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

27
votes

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).

6
c++cfloating-pointdouble
Typically the number of bits for an int is needed because it is encoding some sort of object flags. Why do you need assurances on the precision of your floats? In most cases I've seen people tend to overstate the importance of the sizes of random variables. Often you'd be better off using the machine's default word size than trying to squeeze out 3 bytes of memory or arbitrarily using 32 bit values. – Andrew Khosravian
@Andrew-Khosravian I'm writing a scripting language, and I'd like to be able to make type-size guarantees to my users. That makes code written in my scripting language more portable. – Imagist
Portability is fine, but you need to draw the line somewhere - after all, you're probably not expecting your scripting language to run on a PDP-11. Very few platforms do not support IEEE 754, and if that is supported, then it is a reasonable assumption that double is indeed 64 bits (since it's a double-precision floating point value) - and on the off chance that it is not, 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. – Michael Madsen
int is guaranteed to be at least 16 bits, and long int at least 32 bits (although it's actually defined in terms of the range of values representable) - so if you want a variable that can store any integer from -2147483647 to 2147483647, long int is fine. – caf
@caf And if I want a variable that can store exactly 32 bits cross-platform? int32_t. – Imagist

6 Answers

4
votes

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.

4
votes

While I don't know of a type that guarantees a particular size and format, you do have a few options in C++. You can use the <limits> header and its std::numeric_limits class template to find out the size of a given type, std::numeric_limits::digits tells you the number of bits in the mantissa, and std::numeric_limits::is_iec559 should tell you whether the type follows the IEEE format. (For sample code that manipulates IEEE numbers at the bit level, see the FloatingPoint class template in Google Test's gtest-internal.h.)

1
votes

The other issue is representation of floating point numbers. This is usually based on the hardware on which you are running (but not always). Most system are using IEEE 754 Float point standards, but other can have their own standards as well (an example would be a VAX computer).

Wikipedia explaination of IEEE 754 http://en.wikipedia.org/wiki/IEEE_754-2008

1
votes

There's no variation in float/double that I'm aware of. Float has has been 32 bits for ages and double has been 64. Floating point semantics are pretty complicated, but there do exist constants in 

#include <limits>

boost.numeric.bounds is a simpler interface if you don't need everything in std::numeric_limits

0
votes

Unfortunately, that's not guaranteed either. You have to check numeric_limits< T > in <limits>.

But then again, I've never heard of an implementation where a double wasn't 64 bits long. If you wanted to just assume, you'd probably get away with it.

-4
votes

One of the biggest problems with these kind of "fixed width types" is that it's so easy to get it wrong. You probably didn't want a 32 bits integer. What's the point? WHat you did want is an integer type that can store at least 1>>31. That's long int. You don't even need <stdint.h> for that.

Similarly, your scripting language can implement an FP type that will work as long as the underlying C++ float is at least 32 bits. Note that this still doesn't give you precise behavior. I'm fairly certain C++ doesn't guarantee -1.0/-3.0==1.0/3.0