Define class with fields from the type of the class

Question

Can someone explain me why I can't define something like that:

Class A {  
A a;
//...
};

But I can define something like that:

Class A {  
std::vector<A> vec;
//...
};

What is the difference that allow the second?

Try calculating size of the objects of class `A` (hint `vector` typically stores just several pointers) — user7860670, Jul 15 '18 at 20:00
@Guinzoo Except that the standard says it is undefined behaviour to have containers of incomplete types. Unless that changed in c++17. — juanchopanza, Jul 15 '18 at 20:55
@juanchopanza you right, found some explanaition here https://stackoverflow.com/questions/18672135/why-c-containers-dont-allow-incomplete-types — Guinzoo, Jul 16 '18 at 08:26

score 1 · Answer 1 · answered Jul 15 '18 at 20:07

1

You can't use the first because it is recursive, that is object A contains object A, and the second one you can use because vector doesn't contain object A but a pointer to object A.

answered Jul 15 '18 at 20:07

Zadowolony Pesymista

73
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Define class with fields from the type of the class

1 Answers1