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(sizeof(i));
i = x.i;
}
static void swap(double& d)
{
to_uchar x = d; //[2]
x.byteSwap(sizeof(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.

Thanks in advance

<ma740988@pegasus.cc.ucf.edu> wrote in message
news:1125533764.972952.243820@g49g2000cwa.googlegr oups.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(sizeof(i));
> i = x.i;
> }
> static void swap(double& d)
> {
> to_uchar x = d; //[2]
> x.byteSwap(sizeof(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

ma740988@pegasus.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(reinterpret_cast<char*>(t), reinterpret_cast<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.

|| 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.

ma740988@pegasus.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.

ma740988@pegasus.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:

http://www.parashift.com/c++-faq-lit...alization.html

Cheers! --M

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

ma740988@pegasus.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:

http://boost.org/libs/serialization/doc/index.html

Cheers! --M