hi
i'm trying to shift a double, but i'm getting the
error message '>>' illegal, left operand double.
althought that the manpages say, '>>' and '<<' can
be applied for int's only, i was able to use the
shift operator on unsigned longs without any problems.
does anyone know how to do this properly?
i.e. individually copying bytes into a long array:
--------------------------------
int pointer;
char databuffer[100];
double uint64;
unsigned long uint32;
pointer =0;
// this works:
uint32 = 1111111; // anything
databuffer[pointer++] = uint32 >> 24;
databuffer[pointer++] = uint32 >> 16;
databuffer[pointer++] = uint32 >> 8;
databuffer[pointer++] = uint32 >> 0; // jup, i know..!
// this won't:
uint64 = 1111111; // anything
databuffer[pointer++] = uint64 >> 56;
databuffer[pointer++] = uint64 >> 48;
databuffer[pointer++] = uint64 >> 40;
databuffer[pointer++] = uint64 >> 32;
databuffer[pointer++] = uint64 >> 24;
databuffer[pointer++] = uint64 >> 16;
databuffer[pointer++] = uint64 >> 8;
databuffer[pointer++] = uint64 >> 0;
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.m oderated. First time posters: Do this! ] 25  7883 
Allan Rydberg wrote: hi
i'm trying to shift a double, but i'm getting the
error message '>>' illegal, left operand double.
althought that the manpages say, '>>' and '<<' can
be applied for int's only, i was able to use the
shift operator on unsigned longs without any problems.
does anyone know how to do this properly?
i.e. individually copying bytes into a long array:
--------------------------------
int pointer;
char databuffer[100];
double uint64;
unsigned long uint32;
pointer =0;
// this works:
uint32 = 1111111; // anything
databuffer[pointer++] = uint32 >> 24;
databuffer[pointer++] = uint32 >> 16;
databuffer[pointer++] = uint32 >> 8;
databuffer[pointer++] = uint32 >> 0; // jup, i know..!
// this won't:
uint64 = 1111111; // anything
databuffer[pointer++] = uint64 >> 56;
databuffer[pointer++] = uint64 >> 48;
databuffer[pointer++] = uint64 >> 40;
databuffer[pointer++] = uint64 >> 32;
databuffer[pointer++] = uint64 >> 24;
databuffer[pointer++] = uint64 >> 16;
databuffer[pointer++] = uint64 >> 8;
databuffer[pointer++] = uint64 >> 0;
unsigned long is usually the same as unsigned int, so it works. if you
want to shift 64bit value, you can use long long under gcc and __int64
(?) under msvc++.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.m oderated. First time posters: Do this! ]
Allan Rydberg wrote: hi
i'm trying to shift a double, but i'm getting the
error message '>>' illegal, left operand double.
althought that the manpages say, '>>' and '<<' can
be applied for int's only, i was able to use the
shift operator on unsigned longs without any problems.
does anyone know how to do this properly?
i.e. individually copying bytes into a long array:
--------------------------------
int pointer;
char databuffer[100];
double uint64;
double is floating point type, not integral type
shifting with these operators is allowed on integral types only
(you may want to look for a 64 bit integral type on your platform)
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.m oderated. First time posters: Do this! ]
Allan Rydberg <al****@southte ch.net> writes: int pointer;
char databuffer[100];
double uint64;
unsigned long uint32;
double is a floating point type.
--
Philippe Amarenco, aka Phix
epita 2007 - LSE - EpX
"if new true friend not protected for explicit private union, break
case and try using this." -- Nathan Myers, longest c++ sentence.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.m oderated. First time posters: Do this! ]
Allan Rydberg wrote: hi
i'm trying to shift a double, but i'm getting the
error message '>>' illegal, left operand double.
althought that the manpages say, '>>' and '<<' can
be applied for int's only, i was able to use the
shift operator on unsigned longs without any problems.
does anyone know how to do this properly?
Longs (and chars and shorts) are all integers. Doubles are floats, and
you cannot apply the shift operators to them. Just multiply or divide
by the appropriate power of two instead - instead of "a<<6" do "a*64".
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.m oderated. First time posters: Do this! ]
Allan Rydberg wrote: hi
i'm trying to shift a double, but i'm getting the
error message '>>' illegal, left operand double.
althought that the manpages say, '>>' and '<<' can
be applied for int's only, i was able to use the
shift operator on unsigned longs without any problems.
does anyone know how to do this properly?
i.e. individually copying bytes into a long array:
--------------------------------
int pointer;
char databuffer[100];
double uint64;
unsigned long uint32;
pointer =0;
// this works:
uint32 = 1111111; // anything
databuffer[pointer++] = uint32 >> 24;
databuffer[pointer++] = uint32 >> 16;
databuffer[pointer++] = uint32 >> 8;
databuffer[pointer++] = uint32 >> 0; // jup, i know..!
// this won't:
uint64 = 1111111; // anything
databuffer[pointer++] = uint64 >> 56;
databuffer[pointer++] = uint64 >> 48;
A double is a double float. It is not an integer type, so it cannot be
bit shifted. (It can be divided by a power of 2, of course, but
presumably the bit patterns and not the values stored in dataBuffer are
of interest here).
A long long is an integer type and can be bit shifted.
So I recommend declaring uint64 like this:
typedef unsigned long long uint64;
for the code to compile as written. I would also recommend renaming
"pointer" since it is not a pointer, but an index.
Greg
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.m oderated. First time posters: Do this! ]
Allan Rydberg wrote: i'm trying to shift a double, but i'm getting the
error message '>>' illegal, left operand double.
althought that the manpages say, '>>' and '<<' can
be applied for int's only, i was able to use the
shift operator on unsigned longs without any problems.
does anyone know how to do this properly?
actually the phrase is "...shift operators ... operands shall be
of integral or enumeration type". unsigned long is integral,
double is. You can't do this.
i.e. individually copying bytes into a long array:
--------------------------------
int pointer;
char databuffer[100];
unsigned char databuffer[100];
double uint64;
unsigned long uint32;
pointer =0;
well you could try this:
unsigned char *p;
p = (unsigned char*)&uint64; /* that's an odd name for a double...
*/
for (i = 0; i < sizeof(double); i++)
databuffer [i] = *p++;
this trick of copying one type to another via a cast pointer only
works for unsigned char. Anything else yields undefined behaviour.
Also note that different implementations may represent doubles
differently and hence the above code won't put the same thing in
databuffer.
// this works:
uint32 = 1111111; // anything
databuffer[pointer++] = uint32 >> 24;
databuffer[pointer++] = uint32 >> 16;
databuffer[pointer++] = uint32 >> 8;
databuffer[pointer++] = uint32 >> 0; // jup, i know..!
Undefined Behaviour
// this won't:
uint64 = 1111111; // anything
databuffer[pointer++] = uint64 >> 56;
databuffer[pointer++] = uint64 >> 48;
databuffer[pointer++] = uint64 >> 40;
databuffer[pointer++] = uint64 >> 32;
databuffer[pointer++] = uint64 >> 24;
databuffer[pointer++] = uint64 >> 16;
databuffer[pointer++] = uint64 >> 8;
databuffer[pointer++] = uint64 >> 0;
--
Nick Keighley
"There are 10 types of people in this world.
Those that understand Binary, and those that don't".
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.m oderated. First time posters: Do this! ]
A simple way to do that is to use a union and put both your double and
a char pointer variable or a char array of size double. Thats it. write
data in double and read the value from double but when to stream data
in and out of the char buffer get the data from char poiinter.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.m oderated. First time posters: Do this! ]
Allan Rydberg wrote:
hi
i'm trying to shift a double, but i'm getting the
error message '>>' illegal, left operand double.
althought that the manpages say, '>>' and '<<' can
be applied for int's only, i was able to use the
shift operator on unsigned longs without any problems.
does anyone know how to do this properly?
You've answered your own question. The basic >> and << operators are
only defined to work with integral types.
i.e. individually copying bytes into a long array:
--------------------------------
int pointer;
char databuffer[100];
double uint64;
unsigned long uint32;
pointer =0;
// this works:
uint32 = 1111111; // anything
databuffer[pointer++] = uint32 >> 24;
databuffer[pointer++] = uint32 >> 16;
databuffer[pointer++] = uint32 >> 8;
databuffer[pointer++] = uint32 >> 0; // jup, i know..!
// this won't:
uint64 = 1111111; // anything
databuffer[pointer++] = uint64 >> 56;
databuffer[pointer++] = uint64 >> 48;
databuffer[pointer++] = uint64 >> 40;
databuffer[pointer++] = uint64 >> 32;
databuffer[pointer++] = uint64 >> 24;
databuffer[pointer++] = uint64 >> 16;
databuffer[pointer++] = uint64 >> 8;
databuffer[pointer++] = uint64 >> 0;
If you want access to the bits in the double you might try doing a bit
of wizardry with casting of pointers;
unsigned long long *address; // double is (typically) longer than a
long
address = (unsigned long long)(&uint64);
databuffer[pointer++] = (*address) >> 56;
That will give you access to the bits that (internally) make up the
double variable.
The value of doing this is .... questionable. You might try
specifying what you're *really* trying to achieve by doing this: there
is almost certainly a better way to do it.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.m oderated. First time posters: Do this! ]
Allan Rydberg wrote: i'm trying to shift a double, but i'm getting the error
message '>>' illegal, left operand double.
althought that the manpages say, '>>' and '<<' can be applied
for int's only, i was able to use the shift operator on
unsigned longs without any problems.
The operators are defined for all integral types, not just int.
But becareful with >> on a signed integral type; I tend to only
use it with unsigned types, just to be on the safe side.
does anyone know how to do this properly?
i.e. individually copying bytes into a long array:
--------------------------------
int pointer;
char databuffer[100];
double uint64;
Now that's a confusing name.
unsigned long uint32;
pointer =0;
// this works:
uint32 = 1111111; // anything
databuffer[pointer++] = uint32 >> 24;
databuffer[pointer++] = uint32 >> 16;
databuffer[pointer++] = uint32 >> 8;
databuffer[pointer++] = uint32 >> 0; // jup, i know..!
// this won't:
uint64 = 1111111; // anything
databuffer[pointer++] = uint64 >> 56;
databuffer[pointer++] = uint64 >> 48;
databuffer[pointer++] = uint64 >> 40;
databuffer[pointer++] = uint64 >> 32;
databuffer[pointer++] = uint64 >> 24;
databuffer[pointer++] = uint64 >> 16;
databuffer[pointer++] = uint64 >> 8;
databuffer[pointer++] = uint64 >> 0;
I suppose that you are formatting binary data for output. And
that the specification requires IEEE double, high byte first,
and that you accept the fact that your code will not be portable
to a machine which doesn't use IEEE floating point (IBM or
Unisys mainframe, for example). Given that you don't really
have absolute portability anyway, the obvious answer here is to
use a reinterpret_cas t, e.g.:
double d = 111111 ;
unsigned int const& ui
= reinterpret_cas t< unsigned int const& >( d ) ;
databuffer[ pointer ++ ] = ui >> 56 ;
// ...
If you require absolute portability, so that the code will run
as is even on a Unisys mainframe, you've got your work cut out
for you -- you have to functions like frexp to extract the
necessary information from the floating point format and create
your own IEEE format. But portability to this degree typically
isn't necessary.
--
James Kanze mailto: ja*********@fre e.fr
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 pl. Pierre Sémard, 78210 St.-Cyr-l'École, France +33 (0)1 30 23 00 34
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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