PHP 8.0: Null-safe operator

Null-safe operator is a new syntax in PHP 8.0, that provides optional chaining feature to PHP.

The null-safe operator allows reading the value of property and method return value chaining, where the null-safe operator short-circuits the retrieval if the value is null, without causing any errors.

The syntax is similar to the property/method access operator (->), and following the nullable type pattern, the null-safe operator is ?->.

$foo?->bar?->baz;

Null safe operator silently returns null if the expression to the left side evaluates to null.

class Customer {
    public function getAddress(): ?Address {}
}
class Address {
    public function getCountry(): string {}
}

$country = $customer->getAddress()->getCountry();

In the snippet above, the Customer::getAddress() method return value is nullable; It can return a null value, or an object of Address class.

The $customer->getAddress()->getCountry() chain is not "null safe", because the return value of getAddress can be null, and PHP throws an error when it tries to call getCountry() method:

Fatal error: Uncaught Error: Call to a member function getCountry() on null in ...:...

To safely access the address, it was necessary to check the null return values before further accessing the return value.

$address = $customer->getAddress();
$country = $address ? $address->getCountry() : null;

$address = $customer->getAddress();
if ($address) {
    $country = $address->getCountry();
}
else {
    $country = null;
}

The null-safe operator solves this by short-circuiting the property/method access, and returning null immediately if the left side of the operator is null, without executing the rest of the expression.

- $address = $customer->getAddress();
- $country = $address ? $address->getCountry() : null;
+ $country = $customer->getAddress()?->getCountry(); 

The ?-> null-safe operator can help reduce excessive isset() and ternary checks.

Null-safe operator was added to PHP 8.0 between alpha 3 release and first beta release, right around the time PHP 8.0 reached its feature-freeze.

Read-Only

The null-safe operator is read-only. It cannot write/assign values from it.

class Customer {
    private ?Address $address;

    public function getAddress(): ?Address {
        return $this->address;
    }
}

class Address {
    public string $country;
}

$customer->getAddress()?->country = 'NL';

This snippet attempts to write to the $country property of the Address object. This is not allowed with the null-safe operator.

Fatal error: Can't use nullsafe operator in write context in ... on line ...

Chaining

The null-safe operator can be chained, and the expression as a whole will be short-circuited from the first null-safe operator that encounters null.

$customer->getAddress()?->getCoordinates()->getLongitude()->format();

This chain of calls are null-safe. PHP will stop and immediately return null if any of the null-safe return operators evaluate to null.

{code-ok} Null-safe operator must be present at every step the chain can be short-circuited. Using the ?-> operator will not make the whole chain null-safe.

Evaluated from left to right

The null-safe operator is evaluated from left to right. Most importantly, it does not prioritize other function calls or other access patterns such as array-access.

<?php

class Customer {
    public function getAddress(): ?Address {}
}

class Address {
    public function setCountry(string $country_code): void {}
}

$customer->getAddress()?->setCountry(GeoIP::getCountry());

If the Customer::getAddress method returns a valid Address object, the GeoIP::getCountry() will be executed, and passed to the Address::setCountry() method call. Because the null-safe operators are evaluated from left to right, the GetIP::getCountry() method will never be called if the Customer::getAddress() method returns null.

Note that using braces will make the expression inside the braces execute altogether, as opposed to the left-to-write pattern.

$customer->getAddress()?->setCountry((GeoIP::getAddress()?->getCountry()))

If Customer::getAddress() returns a value other than null, the (GeoIP::getAddress()?->getCountry()) chain will be executed, and the return value will be passed to the Address::setCountry() call.

No References

The null-safe operator cannot be used with references.

$country = &$customer->getAddress()?->getCountry();

Fatal error: Cannot take reference of a nullsafe chain in ... on line ...

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Backwards Compatibility Impact

Null-safe operator is a new syntax, and cannot be back-ported to older PHP versions.

Running code that uses null-safe operator will raise a parse error:

Parse error: syntax error, unexpected '->' (T_OBJECT_OPERATOR) in ... on line ...

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