Default initialization with dynamic allocation

[Marcus Kwok]

Given a user defined class with a default constructor (let's call the
class C), is there any semantic difference between

C* c = new C;

and

C* c = new C();

? I am thinking they are equivalent, as demonstrated by the program
listing below. If not, which style is more commonly preferred?

I know with primitive types, e.g. int, there is a difference.


#include <iostream>

class C {
public:
C() { std::cout << "C()\n"; }
};

int main()
{
C* a = new C;
C* b = new C();

delete a;
delete b;

int* c = new int;
int* d = new int();

std::cout << *c << '\n' << *d << '\n';

delete c;
delete d;

return 0;
}


Output I get is:
C()
C()
3277336
0

--
Marcus Kwok

[Victor Bazarov]

Re: Default initialization with dynamic allocation

Marcus Kwok wrote:[color=blue]
> Given a user defined class with a default constructor (let's call the
> class C), is there any semantic difference between
>
> C* c = new C;
>
> and
>
> C* c = new C();[/color]

No.
[color=blue]
> ? I am thinking they are equivalent, as demonstrated by the program
> listing below. If not, which style is more commonly preferred?[/color]

I prefer the latter when I need to ensure zero-initialisation for PODs and
the former for POD when I need to leave it uninitialised.
[color=blue]
> I know with primitive types, e.g. int, there is a difference.[/color]

The difference is not only with primitive types. The difference is with
all POD types.
[color=blue]
> [..][/color]

V
--
Please remove capital As from my address when replying by mail

[Marcus Kwok]

Re: Default initialization with dynamic allocation

Victor Bazarov <v.Abazarov@com acast.net> wrote:[color=blue]
> Marcus Kwok wrote:[color=green]
>> Given a user defined class with a default constructor (let's call the
>> class C), is there any semantic difference between
>>
>> C* c = new C;
>>
>> and
>>
>> C* c = new C();[/color]
>
> No.[/color]

Thanks, that's what I thought.
[color=blue][color=green]
>> I know with primitive types, e.g. int, there is a difference.[/color]
>
> The difference is not only with primitive types. The difference is with
> all POD types.[/color]

Thanks, sorry for my imprecise language.

--
Marcus Kwok

[davidrubin@warpmail.net]

Re: Default initialization with dynamic allocation


Victor Bazarov wrote:[color=blue]
> Marcus Kwok wrote:[color=green]
> > Given a user defined class with a default constructor (let's call the
> > class C), is there any semantic difference between
> >
> > C* c = new C;
> >
> > and
> >
> > C* c = new C();[/color]
>
> No.
>[color=green]
> > ? I am thinking they are equivalent, as demonstrated by the program
> > listing below. If not, which style is more commonly preferred?[/color]
>
> I prefer the latter when I need to ensure zero-initialisation for PODs and
> the former for POD when I need to leave it uninitialised.[/color]

This makes it sound like there *is* a semantic difference: in the first
case, zero initialization of PODs in not guaranteed, while in the
second case, it is. Can you please elaborate?

[David Lindauer]

Re: Default initialization with dynamic allocation



Victor Bazarov wrote:
[color=blue]
> Marcus Kwok wrote:[color=green]
> > Given a user defined class with a default constructor (let's call the
> > class C), is there any semantic difference between
> >
> > C* c = new C;
> >
> > and
> >
> > C* c = new C();[/color]
>
> No.
>[color=green]
> > ? I am thinking they are equivalent, as demonstrated by the program
> > listing below. If not, which style is more commonly preferred?[/color]
>
> I prefer the latter when I need to ensure zero-initialisation for PODs and
> the former for POD when I need to leave it uninitialised.
>[color=green]
> > I know with primitive types, e.g. int, there is a difference.[/color]
>
> The difference is not only with primitive types. The difference is with
> all POD types.[/color]

can you clarify what a POD type is?

[Peter_Julian]

Re: Default initialization with dynamic allocation


"David Lindauer" <camille@bluegr ass.net> wrote in message
news:43EDFAE5.2 419B1A6@bluegra ss.net...
|
|
| Victor Bazarov wrote:
|
| > Marcus Kwok wrote:
| > > Given a user defined class with a default constructor (let's call
the
| > > class C), is there any semantic difference between
| > >
| > > C* c = new C;
| > >
| > > and
| > >
| > > C* c = new C();
| >
| > No.
| >
| > > ? I am thinking they are equivalent, as demonstrated by the
program
| > > listing below. If not, which style is more commonly preferred?
| >
| > I prefer the latter when I need to ensure zero-initialisation for
PODs and
| > the former for POD when I need to leave it uninitialised.
| >
| > > I know with primitive types, e.g. int, there is a difference.
| >
| > The difference is not only with primitive types. The difference is
with
| > all POD types.
|
| can you clarify what a POD type is?
|
|

Plain Old Data (section 26.7):

http://www.parashift.com/c++-faq-lite/intrinsic-types.html

[David Lindauer]

Re: Default initialization with dynamic allocation



Peter_Julian wrote:
[color=blue]
> "David Lindauer" <camille@bluegr ass.net> wrote in message
> news:43EDFAE5.2 419B1A6@bluegra ss.net...
> |
> |
> | Victor Bazarov wrote:
> |
> | > Marcus Kwok wrote:
> | > > Given a user defined class with a default constructor (let's call
> the
> | > > class C), is there any semantic difference between
> | > >
> | > > C* c = new C;
> | > >
> | > > and
> | > >
> | > > C* c = new C();
> | >
> | > No.
> | >
> | > > ? I am thinking they are equivalent, as demonstrated by the
> program
> | > > listing below. If not, which style is more commonly preferred?
> | >
> | > I prefer the latter when I need to ensure zero-initialisation for
> PODs and
> | > the former for POD when I need to leave it uninitialised.
> | >
> | > > I know with primitive types, e.g. int, there is a difference.
> | >
> | > The difference is not only with primitive types. The difference is
> with
> | > all POD types.
> |
> | can you clarify what a POD type is?
> |
> |
>
> Plain Old Data (section 26.7):
> http://www.parashift.com/c++-faq-lit...sic-types.html[/color]

ok thanks. Now I know that classes get default constructors & assignment
operator if you don't create any, but if I make a POD like this:

struct aa
{
int a;
int b;
} ;

I assume it wouldn't get default constructors? But if I do anything to
make it a non-POD it would?

Thanks,

David

[Victor Bazarov]

Re: Default initialization with dynamic allocation

davidrubin@warp mail.net wrote:[color=blue]
> Victor Bazarov wrote:[color=green]
>> Marcus Kwok wrote:[color=darkred]
>>> Given a user defined class with a default constructor (let's call
>>> the class C), is there any semantic difference between
>>>
>>> C* c = new C;
>>>
>>> and
>>>
>>> C* c = new C();[/color]
>>
>> No.
>>[color=darkred]
>>> ? I am thinking they are equivalent, as demonstrated by the program
>>> listing below. If not, which style is more commonly preferred?[/color]
>>
>> I prefer the latter when I need to ensure zero-initialisation for
>> PODs and the former for POD when I need to leave it uninitialised.[/color]
>
> This makes it sound like there *is* a semantic difference: in the
> first case, zero initialization of PODs in not guaranteed, while in
> the second case, it is. Can you please elaborate?[/color]

Paragraph 5.3.4/15 of the Standard elaborates on that. What else do
you expect me to add?

V
--
Please remove capital As from my address when replying by mail

[davidrubin@warpmail.net]

Re: Default initialization with dynamic allocation

Do you feel like adding more? Otherwise, that is fine for me. Thanks.