If a double is demoted to a float or a long double is demoted to a double or a float, and the initial value exceeds the maximum of the demoted type, then the value of the result undefined.
Non-Compliant Code Example
This non-compliant code illustrates possible undefined behavior associated with demoting floating point represented numbers.
long double ld; double d1; double d2; float f1; float f2; ... f1 = (float)d1; f2 = (float)ld; d2 = (double)ld;
In the assignments above, it is possible that the variable d1 exceeds the maximum value that can be stored by a float or that the variable ld exceeds the maximum value that can be represented as either a float or a double.
Compliant Code Example
This compliant code properly checks to see whether the values to be stored are too large to be represented.
#include <float.h>
...
long double ld;
double d1;
double d2;
float f1;
float f2;
...
if(d1 > FLT_MAX) {
/* Handle error condition */
} else {
f1 = (float)d1;
}
if(ld > FLT_MAX) {
/* Handle error condition */
} else {
f2 = (float)ld;
}
if(ld > DBL_MAX) {
/* Handle error condition */
} else {
d2 = (double)ld;
}
References
[WG14 N1124] Section 6.3.1.5, "Real floating types"