Best way to store currency values in C++

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.

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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).

The biggest issue is rounding itself!

19% of 42,50 € = 8,075 €. Due to the German rules for rounding this is 8,08 €. The problem is, that (at least on my machine) 8,075 can't be represented as double. Even if I change the variable in the debugger to this value, I end up with 8,0749999....

And this is where my rounding function (and any other on floating point logic that I can think of) fails, since it produces 8,07 €. The significant digit is 4 and so the value is rounded down. And that is plain wrong and you can't do anything about it unless you avoid using floating point values wherever possible.

It works great if you represent 42,50 € as Integer 42500000.

42500000 * 19 / 100 = 8075000. Now you can apply the rounding rule above 8080000. This can easily be transformed to a currency value for display reasons. 8,08 €.

But I would always wrap that up in a class.

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++.

You say you've looked in the boost library and found nothing about there. But there you have multiprecision/cpp_dec_float which says:

> The radix of this type is 10. As a result it can behave subtly differently from base-2 types.

So if you're already using Boost, this should be good to currency values and operations, as its base 10 number and 50 or 100 digits precision (a lot).

See:

#include <iostream>
#include <iomanip>
#include <boost/multiprecision/cpp_dec_float.hpp>

int main()
{
    float bogus = 1.0 / 3.0;
    boost::multiprecision::cpp_dec_float_50 correct = 1.0 / 3.0;

    std::cout << std::setprecision(16) << std::fixed 
              << "float: " << bogus << std::endl
              << "cpp_dec_float: " << correct << std::endl;
          
    return 0;
}

Output:

> float: 0.3333333432674408 > > cpp_dec_float: 0.3333333333333333

*I'm not saying float (base 2) is bad and decimal (base 10) is good. They just behave differently...

** I know this is an old post and boost::multiprecision was introduced in 2013, so wanted to remark it here.

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.

Store the dollar and cent amount as two separate integers.

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 solution is simple, store to whatever accuracy is required, as a shifted integer. But when reading in convert to a double float, so that calculations suffer fewer rounding errors. Then when storing in the database multiply to whatever integer accuracy is needed, but before truncating as an integer add +/- 1/10 to compensate for truncation errors, or +/- 51/100 to round. Easy peasy.

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)

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.

I'd use signed long for 32-bit and signed long long for 64-bit. This will give you maximum storage capacity for the underlying quantity itself. I would then develop two custom manipulators. One that converts that quantity based on exchange rates, and one that formats that quantity into your currency of choice. You can develop more manipulators for various financial operations / and rules.

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", &cents);

    // 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);
}

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.