How does constructor injection work in TYPO3 Extbase?

I just realized that TYPO3 Extbase can (in some situations?) inject dependencies even if you do not pass them to the constructor when you call e.g. $objectManager->get(Foo::class)? I am confused how and when this works. I could not find the answer myself by reading the source code.

Have a look at \TYPO3\CMS\Extbase\Object\Container\Container::getConstructorArguments().

It works as follows:

if

• argument was not supplied when calling ObjectManager->get() and
• there‘s no default value (in constructor signature, that‘s when $argumentInformation['defaultValue'] is set) and
• the required argument is a class (that‘s when $argumentInformation['dependency'] is set)

then this class will be instantiated and supplied as a parameter to the constructor call