bit field usage question

[ajcrm125]

I'm trying to utilize bitfields to make my file easier. But for some
reason, I can't assign a value to this structure. I'm sure I must be
missing something:

typedef unsigned short WORD;
WORD data;

union uVCTR {
WORD Ypos : 10; // Y position
WORD Ysign : 1; // Y position sign bit
WORD temp : 1; //
WORD opCode : 4; // 0..100 (8 bits)
} VCTR;

VCTR = data;


"error C2679: binary '=' : no operator defined which takes a right-
hand operand of type 'unsigned short' (or there is no acceptable
conversion)"

Thanks for any info.




VCTR = data

 

[Victor Bazarov]

I'm trying to utilize bitfields to make my file easier. But for some
reason, I can't assign a value to this structure. I'm sure I must be
missing something:

typedef unsigned short WORD;
WORD data;

union uVCTR {
WORD Ypos : 10; // Y position
WORD Ysign : 1; // Y position sign bit
WORD temp : 1; //
WORD opCode : 4; // 0..100 (8 bits)
} VCTR;

VCTR = data;


"error C2679: binary '=' : no operator defined which takes a right-
hand operand of type 'unsigned short' (or there is no acceptable
conversion)"

First off, your 'uVCTR' should probably be a struct and not a union.
Second, you cannot assign an unsigned short to a union or a struct,
their default assignment operator takes only the same type. You might
try

union uVCTR {
struct bits {
WORD Ypos : 10; // Y position
WORD Ysign : 1; // Y position sign bit
WORD temp : 1; //
WORD opCode : 4; // 0..100 (8 bits)
};
WORD whole;
} VCTR;

and then do

VCTR.whole = data;

but using the bit fields after that can cause undefined behaviour.

V

 

[Victor Bazarov]

First off, your 'uVCTR' should probably be a struct and not a union.
Second, you cannot assign an unsigned short to a union or a struct,
their default assignment operator takes only the same type. You might
try

union uVCTR {
struct bits {
WORD Ypos : 10; // Y position
WORD Ysign : 1; // Y position sign bit
WORD temp : 1; //
WORD opCode : 4; // 0..100 (8 bits)
};

This is nonsense of course, should be

struct {
WORD Ypos : 10; // Y position
WORD Ysign : 1; // Y position sign bit
WORD temp : 1; //
WORD opCode : 4; // 0..100 (8 bits)
} bits;

WORD whole;
} VCTR;

and then do

VCTR.whole = data;

but using the bit fields after that can cause undefined behaviour.

V

 

[James Kanze]

I'm trying to utilize bitfields to make my file easier. But for some
reason, I can't assign a value to this structure. I'm sure I must be
missing something:

Several things, actually.

typedef unsigned short WORD;
WORD data;

union uVCTR {
WORD Ypos : 10; // Y position
WORD Ysign : 1; // Y position sign bit
WORD temp : 1; //
WORD opCode : 4; // 0..100 (8 bits)
} VCTR;

First, I'm not sure what bitfields are supposed to mean in a
union. I can't find anything offhand, but I would have assumed
that it was illegal. (There's a guarantee somewhere that all of
the elements of a union have the same address. Bitfields don't
have an address.)

Second, supposing you mean struct instead of union, you
mentioned files. Bitfield's define a purely internal format,
and can't be used when external formats are involved.

VCTR = data;

"error C2679: binary '=' : no operator defined which takes a right-
hand operand of type 'unsigned short' (or there is no acceptable
conversion)"

And finally, why should it work? On one side of the assignment,
you have a class type (uVCTR), on the other an unsigned short.
Two totally unrelated types.