What can be a reason for converting an integer to a boolean in this way?
bool booleanValue = !!integerValue;
instead of just
bool booleanValue = integerValue;
All I know is that in VC++7 the latter will cause C4800 warning and the former will not. Is there any other difference between the two?
The answer of user143506 is correct but for a possible performance issue I compared the possibilies in asm:
return x;, return x != 0;, return !!x; and even return boolean_cast<bool>(x) results in this perfect set of asm instructions:
test edi/ecx, edi/ecx
setne al
ret
This was tested for GCC 7.1 and MSVC 19 2017. (Only the boolean_converter in MSVC 19 2017 results in a bigger amount of asm-code but this is caused by templatization and structures and can be neglected by a performance point of view, because the same lines as noted above may just duplicated for different functions with the same runtime.)
This means: There is no performance difference.
PS: This boolean_cast was used:
#define BOOL int
// primary template
template< class TargetT, class SourceT >
struct boolean_converter;
// full specialization
template< >
struct boolean_converter<bool, BOOL>
{
static bool convert(BOOL b)
{
return b ? true : false;
}
};
// Type your code here, or load an example.
template< class TargetT, class SourceT >
TargetT boolean_cast(SourceT b)
{
typedef boolean_converter<TargetT, SourceT> converter_t;
return converter_t::convert(b);
}
bool is_non_zero(int x) {
return boolean_cast< bool >(x);
}