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edited Feb 16, 2022 at 15:23

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I'll give you a very simple example where you run into trouble. You have classes Person, Teacher::Person, and Student::Person. You assume that the same person cannot be both a Teacher and a Student. This is definitely wrong if you go to a university: Your maths professor could be a music student (if he is interested in the subject).

But here's a case where it's obvious: A FirstAider is also a person. And quite obviously, both teachers and students can be first aiders. So what do you do know: If Teacher, Student, and FirstAider are subclasses of Person, then a Person object for the same person will actually appear twice. Now you add a class Employee::Person. Teachers are usually employees. Students are usually not employees, but there may be one who makes a bit of extra money helping out in the kitchen, or in IT.

So suddenly you are in trouble. You asked "And also, what are those "very painful problems" that can appear?". With some experience, it may be difficult to pick out exactly what problems can appear, but you will know that the subclassing design is just "asking for trouble".

You could also read up on Edgar F. Code, 1981 Turing Award winner for his work on database normalisation - which covers the exact same principles, just 40 years earlier and from a very different angle.

I'll give you a very simple example where you run into trouble. You have classes Person, Teacher::Person, and Student::Person. You assume that the same person cannot be both a Teacher and a Student. This is definitely wrong if you go to a university: Your maths professor could be a music student (if he is interested in the subject).

But here's a case where it's obvious: A FirstAider is also a person. And quite obviously, both teachers and students can be first aiders. So what do you do know: If Teacher, Student, and FirstAider are subclasses of Person, then a Person object for the same person will actually appear twice. Now you add a class Employee::Person. Teachers are usually employees. Students are usually not employees, but there may be one who makes a bit of extra money helping out in the kitchen, or in IT.

So suddenly you are in trouble. You asked "And also, what are those "very painful problems" that can appear?". With some experience, it may be difficult to pick out exactly what problems can appear, but you will know that the subclassing design is just "asking for trouble".

I'll give you a very simple example where you run into trouble. You have classes Person, Teacher::Person, and Student::Person. You assume that the same person cannot be both a Teacher and a Student. This is definitely wrong if you go to a university: Your maths professor could be a music student (if he is interested in the subject).

But here's a case where it's obvious: A FirstAider is also a person. And quite obviously, both teachers and students can be first aiders. So what do you do know: If Teacher, Student, and FirstAider are subclasses of Person, then a Person object for the same person will actually appear twice. Now you add a class Employee::Person. Teachers are usually employees. Students are usually not employees, but there may be one who makes a bit of extra money helping out in the kitchen, or in IT.

So suddenly you are in trouble. You asked "And also, what are those "very painful problems" that can appear?". With some experience, it may be difficult to pick out exactly what problems can appear, but you will know that the subclassing design is just "asking for trouble".

You could also read up on Edgar F. Code, 1981 Turing Award winner for his work on database normalisation - which covers the exact same principles, just 40 years earlier and from a very different angle.

answered Feb 16, 2022 at 15:06

49.4k
4
71
137

I'll give you a very simple example where you run into trouble. You have classes Person, Teacher::Person, and Student::Person. You assume that the same person cannot be both a Teacher and a Student. This is definitely wrong if you go to a university: Your maths professor could be a music student (if he is interested in the subject).

But here's a case where it's obvious: A FirstAider is also a person. And quite obviously, both teachers and students can be first aiders. So what do you do know: If Teacher, Student, and FirstAider are subclasses of Person, then a Person object for the same person will actually appear twice. Now you add a class Employee::Person. Teachers are usually employees. Students are usually not employees, but there may be one who makes a bit of extra money helping out in the kitchen, or in IT.

So suddenly you are in trouble. You asked "And also, what are those "very painful problems" that can appear?". With some experience, it may be difficult to pick out exactly what problems can appear, but you will know that the subclassing design is just "asking for trouble".