Forthcoming Decimal Library review (15th Jan)

From: John Maddock (jz.maddock_at_[hidden])
Date: 2025-01-08 12:06:23

Just a gentle reminder that the review of the decimal number library by 
Matt Borland and Chris Kormanyos will start NEXT WEDNESDAY 15th Jan.

The library is an implementation of IEEE 754 Decimal Floating Point Numbers.

Documentation: https://cppalliance.org/decimal/decimal.html

Code: https://github.com/cppalliance/decimal/

Motivation:

Current C++ floating point types store the significand (often 
incorrectly referred to as the mantissa) as binary digits. Famously this 
leads to representation errors: https://0.30000000000000004.com 
<https://0.30000000000000004.com>. Decimal floating point numbers avoid 
this issue by storing the significand in base-10 (decimal). The other 
major difference between binary and decimal floating point types is that 
the latter allows for multiple representations of the same number. For 
example 1e5 could also be stored as 0.1e6, 0.01e7, so on and so forth. 
These are referred to as cohorts which binary does not have as there is 
only one way to represent each number in binary floating point.

Use Cases:

The use case for Decimal Floating Point numbers is where rounding errors 
are significantly impactful such as finance. In applications where 
integer or fixed-point arithmetic are used to combat this issue Decimal 
Floating Point numbers can provide a significantly greater range of 
values. For example, while a fixed-point representation that allocates 8 
decimal digits and 2 decimal places can represent the numbers 123456.78, 
8765.43, 123.00, and so on, a floating-point representation with 8 
decimal digits could also represent 1.2345678, 1234567.8, 
0.000012345678, 12345678000000000, and so on.

I believe Matt has some minor doc updates to push between now and next 
week, but other than that the library is good to review (but hold your 
fire until we start next week unless you're away etc).

Best, John.