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From: "REH" <bogus@nowhere.net>
Newsgroups: comp.lang.ada,comp.lang.c++,comp.realtime,comp.software-eng
Subject: Re: [OT] Re: Teaching new tricks to an old dog (C++ -->Ada)
Date: Mon, 28 Mar 2005 16:52:50 -0500
Organization: Earth
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Date: 2005-03-28T16:52:50-05:00
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"Ioannis Vranos" <ivr@remove.this.grad.com> wrote in message
news:1112045099.679682@athnrd02...

> However the following code *is* portable:
>
>
> #include <iostream>
> #include <cstring>
>
> class SomeClass
> {
>      public:
>
>      double d;
>      int i;
>      float f;
>      long l;
> };
>
>
> int main()
> {
>      using namespace std;
>
>      unsigned char array[sizeof(SomeClass)];
>
>      SomeClass obj= {1, 2, 3, 4};
>
>      memcpy(array, &obj, sizeof(obj));
>
>      SomeClass *p= reinterpret_cast<SomeClass *>(array);
>
>      cout<<p->d<<" "<<p->i<<" "<<p->f<<" "<<p->l<<"\n";
> }
>
>
> C:\c>temp
> 1 2 3 4
>
> C:\c>
>
>
> So since this is *guaranteed* to be portable, why this isn't?
>
>
> #include <iostream>
> #include <cstring>
>
> class SomeClass
> {
>      public:
>
>      double d;
>      int i;
>      float f;
>      long l;
> };
>
>
> int main()
> {
>      using namespace std;
>
>      unsigned char array[sizeof(SomeClass)];
>
>      SomeClass obj= {1, 2, 3, 4};
>
>      SomeClass *p= new(array)SomeClass(obj);
>
>      cout<<p->d<<" "<<p->i<<" "<<p->f<<" "<<p->l<<"\n";
> }
>
>
> C:\c>temp
> 1 2 3 4
>
> C:\c>
>
>
> -- 
> Ioannis Vranos
>
No, NEITHER is portable!  That both have alignment issues!  Both exihibit
undefined behavior.  Show me where in the standard is says you may cast a
pointer to a char array to a pointer of any other type (other than another
signed/unsigned char*) and access it.  You do this on a system that cannot
handle misaligned accesses, and your program will most likely crash!