experimenting with unions - byteswap approach..

Re: experimenting with unions - byteswap approach..

<ma740988@pegas us.cc.ucf.edu> wrote in message
news:1125533764 .972952.243820@ g49g2000cwa.goo glegroups.com.. .[color=blue]
>
> Consider the source snippet:
>
> union to_uchar
> {
> int i;
> unsigned int ui;
> long l;
> unsigned long ul;
> short s;
> unsigned short us;
> double d;
> unsigned char uc[8];
>
> void byteSwap(int siz)
> {
> int idx = 0;
> int jdx = siz-1;
> while (idx < jdx)
> {
> std::swap(uc[idx], uc[jdx]);
> idx++, jdx--;
> }
> }
> to_uchar(int ii) : i(ii) { swap(i); }
> to_uchar(double dd) : d(dd) { swap(d); }
> unsigned char* getBuf() { return uc; }
>
> static void swap(int& i)
> {
> to_uchar x = i; //[1]
> x.byteSwap(size of(i));
> i = x.i;
> }
> static void swap(double& d)
> {
> to_uchar x = d; //[2]
> x.byteSwap(size of(d));
> d = x.d;
> }
>
> };
>
>
> int main()
> {
> double d = 12345.678;
> to_uchar::swap( d);
> int i = 12345;
> to_uchar::swap( i);
>
> to_uchar iuc(12345);
> unsigned char* bufi = iuc.getBuf();
> to_uchar duc(12345.678);
> unsigned char* bufd = duc.getBuf();
> }
>
> the lines indicated by [1] and [2] above results in exceptions but this
> is one of those cases where I dont fully understand why or what teh
> solution is.[/color]

I'm not sure what you're trying to do here, but your constructors are a
little screwy, and it has nothing to do with unions.

This constructor...

to_uchar(int ii) : i(ii) { swap(i); }

calls void to_uchar::swap( int& i)...

to_uchar x = i; //[1]

which constructs a to_uchar from an int (invokes the to_uchar(int ii)
constructor)...

to_uchar(int ii) : i(ii) { swap(i); }

which calls void to_uchar::swap( int& i)...

to_uchar x = i; //[1]

which constructs a to_uchar from an int (invokes the to_uchar(int ii)
constructor)...

I think you can see where this is going (infinite recursion). It looks like
the other constructor suffers from the same problem.

--
David Hilsee

Re: experimenting with unions - byteswap approach..


ma740988@pegasu s.cc.ucf.edu wrote:

[]
[color=blue]
> int main()
> {
> double d = 12345.678;
> to_uchar::swap( d);
> int i = 12345;
> to_uchar::swap( i);
>
> to_uchar iuc(12345);
> unsigned char* bufi = iuc.getBuf();
> to_uchar duc(12345.678);
> unsigned char* bufd = duc.getBuf();
> }[/color]

Why don't you use std::reverse()?

template<class T>
void byteswap(T* t)
{
std::reverse(re interpret_cast< char*>(t), reinterpret_cas t<char*>(t
+ 1));
}

By the way, you can't byteswap a floating point number (float, double,
....), since not any bit pattern is a valid floating point number binary
representation.

Re: experimenting with unions - byteswap approach..


|| By the way, you can't byteswap a floating point number (float,
double,
|| ...), since not any bit pattern is a valid floating point number
binary
|| representation.

Maxim, thanks for the suggestion.... So what do you do with floats
(doubles) when you have endian issues across platforms? IOW the
sender transmits - say 9.2 and the receiver sees some convoluted number.

Re: experimenting with unions - byteswap approach..


ma740988@pegasu s.cc.ucf.edu wrote:[color=blue]
> || By the way, you can't byteswap a floating point number (float,
> double,
> || ...), since not any bit pattern is a valid floating point number
> binary
> || representation.
>
> Maxim, thanks for the suggestion.... So what do you do with floats
> (doubles) when you have endian issues across platforms? IOW the
> sender transmits - say 9.2 and the receiver sees some convoluted number.[/color]

I use some binary format and a library, such as XDR.

Alternatively, you may extract mantissa and exponent as integers, send
and reassemble the number on the receiving side.

Re: experimenting with unions - byteswap approach..

ma740988@pegasu s.cc.ucf.edu wrote:[color=blue]
> Maxim, thanks for the suggestion.... So what do you do with floats
> (doubles) when you have endian issues across platforms? IOW the
> sender transmits - say 9.2 and the receiver sees some convoluted number.[/color]

You could also use text since different processors sometimes use
different floating point formats. See these FAQs:



Cheers! --M

Re: experimenting with unions - byteswap approach..

Ironically, you brought this up becasue serilization is the approach
used.
I need to delve more into the text 'approach' so I suspect I'll peruse
the web to get a feel for how this all works (somebody should have
something that shows this).

Being a little more than an intermediate apprentice :) it'll take me a
few to get this right.

Thanks

Re: experimenting with unions - byteswap approach..

ma740988@pegasu s.cc.ucf.edu wrote:[color=blue]
> Ironically, you brought this up becasue serilization is the approach
> used.
> I need to delve more into the text 'approach' so I suspect I'll peruse
> the web to get a feel for how this all works (somebody should have
> something that shows this).
>
> Being a little more than an intermediate apprentice :) it'll take me a
> few to get this right.
>
> Thanks[/color]

You might also want to consider using the Boost serialization library:



Cheers! --M