Calling one constructor from another in VC++

[Peter E. Granger]

I'm fairly new to C++ and VC++, but for the most part it seems to do most of
the same things that can be done in Java, with just some syntactic and
structural adjustments. However, one thing I haven't been able to figure out
is how to call one constructor from another within a class. It's easy enough
to call the base class's constructor from the derived class, but that's not
what I'm trying to do.

For example, in Java (or J#) it's easy to do this:

public class Circle
{
private int PointX;
private int PointY;
private int Radius;

public Circle()
{
this.PointX = 0;
this.PointY = 0;
this.Radius = 0;
}

public Circle(int x, int y)
{
this();
this.PointX = x;
this.PointY = y;
}

public Circle(int x, int y, int r)
{
this(x, y);
this.Radius = r;
}
}

However, trying to do the same thing in VC++ doesn't work:

// Sphere.h (partial file)
class Sphere
{
public:
Sphere(void);
Sphere(int x, int y);
Sphere(int x, int y, int r);
~Sphere(void);

private:
int PointX;
int PointY;
int Radius;
};

// Circle.cpp (partial file)
#include ".\circle.h "

Circle::Circle( void)
{
this->PointX = 0;
this->PointY = 0;
this->Radius = 0;
}

Circle::Circle( int x, int y)
{
Circle();
this->PointX = x;
this->PointY = y;
}

Circle::Circle( int x, int y, int r)
{
Circle(x, y);
this->Radius = r;
}

In the C++ version, I also tried using this() instead of Circle(), which
caused a compiler error. I also tried doing it without the this-> specifier,
which made no difference. The end result if you call the second constructor
is that the X and Y values have been set, and the Radius is uninitialized.
If you use the third constructor, you get exactly the opposite results. (And
I don't mean they have values of zero; looking at them with the debugger, I
see values of -842150451 for the uninitialized members.)

So is there any way in C++ to let one constructor build on another like
this?

Thanks for any help.

[Carl Daniel [VC++ MVP]]

Peter E. Granger wrote:

I'm fairly new to C++ and VC++, but for the most part it seems to do
most of the same things that can be done in Java, with just some
syntactic and structural adjustments.
Actually, there are many things you can do in C++ that have no counterpart
in Java at all. You'll discover them in time.
However, one thing I haven't
been able to figure out is how to call one constructor from another
within a class. It's easy enough to call the base class's constructor
from the derived class, but that's not what I'm trying to do.

There's no way to do this in C++. Unlike Java, in C++ at the start of the
first statement of a constructor's body all of the members have been
initialized (their constructors have been run if they have them). In Java
everything gets default initialized before the constructor body is run, and
then the constructor body (perhaps redundantly) initializes everything.

To get the same "constructo r re-use" effect in C++, you have to put the
initialization code into a (private/protected) member function and call it
from the constructor(s).

-cd

[Doug Harrison [MVP]]

Peter E. Granger wrote:

I'm fairly new to C++ and VC++, but for the most part it seems to do most of
the same things that can be done in Java, with just some syntactic and
structural adjustments. However, one thing I haven't been able to figure out
is how to call one constructor from another within a class. It's easy enough
to call the base class's constructor from the derived class, but that's not
what I'm trying to do.
<snip>
Circle::Circle (int x, int y)
{
Circle();
The statement above creates a temporary Circle object, which is immediately
destroyed at the end of the statement.
this->PointX = x;
this->PointY = y;
}

Circle::Circle (int x, int y, int r)
{
Circle(x, y);
The statement above creates a temporary Circle object, which is immediately
destroyed at the end of the statement.
this->Radius = r;
}

In the C++ version, I also tried using this() instead of Circle(), which
caused a compiler error. I also tried doing it without the this-> specifier,
which made no difference. The end result if you call the second constructor
is that the X and Y values have been set, and the Radius is uninitialized.
If you use the third constructor, you get exactly the opposite results. (And
I don't mean they have values of zero; looking at them with the debugger, I
see values of -842150451 for the uninitialized members.)

So is there any way in C++ to let one constructor build on another like
this?

Sorry, no. Sometimes you can factor the common initialization out into a
separate init() function, but of course members you don't mention in the
member-initialization list will have been default-constructed by the time
you can call init(). If you have const members, reference members, or
members that don't have default ctors, you must initialize them in each
ctor's member initialization list.

--
Doug Harrison
Microsoft MVP - Visual C++

[Peter E. Granger]

"Doug Harrison [MVP]" <ds*@mvps.org > wrote in message
news:ct******** *************** *********@4ax.c om...

Peter E. Granger wrote:

I'm fairly new to C++ and VC++, but for the most part it seems to do most ofthe same things that can be done in Java, with just some syntactic and
structural adjustments. However, one thing I haven't been able to figure outis how to call one constructor from another within a class. It's easy enoughto call the base class's constructor from the derived class, but that's notwhat I'm trying to do.
<snip>

Circle::Circle (int x, int y)
{
Circle();

The statement above creates a temporary Circle object, which is

immediately destroyed at the end of the statement.

Right -- that's what I expected would happen. In fact, I thought I'd said
that in my original message, but I see I omitted it.

So is there any way in C++ to let one constructor build on another like
this?

Sorry, no. Sometimes you can factor the common initialization out into a
separate init() function, but of course members you don't mention in the
member-initialization list will have been default-constructed by the time
you can call init(). If you have const members, reference members, or
members that don't have default ctors, you must initialize them in each
ctor's member initialization list.

That's unfortunate. It's really a handy feature. But you can't get
everything in one, language, I guess.

Thanks for the reply.

- Peter

[Peter E. Granger]

"Carl Daniel [VC++ MVP]" <cp************ *************** **@mvps.org.nos pam>
wrote in message news:uq******** ******@TK2MSFTN GP10.phx.gbl...

Peter E. Granger wrote:

I'm fairly new to C++ and VC++, but for the most part it seems to do
most of the same things that can be done in Java, with just some
syntactic and structural adjustments.
Actually, there are many things you can do in C++ that have no counterpart
in Java at all. You'll discover them in time.

And vice-versa, presumably (like this case).

However, one thing I haven't
been able to figure out is how to call one constructor from another
within a class. It's easy enough to call the base class's constructor
from the derived class, but that's not what I'm trying to do.

There's no way to do this in C++. Unlike Java, in C++ at the start of the
first statement of a constructor's body all of the members have been
initialized (their constructors have been run if they have them). In Java
everything gets default initialized before the constructor body is run,

and then the constructor body (perhaps redundantly) initializes everything.

To get the same "constructo r re-use" effect in C++, you have to put the
initialization code into a (private/protected) member function and call it
from the constructor(s).

Thanks, I figured I might end up having to do that. Bit of a nuisance, but
manageable.

Thanks for the reply.

- Peter

[Hendrik Schober]

Peter E. Granger <PE******@hotma il.com> wrote:

[...]

Actually, there are many things you can do in C++ that have no counterpart
in Java at all. You'll discover them in time.
And vice-versa, presumably (like this case).

I wouldn't expect that much, though.
[...]

To get the same "constructo r re-use" effect in C++, you have to put the
initialization code into a (private/protected) member function and call it
from the constructor(s).
Thanks, I figured I might end up having to do that. Bit of a nuisance, but
manageable.

Note that C++ has default parameters which,
AFAIK, where introduced especially to solve
this problem:

class Circle {
public:
Circle(int x=0, int y=0, int r=0)
: PointX(x), PointY(y), Radius(r)
{
}
// ...
};
Thanks for the reply.
HTH,
- Peter

Schobi

--
Sp******@gmx.de is never read
I'm Schobi at suespammers dot org

"Sometimes compilers are so much more reasonable than people."
Scott Meyers

[Mihajlo Cvetanovic]

Peter E. Granger wrote:

"Carl Daniel [VC++ MVP]" <cp************ *************** **@mvps.org.nos pam>
wrote in message news:uq******** ******@TK2MSFTN GP10.phx.gbl...

Actually, there are many things you can do in C++ that have no counterpart
in Java at all. You'll discover them in time.

And vice-versa, presumably (like this case).

There is something called "placement new" which in this context
behaves exactly like calling one c-tor from another, but programmers
are discouraged from using it.

[Ken Alverson]

"Hendrik Schober" <Sp******@gmx.d e> wrote in message
news:%2******** ********@TK2MSF TNGP11.phx.gbl. ..

Note that C++ has default parameters which,
AFAIK, where introduced especially to solve
this problem:

Default parameters were introduced because, at the time, there was no such
thing as method overloading. They just happen to also solve some of the
simpler cases of redundant constructors.

Ken

[Ken Alverson]

"Mihajlo Cvetanovic" <ma*@RnEeMtOsVe Et.co.yu> wrote in message
news:40******** ******@RnEeMtOs VeEt.co.yu...

There is something called "placement new" which in this context
behaves exactly like calling one c-tor from another, but programmers
are discouraged from using it.

Placement new is only designed to be used on uninitialized memory. By the
time you get to the first line of a constructor, you have a semi-valid object
in that memory that would be wiped out.

Ken

[Hendrik Schober]

Ken Alverson <US********@Alv erson.net> wrote:

"Hendrik Schober" <Sp******@gmx.d e> wrote in message
news:%2******** ********@TK2MSF TNGP11.phx.gbl. ..

Note that C++ has default parameters which,
AFAIK, where introduced especially to solve
this problem:
Default parameters were introduced because, at the time, there was no such
thing as method overloading. They just happen to also solve some of the
simpler cases of redundant constructors.

"The original motivation for default parameters
was exactly because a ctor cannot call another
ctor for the same type." (Francis Glassborow)
http://www.google.de/groups?selm=pM5...on.demon.co.uk
I don't have D&E handy here, but I believe that
function overloading was among the very first
features of C++. Wasn't there even an 'overload'
keyword (considered?) ones but later dismissed
because it wasn't needed?

OTOH, I know the first one only from Francis'
posting and the second only from my memory, so
i might be wrong.
Ken

Schobi

--
Sp******@gmx.de is never read
I'm Schobi at suespammers dot org

"Sometimes compilers are so much more reasonable than people."
Scott Meyers