
> what is the notion for one constructor to call another?
>
> sorry for this bloody basic question.
> ...[/color]

There's no such syntax in C++. In some cases the common code can be
transferred from constructors to a separate function (regular function,
not a constructor), which then can be called from all constructors. In
other cases there is no other way but to repeat the same code in all
constructors.

--
Best regards,
Andrey Tarasevich

[tom_usenet]

Re: another bloody basic question

On Wed, 05 Nov 2003 18:12:54 -0800, jesse <jessezx@yahoo. com> wrote:
[color=blue]
>In java, one constructor can call another constructor through this(...)
>
>for instance
>
>class foo
>{
>public:
> foo(int k) { this(k,false)};
> foo(int k, boolean m){...};
>}
>
>It seems i can't find similar syntax in c++.
>what is the notion for one constructor to call another?
>
>sorry for this bloody basic question.[/color]

Basically you can't. The C++ construction model is very different to
Javas. By the time you enter the {} part of your constructor, all base
classes and all member variables have already been constructed. This
is because of the existence of user-defined value types, a concept
that Java doesn't have.

Have you come across initializer lists?

foo(int k)
:m_k(k)
{}

foo(int k, boolean m)
:m_k(k), m_m(m)
{}

With initializer lists, the constructor body is often empty anyway, so
what is there to share?

That said, I think a syntax like this has been proposed at some point
in the past, but I don't think any compiler supports it:

foo(int k)
:foo(k, false)
{
//anything extra
}

Tom

[Nils Petter Vaskinn]

Re: another bloody basic question

On Wed, 05 Nov 2003 18:12:54 -0800, jesse wrote:
[color=blue]
> In java, one constructor can call another constructor through this(...)
>
> for instance
>
> class foo
> {
> public:
> foo(int k) { this(k,false)};
> foo(int k, boolean m){...};
> }
>
> It seems i can't find similar syntax in c++.
> what is the notion for one constructor to call another?[/color]

You can make an init() function that is called from the constructors,
though for your example the simplest would be:

class Foo {
public:
Foo(int k, bool m = false);
}

When you call do new Foo(42) the default is used for m since you didn't
supply a value.

--
NPV

"the large print giveth, and the small print taketh away"
Tom Waits - Step right up

[Gary Labowitz]

Re: another bloody basic question

"tom_usenet " <tom_usenet@hot mail.com> wrote in message
news:448kqvsu0j a2vesvc5t9c1g2d oorkgkpen@4ax.c om...[color=blue]
> On Wed, 05 Nov 2003 18:12:54 -0800, jesse <jessezx@yahoo. com> wrote:
>[color=green]
> >In java, one constructor can call another constructor through this(...)
> >
> >for instance
> >
> >class foo
> >{
> >public:
> > foo(int k) { this(k,false)};
> > foo(int k, boolean m){...};
> >}
> >
> >It seems i can't find similar syntax in c++.
> >what is the notion for one constructor to call another?
> >
> >sorry for this bloody basic question.[/color]
>
> Basically you can't. The C++ construction model is very different to
> Javas. By the time you enter the {} part of your constructor, all base
> classes and all member variables have already been constructed. This
> is because of the existence of user-defined value types, a concept
> that Java doesn't have.
>
> Have you come across initializer lists?
>
> foo(int k)
> :m_k(k)
> {}
>
> foo(int k, boolean m)
> :m_k(k), m_m(m)
> {}
>
> With initializer lists, the constructor body is often empty anyway, so
> what is there to share?[/color]

The equivalent in C++ is to use the initializer list to indicate the
constructor of the base class to be used. It is selected by the normal
overloading operation.
Here is a simple example showing construction of a derived class with no,
one, or two arguments.

#include <iostream>
using namespace std;

class B
{public:
int b;
B( ):b(2){}
B(int x):b(x){}
};
class A : public B
{public:
int a;
A( ):a(1){}
A(int x):a(x){}
A(int x, int y):a(x),B(y){}
};
int main ( )
{
A myA;
cout <<myA.a<<myA.b< <endl;
A myA1(3);
cout<<myA1.a<<m yA1.b<<endl;
A myA2(4,5);
cout << myA2.a<<myA2.b< <endl;
return 0;
}

--
Gary

[Ron Natalie]

Re: another bloody basic question


"jesse" <jessezx@yahoo. com> wrote in message news:3FA9AE26.6 070308@yahoo.co m...
[color=blue]
> It seems i can't find similar syntax in c++.
> what is the notion for one constructor to call another?[/color]

There is no similar syntax in C++. You can't call constructors in C++.
The cleanest work around is to put the common code in a seperate
member function and call it from both constructors.

[Ron Natalie]

Re: another bloody basic question


"Andrey Tarasevich" <andreytarasevi ch@hotmail.com> wrote in message news:vqjc6lbo6k 3fab@news.super news.com...
[color=blue]
>
> There's no such syntax in C++. In some cases the common code can be
> transferred from constructors to a separate function (regular function,
> not a constructor), which then can be called from all constructors. In
> other cases there is no other way but to repeat the same code in all
> constructors.[/color]

There's nothing that ever prevents the constructor body from being
moved. What's a pain in the butt is that there is no way to share
the initializer lists between two constructors.