# Object construction

During the mapping, instances of objects are recursively created and hydrated
with values coming from the input.

The values of an object are filled either with a constructor — which is the
recommended way — or using the class properties. If a constructor exists, it
will be used to create the object, otherwise the properties will be filled
directly.

By default, the library will use a native constructor of a class if it is
public; for advanced use cases, the library also allows the [usage of custom
constructors](../how-to/use-custom-object-constructors.md).

## Class with a single value

When an object needs only one value (one constructor argument or one property),
the source given to the mapper can match the type of the value — it does not
need to be an array with one value with a key matching the argument/property
name.

This can be useful when the application has control over the format of the 
source given to the mapper, in order to lessen the structure of input.

```php
final class Identifier
{
    public readonly string $value;
}

final class SomeClass
{
    public readonly Identifier $identifier;

    public readonly string $description;
}

$mapper = (new \CuyZ\Valinor\MapperBuilder())->mapper();

$mapper->map(SomeClass::class, [
    'identifier' => [
        // 👎 The `value` key feels a bit excessive
        'value' => 'some-identifier'
    ],
    'description' => 'Lorem ipsum…',
]); 

$mapper->map(SomeClass::class, [
    // 👍 The input has been flattened and is easier to read
    'identifier' => 'some-identifier',
    'description' => 'Lorem ipsum…',
]);
```