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I know that a float isn't appropriate to store currency values because of rounding errors. Is there a standard way to represent money in C++?
I've looked in the boost library and found nothing about it. In java, it seems that BigInteger is the way but I couldn't find an equivalent in C++. I could write my own money class, but prefer not to do so if there is something tested.
Don't store it just as cents, since you'll accumulate errors when multiplying for taxes and interest pretty quickly. At the very least, keep an extra two significant digits: $12.45 would be stored as 124,500. If you keep it in a signed 32 bit integer, you'll have $200,000 to work with (positive or negative). If you need bigger numbers or more precision, a signed 64 bit integer will likely give you all the space you'll need for a long time.
It might be of some help to wrap this value in a class, to give you one place for creating these values, doing arithmetic on them, and formatting them for display. This would also give you a central place to carry around which currency it being stored (USD, CAD, EURO, etc).
Having dealt with this in actual financial systems, I can tell you you probably want to use a number with at least 6 decimal places of precision (assuming USD). Hopefully since you're talking about currency values you won't go way out of whack here. There are proposals for adding decimal types to C++, but I don't know of any that are actually out there yet.
The best native C++ type to use here would be long double.
The problem with other approaches that simply use an int is that you have to store more than just your cents. Often financial transactions are multiplied by non-integer values and that's going to get you in trouble since $100.25 translated to 10025 * 0.000123523 (e.g. APR) is going cause problems. You're going to eventually end up in floating point land and the conversions are going to cost you a lot.
Now the problem doesn't happen in most simple situations. I'll give you a precise example:
Given several thousand currency values, if you multiply each by a percentage and then add them up, you will end up with a different number than if you had multiplied the total by that percentage if you do not keep enough decimal places. Now this might work in some situations, but you'll often be several pennies off pretty quickly. In my general experience making sure you keep a precision of up to 6 decimal places (making sure that the remaining precision is available for the whole number part).
Also understand that it doesn't matter what type you store it with if you do math in a less precise fashion. If your math is being done in single precision land, then it doesn't matter if you're storing it in double precision. Your precision will be correct to the least precise calculation.
Now that said, if you do no math other than simple addition or subtraction and then store the number then you'll be fine, but as soon as anything more complex than that shows up, you're going to be in trouble.
Look in to the relatively recent Intelr Decimal Floating-Point Math Library. It's specifically for finance applications and implements some of the new standards for binary floating point arithmetic (IEEE 754r).
I would suggest that you keep a variable for the number of cents instead of dollars. That should remove the rounding errors. Displaying it in the standards dollars/cents format should be a view concern.
You can try decimal data type:
https://github.com/vpiotr/decimal_for_cpp
Designed to store money-oriented values (money balance, currency rate, interest rate), user-defined precision. Up to 19 digits.
It's header-only solution for C++.
Know YOUR range of data.
A float is only good for 6 to 7 digits of precision, so that means a max of about +-9999.99 without rounding. It is useless for most financial applications.
A double is good for 13 digits, thus: +-99,999,999,999.99, Still be careful when using large numbers. Recognize the subtracting two similar results strips away much of the precision (See a book on Numerical Analysis for potential problems).
32 bit integer is good to +-2Billion (scaling to pennies will drop 2 decimal places)
64 bit integer will handle any money, but again, be careful when converting, and multiplying by various rates in your app that might be floats/doubles.
The key is to understand your problem domain. What legal requirements do you have for accuracy? How will you display the values? How often will conversion take place? Do you need internationalization? Make sure you can answer these questions before you make your decision.
Whatever type you do decide on, I would recommend wrapping it up in a "typedef" so you can change it at a different time.
It depends on your business requirements with regards to rounding. The safest way is to store an integer with the required precision and know when/how to apply rounding.
Integers, always--store it as cents (or whatever your lowest currency is where you are programming for.) The problem is that no matter what you do with floating point someday you'll find a situation where the calculation will differ if you do it in floating point. Rounding at the last minute is not the answer as real currency calculations are rounded as they go.
You can't avoid the problem by changing the order of operations, either--this fails when you have a percentage that leaves you without a proper binary representation. Accountants will freak if you are off by a single penny.
I would recommend using a long int to store the currency in the smallest denomination (for example, American money would be cents), if a decimal based currency is being used.
Very important: be sure to name all of your currency values according to what they actually contain. (Example: account_balance_cents) This will avoid a lot of problems down the line.
(Another example where this comes up is percentages. Never name a value "XXX_percent" when it actually contains a ratio not multiplied by a hundred.)
The GMP library has "bignum" implementations that you can use for arbitrary sized integer calculations needed for dealing with money. See the documentation for mpz_class (warning: this is horribly incomplete though, full range of arithmetic operators are provided).
One option is to store $10.01 as 1001, and do all calculations in pennies, dividing by 100D when you display the values.
Or, use floats, and only round at the last possible moment.
Often the problems can be mitigated by changing order of operations.
Instead of value * .10 for a 10% discount, use (value * 10)/100, which will help significantly. (remember .1 is a repeating binary)
This is a very old post, but I figured I update it a little since it's been a while and things have changed. I have posted some code below which represents the best way I have been able to represent money using the long long integer data type in the C programming language.
#include <stdio.h>
int main()
{
// make BIG money from cents and dollars
signed long long int cents = 0;
signed long long int dollars = 0;
// get the amount of cents
printf("Enter the amount of cents: ");
scanf("%lld", ¢s);
// get the amount of dollars
printf("Enter the amount of dollars: ");
scanf("%lld", &dollars);
// calculate the amount of dollars
long long int totalDollars = dollars + (cents / 100);
// calculate the amount of cents
long long int totalCents = cents % 100;
// print the amount of dollars and cents
printf("The total amount is: %lld dollars and %lld cents\n", totalDollars, totalCents);
}
As other answers have pointed out, you should either:
Use an integer type to store whole units of your currency (ex: $1) and fractional units (ex: 10 cents) separately.
Use a base 10 decimal data type that can exactly represent real decimal numbers such as 0.1. This is important since financial calculations are based on a base 10 number system.
The choice will depend on the problem you are trying to solve. For example, if you only need to add or subtract currency values then the integer approach might be sensible. If you are building a more complex system dealing with financial securities then the decimal data type approach may be more appropriate.
As another answer points out, Boost provides a base 10 floating point number type that serves as a drop-in replacement for the native C++ floating-point types, but with much greater precision. This might be convenient to use if your project already uses other Boost libraries.
The following example shows how to properly use this decimal type:
#include <iostream>
#include <boost/multiprecision/cpp_dec_float.hpp>
using namespace std;
using namespace boost::multiprecision;
int main() {
std::cout << std::setprecision(std::numeric_limits<cpp_dec_float_50>::max_digits10) << std::endl;
double d1 = 1.0 / 10.0;
cpp_dec_float_50 dec_incorrect = 1.0 / 10.0; // Incorrect! We are constructing our decimal data type from the binary representation of the double value of 1.0 / 10.0
cpp_dec_float_50 dec_correct(cpp_dec_float_50(1.0) / 10.0);
cpp_dec_float_50 dec_correct2("0.1"); // Constructing from a decimal digit string.
std::cout << d1 << std::endl; // 0.1000000000000000055511151231257827021181583404541015625
std::cout << dec_incorrect << std::endl; // 0.1000000000000000055511151231257827021181583404541015625
std::cout << dec_correct << std::endl; // 0.1
std::cout << dec_correct2 << std::endl; // 0.1
return 0;
}
Notice how even if we define a decimal data type but construct it from a binary representation of a double, then we will not obtain the precision that we expect. In the example above, both the double d1 and the cpp_dec_float_50 dec_incorrect are the same because of this. Notice how they are both "correct" to about 17 decimal places which is what we would expect of a double in a 64-bit system.
Finally, note that the boost multiprecision library can be significantly slower than the fastest high precision implementations available. This becomes evident at high digit counts (about 50+); at low digit counts the Boost implementation can be comparable other, faster implementations.
Sources:
Our financial institution uses "double". Since we're a "fixed income" shop, we have lots of nasty complicated algorithms that use double anyway. The trick is to be sure that your end-user presentation does not overstep the precision of double. For example, when we have a list of trades with a total in trillions of dollars, we got to be sure that we don't print garbage due to rounding issues.
go ahead and write you own money (http://junit.sourceforge.net/doc/testinfected/testing.htm) or currency () class (depending on what you need). and test it.