When an object is defined without an argument list for its constructor, the compiler automatically calls the object's default constructor?

Yes, but your wording is a little off. When an object is created without an argument list / parameters, the default constructor is called. The one caveat is if the default constructor has been deleted. Then a compilation error will be thrown. I'll also add that it depends on the language and compiler. So if you want more details, you need to tell us what language you are referring too.