make this snippet efficient

[KK]

/* Target - read an integer from a binary file */
unsigned int Byte2Int(char *buff)
{
unsigned char* byte = reinterpret_cas t<unsigned char*> (buff);
return ((byte[0]<<24)|(byte[1]<<16)|(byte[2]<<8)|(byte[3]));
}
/* part of main funciton */

ifstream fp("in.bin",ios ::binary);
char buff[4];
fp.read(buff,4) ;
unsigned int loadSize = Byte2Int(buff);

Thank you.
KK

[Victor Bazarov]

KK wrote:

/* Target - read an integer from a binary file */
unsigned int Byte2Int(char *buff)
{
unsigned char* byte = reinterpret_cas t<unsigned char*> (buff);
return ((byte[0]<<24)|(byte[1]<<16)|(byte[2]<<8)|(byte[3]));
}
/* part of main funciton */

ifstream fp("in.bin",ios ::binary);
char buff[4];
fp.read(buff,4) ;
unsigned int loadSize = Byte2Int(buff);

What's *INefficient* about it?

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[pedagani]

Victor Bazarov wrote:

KK wrote:

/* Target - read an integer from a binary file */
unsigned int Byte2Int(char *buff)
{
unsigned char* byte = reinterpret_cas t<unsigned char*> (buff);
return ((byte[0]<<24)|(byte[1]<<16)|(byte[2]<<8)|(byte[3]));
}
/* part of main funciton */

ifstream fp("in.bin",ios ::binary);
char buff[4];
fp.read(buff,4) ;
unsigned int loadSize = Byte2Int(buff);

What's *INefficient* about it?

V
--

Must I use reinterpret_cas t operator ? How can I avoid it?

[Frederick Gotham]

KK posted:

/* Target - read an integer from a binary file */
unsigned int Byte2Int(char *buff)
{
unsigned char* byte = reinterpret_cas t<unsigned char*> (buff);
return ((byte[0]<<24)|(byte[1]<<16)|(byte[2]<<8)|(byte[3]));
}
/* part of main funciton */

ifstream fp("in.bin",ios ::binary);
char buff[4];
fp.read(buff,4) ;
unsigned int loadSize = Byte2Int(buff);

Thank you.
KK

You don't specify the amount of bits in a byte, however, looking at your
code, we can make an educated guess of 8.

You don't specify the amount of bytes in an int, however, looking at your
code, we can make an educated guess of 4.

You don't specify the byte order of the integer stored in the file, so we
can only hope that it's the same as the system's.

You don't specify the negative number system used to represent the number
in the file, so we can only hope that it's the same as the system's.

You don't specify whether the integer in the file contains padding bits, or
where they're located, nor do you specify whether the system stores
integers with padding bits, or where they're located.

Working with the scraps being given, try this:

unsigned Func( char (&array)[4] )
{
return reinterpret_cas t<int&>( array );
}
--

Frederick Gotham

[chandu]

> > > unsigned char* byte = reinterpret_cas t<unsigned char*> (buff);

Must I use reinterpret_cas t operator ? How can I avoid it?

use : unsigned char* byte = (unsigned char*)buff;

[Frederick Gotham]

Frederick Gotham posted:

unsigned Func( char (&array)[4] )
{
return reinterpret_cas t<int&>( array );
}

Should have casted to unsigned&, rather than int&.
--

Frederick Gotham

[Victor Bazarov]

chandu wrote:

unsigned char* byte = reinterpret_cas t<unsigned char*> (buff);

Must I use reinterpret_cas t operator ? How can I avoid it?

use : unsigned char* byte = (unsigned char*)buff;

Which means exactly the same thing without the keyword. How is it
any better?

[Markus Svilans]

Frederick Gotham wrote:

KK posted:

/* Target - read an integer from a binary file */
unsigned int Byte2Int(char *buff)
{
unsigned char* byte = reinterpret_cas t<unsigned char*> (buff);
return ((byte[0]<<24)|(byte[1]<<16)|(byte[2]<<8)|(byte[3]));
}
/* part of main funciton */

ifstream fp("in.bin",ios ::binary);
char buff[4];
fp.read(buff,4) ;
unsigned int loadSize = Byte2Int(buff);

Thank you.
KK

You don't specify the amount of bits in a byte, however, looking at your
code, we can make an educated guess of 8.

When programming in C++, could one realistically expect to encounter a
system that does not have 8 bits in a byte?

Markus.

[Frederick Gotham]

Markus Svilans posted:

When programming in C++, could one realistically expect to encounter a
system that does not have 8 bits in a byte?

You're on a Standard C++ newsgroup, and people here like to be pedantic. It
pays off in the long run, you end up with code that will run perfectly for
eons.

Here's a few things that the Standard allows:

(1) Machines need not use two's complement.
(2) Null pointers need not be all-bits-zero.
(3) Bytes need not be eight bits.
(4) Primitive types may contain padding bits.

Either you take all these things into account, and write FULLY-portable and
Standard-compliant code, or you don't.

If it ever got to a point where an old-fashioned constraint was hindering
efficiency or functionality, the constraint would be lifted. But until
then, you use the following macros to tell you how many bits you have in a
byte:
#define CHAR_BIT \
(((unsigned char)-1)/(((unsigned char)-1)%0x3fffffffL+ 1) \
/0x3fffffffL%0x3 fffffffL*30+((u nsigned char)-1)%0x3fffffffL \
/(((unsigned char)-1)%31+1)/31%31*5 + 4-12/(((unsigned char)\
-1)%31+3))

--

Frederick Gotham

[Luke Meyers]

Frederick Gotham wrote:

use the following macros to tell you how many bits you have in a
byte:
#define CHAR_BIT \
(((unsigned char)-1)/(((unsigned char)-1)%0x3fffffffL+ 1) \
/0x3fffffffL%0x3 fffffffL*30+((u nsigned char)-1)%0x3fffffffL \
/(((unsigned char)-1)%31+1)/31%31*5 + 4-12/(((unsigned char)\
-1)%31+3))

Why provide an implementation (especially one so... urk), rather than
just explain that this macro is available in the standard header
<climits>?

Luke