Author: Gerd Isenberg
Date: 05:58:16 11/19/02
Go up one level in this thread
<snip>
>>Btw. the build in C++ type "bool" is often implemented with byte size.
>>Due to performace reasons, it might be advantageous to use an own typedef like
>
>Yes, that's it, it's a performance warning.
solvable by including an != 0 without any code size effects...
int i = 1234;
bool b = i; // the value range of bool is [0,1] resp [false,true]
// so the (ms)compiler warns, but implicitly assignes (i != 0)
>
>>typedef int BOOL;
>>
>>#ifndef TRUE
>>#define FALSE 0
>>#define TRUE 1
>>#endif
>
>I don't think I get it, you suggest to cast to int instead, what is the rest
>for?
Confusion :)
You find the BOOL typedef it a lot in ms-header files, maybe also for historical
reasons, because in older days there was no bool type in C.
But even today, "bool" may have some performance problems due to byte-size.
Look in MFC-functions you find mostly BOOL instead of bool, because sizeof(BOOL)
is 4, which is the natural register size of x86 processors. Another point is
pushing bool/char parameters may result in some misalignment on the stack.
The rest, i guess you mean the defines?
MS simply use the old fashioned preprocessor to define TRUE,FALSE literals, if
not already defined.
Of course this BOOL type, behaves exactly like an int.
>
>Anyway, I hardly ever do cast to bool, it's working fine without it so it's
>probably just wasted clocks?
>
It's a good idea to assign only other boolean expressions to variables of type
bool incuding the literals "true" or "false". If you wan't to assign other types
to bool you should always use explicite relational operators, most likely != 0.
Assigning bool to int is another thing, because bool is a kind of pragmatical
subset of int, due to "true" maps to "one" and "false" to "zero".
Therefore expressions like that are possible:
int a,b,c;
...
bool bExpression = (b>c);
int nTrueConditions = (a>b) + (a>c) + bExpression;
with a value range of [0...3]
In strict and for didactical purposes more cleaner languages like Java or Pascal
you can't simply do such performance relevant "dirty" tricks like in the
"pragmatic" C-language.
Gerd
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