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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 15:42:07 -0500
Organization: Earth
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Date: 2005-03-28T15:42:07-05:00
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"Ioannis Vranos" <ivr@remove.this.grad.com> wrote in message
news:1112039976.169686@athnrd02...

> >  I was just pointing out that what you wrote was
> > not portable, and would not work for any arbitrary class.  Besides, why
> > would you incur the extra time and complexity of manually moving the
data
> > back and forth, byte-by-byte instead of just insuring the correct
alignment
> > and letting the compiler take care of it?
>
>
> This question can be rephrased to: "Why is memcpy() needed?".
>
>
>
> -- 
No, it is not about whether memcpy() is a necessary function.  You purposely
over simplified what I did.  I don't want to appear rude, but your whole
statement about moving a double byte-by-byte to avoid alignment issues is
just nonsense.  You didn't take portability into account when you wrote the
snippets of code using placement new, and now you wish to backpedal.  You
really are not going to convince me that you intended write all your classes
to copy all the bytes of variables with alignment needs greater than one,
instead of just fixing the alignment.  Do you really expect my to believe
you would write:

struct foo {
    foo(double d) : m_dbl(d) {}
    double get() const
    {
        double tmp;
        memcpy(&tmp, &m_dbl, sizeof(tmp));
        return tmp;
    }

private:
    double m_dbl;
}

char buf[sizeof(foo)];

foo* pf = new(buf) foo();
double d = pf->get();


instead of:

struct foo {
    foo(double d) : m_dbl(d) {}
    double get() const {return m_dbl;}

private:
    double m_dbl;
}

union data_type {
    double align;
    char buf[sizeof(foo)];
} data;

foo* pf = new(data.buf) foo();
double d = pf->get();