default value of a pointer type

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George - 26 Oct 2004 09:45 GMT

Hi!

What is the default value of a pointer type?

Is it "null"? since a pointer type is in particular a reference type

Thanks. George

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Mattias Sj?gren - 26 Oct 2004 12:00 GMT

>What is the default value of a pointer type?
>
>Is it "null"?

Zero, or null of you see it from a C# point of view.

Mattias

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George - 26 Oct 2004 12:09 GMT

Hi Mattias,

Thanks.

> Zero, or null of you see it from a C# point of view.

from the CIL point of view, is it zero?

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Brian Tyler - 26 Oct 2004 15:01 GMT

Zero. Only object references are assigned null.

For example:

static unsafe void Main(string[] args)
{
Int32* x = null;
Object y = null;
}

results in

.method private hidebysig static void Main(string[] args) cil managed
{
.entrypoint
// Code size 6 (0x6)
.maxstack 1
.locals ([0] int32* x,
[1] object y)
IL_0000: ldc.i4.0
IL_0001: conv.u
IL_0002: stloc.0
IL_0003: ldnull
IL_0004: stloc.1
IL_0005: ret
} // end of method Class1::Main

> Hi Mattias,
>
[quoted text clipped - 3 lines]
>
> from the CIL point of view, is it zero?

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George - 26 Oct 2004 15:11 GMT

Thank you Brian! It's clear now.

George

> Zero. Only object references are assigned null.
>
[quoted text clipped - 30 lines]
> >
> > from the CIL point of view, is it zero?

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Brian Tyler - 26 Oct 2004 15:43 GMT

Actually, now that I have written this, I am confused. My understading is
that ldnull has the very nice property of not actually specifying the size
of the "null" to load into the location. Very handy since a reference size
is dependent on the platform (32 vs. 64 bit).

However, a pointer is itself a reference and therefore it would seem that
for the IL to be portable to 64-bit systems, they should be initialized with
ldnull also. This IL was generated with a 1.1 C# compiler, so I don't know
what the 2.0 system does - need to try that next. Granted pointers are
"unsafe", but that isn't the same as "unportable".

Ideas?

> Zero. Only object references are assigned null.
>
[quoted text clipped - 30 lines]
> >
> > from the CIL point of view, is it zero?

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Brian Tyler - 26 Oct 2004 16:03 GMT

Okay, my bad. The conv.u is not conv.u4 which is what my brain was seeing,
thus the Int32(0) is converted to a native unsigned int(0).

> Actually, now that I have written this, I am confused. My understading is
> that ldnull has the very nice property of not actually specifying the size
[quoted text clipped - 43 lines]
> > >
> > > from the CIL point of view, is it zero?

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default value of a pointer type

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