Define an element that points to a same element type in C++

Question

I want to define an element that points to a same element type, something like this:

#define Foo { Foo*, ..., ... }

For example, this helps me to create a simple tree without creating any auxiliar class:

#define TreeNode std::deque(std::pair(char, TreeNode*))

TreeNode mRootNode;

However, this cannot be done, because a Foo isn't still declared when making a reference to itself in its declaration.

My question is... why the same feature is allowed when implementing it with classes? Is there another simpler way, not Object Oriented, to define my Foo?

You don't want to do this, or almost anything else. using preprocessor macros in C++. — , Jan 20 '17 at 19:00
#define creates a macro, not a type. If you try to expand that macro as shown you get an infinite self-recursion. — zstewart, Jan 20 '17 at 19:02
Step two - Do not use macros - They do not have type safety. They should be used liberally. Why use them in C++? — Ed Heal, Jan 20 '17 at 19:03
@FrançoisAndrieux I mean theoretically. I wasn't concerned with the actual behavior because infinite recursion is the (or at least a) reason you're not allowed to do this/it doesn't work. — zstewart, Jan 20 '17 at 19:05
@FrançoisAndrieux - pre-processor is just text replacement. The compiler uses semantics and syntax as well. That is why they are different — Ed Heal, Jan 20 '17 at 19:09
@latedeveloper You're probably right. I do not think he is talking about naked code ;) — NathanOliver, Jan 20 '17 at 19:11
_not Object Oriented_. Why are you using C++ in the first place? — mascoj, Jan 20 '17 at 19:12
@Borgleader - Poor choice of word - The should be used under duress — Ed Heal, Jan 20 '17 at 19:12
@NathanOliver Why tie your hands behind your back though? If he doesn't want OOP, just use C. — mascoj, Jan 20 '17 at 19:14
@mascoj Because you still have plenty of good things that aren't necessarily OOP but are still objects. — NathanOliver, Jan 20 '17 at 19:19

score 2 · Accepted Answer · answered Jan 20 '17 at 19:12

Define an element that points to a same element type in C++

A simple way to define a type that can refer to another object of the same type is to define a datastructure like this:

struct Foo {
    Foo* link;
};

No need for a macro.

This is allowed, because the Foo has been declared by the time the member pointer is declared. The rules of the language allow recursive classes, but not recursive pre-processor macros.

P.S. To clarify some confusion in the question: While classes in C++ support object oriented features, you can choose to not use those features and instead use classes just like you would use structures in C.

Define an element that points to a same element type in C++

1 Answers1