Fixed-width Floating-Point Numbers in C/C++

Question

int is usually 32 bits, but in the standard, int is not guaranteed to have a constant width. So if we want a 32 bit int we include stdint.h and use int32_t.

Is there an equivalent for this for floats? I realize it's a bit more complicated with floats since they aren't stored in a homogeneous fashion, i.e. sign, exponent, significand. I just want a double that is guaranteed to be stored in 64 bits with 1 sign bit, 10 bit exponent, and 52/53 bit significand (depending on whether you count the hidden bit).

Answer 1

According to the current C99 draft standard, annex F, that should be double. Of course, this is assuming your compilers meet that part of the standard.

For C++, I've checked the 0x draft and a draft for the 1998 version of the standard, but neither seem to specify anything about representation like that part of the C99 standard, beyond a bool in numeric_limits that specifies that IEEE 754/IEC 559 is used on that platform, like Josh Kelley mentions.

Very few platforms do not support IEEE 754, though - it generally does not pay off to design another floating-point format since IEEE 754 is well-defined and works quite nicely - and if that is supported, then it is a reasonable assumption that double is indeed 64 bits (IEEE 754-1985 calls that format double-precision, after all, so it makes sense).

On the off chance that double isn't double-precision, build in a sanity check so users can report it and you can handle that platform separately. If the platform doesn't support IEEE 754, you're not going to get that representation anyway unless you implement it yourself.