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Currency and Money Example

The Currency and Money Value Objects are probably the most used examples for explaining Value Objects, thanks to the Money pattern. This design pattern provides a solution for modeling a problem that avoids a floating-point rounding issue, which in turn allows for deterministic calculations to be performed.

In the real world, a currency describes monetary units in the same way that meters and yards describe distance units. Each currency is represented with a three-letter uppercase ISO code:

class Currency 
{ 
    private $isoCode;

    public function __construct($anIsoCode)
    {
        $this->setIsoCode($anIsoCode);
    }

    private function setIsoCode($anIsoCode)
    {
        if (!preg_match('/^[A-Z]{3}$/', $anIsoCode)) {
           throw new InvalidArgumentException();
        }

        $this->isoCode = $anIsoCode;
    }

    public function isoCode()
    {
        return $this->isoCode;
    }
}

One of the main goals of Value Objects is also the holy grail of Object-Oriented design: encapsulation. By following this pattern, you'll end up with a dedicated location to put all the validation, comparison logic, and behavior for a given concept.

Extra Validations for Currency
In the previous code example, we can build a Currency with an AAA Currency ISO code. That isn't valid at all. Write a more specific rule that will check if the ISO Code is valid. A full list of valid currency ISO codes can be found here. If you need help, take a look at the Money packagist library.

Money is used to measure a specific amount of currency. It's modeled using an amount and a currency. Amount, in the case of the Money pattern, is implemented using an integer representation of the Currency's least-valuable fraction — For example in the case of USD or EUR, cents.

As a bonus, you might also notice that we're using self encapsulation to set the ISO code, which centralizes changes in the Value Object itself:

class Money 
{ 
    private $amount; 
    private $currency;

    public function __construct($anAmount, Currency $aCurrency)
    {
        $this->setAmount($anAmount);
        $this->setCurrency($aCurrency);
    }

    private function setAmount($anAmount)
    {
        $this->amount = (int) $anAmount;
    }

    private function setCurrency(Currency $aCurrency)
    {
        $this->currency = $aCurrency;
    }

    public function amount()
    {
        return $this->amount;
    }

    public function currency()
    {
        return $this->currency;
    }
}

Now that you know the formal definition of Value Objects, let's dive deeper into some of the powerful features they offer.