Re: sizeof on arrays and pointers
From: Doug Harrison [MVP] (dsh_at_mvps.org)
Date: 01/17/05
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Date: Mon, 17 Jan 2005 10:29:10 -0600
BRG wrote:
>The following program produces different results on the five different
>compilers I am using:
>
>---------------------------------
>#include <stdio.h>
>typedef char ctype[100];
>int main()
>{ char x[100], *y;
> ctype c;
> printf("\n%3ld %3ld", sizeof(x), sizeof(&x));
> printf("\n%3ld %3ld", sizeof(y), sizeof(&y));
> printf("\n%3ld %3ld", sizeof(c), sizeof(&c));
> return 0;
>}
>---------------------------------
>
>These results are as follows:
>
>Compiler 1 Compiler 2 Compiler 3 Compiler 4 Compiler 5
>100 100 100 100 100 4 100 2 100 100
> 4 4 4 4 4 4 2 2 4 4
>100 100 100 100 100 4 100 2 100 100
>
>VC++ produces the first of these and is claimed to be wrong by some who
>think that the right answer is number three.
>
>Is this a bug in VC++?
Yes. The type of the expressions &x and &c above is char (*)[100], or
pointer to array of char of size 100. This bug also affects the Whidbey
beta, so you might want to go here and report it:
http://lab.msdn.microsoft.com/vs2005/default.aspx
It might also be interesting to print typeid(&x).name(), e.g.
// a.cpp
#include <stdio.h>
#include <typeinfo>
int main()
{
char x[100];
printf("sizeof(%s) = %d\nsizeof(%s) = %d\n",
typeid(x).name(),
(int) sizeof(x),
typeid(&x).name(),
(int) sizeof(&x));
return 0;
}
Here is a copy of my Whidbey beta compile session and output for this
program:
C>cl -W4 a.cpp
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 14.00.41202 for 80x86
Copyright (C) Microsoft Corporation. All rights reserved.
a.cpp
Microsoft (R) Incremental Linker Version 8.00.41202
Copyright (C) Microsoft Corporation. All rights reserved.
/out:a.exe
a.obj
C>a
sizeof(char [100]) = 100
sizeof(char (*)[100]) = 100
-- Doug Harrison Microsoft MVP - Visual C++
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