How to have both 32bit and 64bit float in C++

Question

I have a file spec (here: http://www.septentrio.com/secure/asterx1v_2_1/SBF%20Reference%20Guide.pdf) that has fields marked as both 32-bit and 64-bit floats (see page 8). How can I use both widths in my program? I am developing on Mac OSX right now but I will also deploy on a Linux machine.

More details:

I know I could tell the compiler the width, but how could I distinguish two different float widths? Maybe someone also has a suggestion for changing the way I parse, which is to reinterpret_cast(buffer+offset) and then use the values. These file sizes are huge (4GB) so I need performance.

Might be float is 32 bits and double is 64 bits on your platform - it's pretty common. — nos, Jan 18 '14 at 01:34
Yes, however, but floats and doubles have different internal structure correct? — Brian, Jan 18 '14 at 01:35
Maybe this could help: http://stackoverflow.com/a/19466500/2293156 — Amadeus, Jan 18 '14 at 01:42
The document says the 4 and 8 bytes floats use IEEE format. Many, but not all, machines use the same format for float and double types. — brian beuning, Jan 18 '14 at 01:44
Okay, I assumed doubles and floats were completely different data types...but I guess they're both IEEE floating point standard.. — Brian, Jan 18 '14 at 02:08
They are different data types, and different formats, but usually both are IEEE-754. IEEE-754 defines both 32 bit and 64 bit binary floating point formats. — Patricia Shanahan, Jan 18 '14 at 03:53

Kirill Kobelev · Accepted Answer · 2014-01-18T02:41:52.217

1

This might seem obvious, nevertheless:

On Intel platform and many others float is 32-bit floating point value, and double is 64-bit floating point value. Try this approach. Most likely it will work.

To be absolutely sure check sizeof of your types at the start of your program or statically during compilation if your compiler allows this.

Once again, try the simple solution first.

Float and double arithmetic is both implemented on Intel and it is fast. In any case native arithmetic is the fastest of what you can get from the CPU.

IEEE 754 (http://en.wikipedia.org/wiki/IEEE_floating_point) defines not one floating point format, but several, like 4, 8, 16 bytes, etc. They all have different range and precision but they are all still IEEE values.

edited Jan 18 '14 at 02:41

answered Jan 18 '14 at 01:54

Kirill Kobelev

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Can you clarify whether floats and doubles are both stored in the same IEEE format given in the spec (page 8)? – Brian Jan 18 '14 at 02:01
I do not have MAC experience but know that our days it is Intel based. This means that you just define float and double fields in your structures and it will work. You only need to ensure proper fields alignment. – Kirill Kobelev Jan 18 '14 at 02:39

How to have both 32bit and 64bit float in C++

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