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Is it guaranteed by modern C++ compilers that a caste from bool to double will always result only in 0.0 or 1.0?

Posted on 2011-09-30
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Last Modified: 2012-05-12
double d;
bool b;
int i;

b = true:
d = (double) b;

// how about
i = 2;
b = (bool) i;
d = (double) b;
     
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Question by:47pitch
6 Comments
 
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for_yan earned 25 total points
ID: 36894671


see here:
http://msdn.microsoft.com/en-us/library/aa325135%28v=vs.71%29.aspx

public: static double ToDouble(
 bool value
);

The number 1 if value is true; otherwise, 0.
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Expert Comment

by:for_yan
ID: 36894682

http://bytes.com/topic/c/answers/674697-bool-int-cast
From bool to int, false is always 0, true is always 1.
From int to bool, 0 is always false, anything other than 0 is true.


// how about
i = 2;
b = (bool) i;
d = (double) b;

therefore d should become 1.0

as b becomes true
and then it is cast to double as 1.0


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LVL 47

Expert Comment

by:for_yan
ID: 36894863
I actually tried this, using gcc compiler on solaris
and got expected result : 1.000

I think such things should be fairly standardized
 

#include <stdio.h>
#include <stdbool.h>

void main(){
int i;

double d;
bool b;
i =2;
b = (bool) i;
d = (double) b;
printf("%3f\n",d);

}

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Expert Comment

by:Subrat (C++ windows/Linux)
ID: 36895037


Compiler implicitly do the casting for you.

But better to use static_cast instead of simple c style casting.
Ex:
b = static_cast<bool>(i);
d = static_cast<double>(b);
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Assisted Solution

by:evilrix
evilrix earned 25 total points
ID: 36895047
>> I think such things should be fairly standardized

Actually, the result is very well defined as far as the standard is concerned...

4.9 Floating-integral conversions

An rvalue of an integer type or of an enumeration type can be converted to an rvalue of a floating point
type. The result is exact if possible. Otherwise, it is an implementation-defined choice of either the next
lower or higher representable value. [Note: loss of precision occurs if the integral value cannot be represented
exactly as a value of the floating type. ] If the source type is bool, the value false is converted to
zero and the value true is converted to one.
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Author Closing Comment

by:47pitch
ID: 36895115
Thanks!
I was looking for authoritative reference to show my colleagues
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