MindlessCode.net
"Life, The Universe, and Everything through an ADSL connection."

MindlessCode.net: Fixed Point Decimal Class

I recently needed a fixed point decimal support in C++ for an inventory program I'm writing. Normal floating point numbers are not accurate enough to store money or quantities, and so this class was born:

The code is public domain--do what you want to with it.

The class is fairly natural to use. Here's a short example:

#include <iostream>
#include "decimal.h"

int main(int argc, char *argv[])
{
	//create decimal with a precision of 2 decimal places and an initial value of 1000.00
	decimal bank_accout(2,1000);
	
	bank_accout -= decimal(2,25.50); //deduct 25.50
	decimal a_check(2,50.78);
	bank_accout -= a_check;
	bank_accout += 100;
	std::cout<<"Accout Balance: "<<bank_accout<<std::endl;
	
	return 0;
}

Implementation Details

The value of the decimal is stored in an unsigned 64 bit integer. A second variable keeps track of the location of the decimal point (m_precision). For example, 25.75 (with a precision of 4) is stored as 257500.

Since it uses a 64 bit int, you should be able to store a fairly large number with quite a bit of accuracy (264-1). Be careful of overflowing the storage integer--there is no bounds checking. If you cause it to wrap then you will get strange results.

Things You Should Know

When executing the operations +, -, *, /, <, and >, they are executed using the greater precision of the two. Then, after calculation, the precision is changed back to the initial precision of the left-hand side. For example:

decimal dec1(2,25.75), dec2(4, 34.3321);
dec1+=dec2; // 60.08, (answer at 4 places is 60.0821)

You can control the rounding method used by the decimal by passing a RoundingMethod (an enum) to the constructor. See the docs below.

Contact

Questions? Comments? Patches? Send them here!

Documentation

The code is pretty well commented (some functions are documented with Doxygen documentation). If you have any questions, just send me an email and I'll try to answer them (see above). Here's a short synopsis: