Are C++ recursive type definitions possible, in particular can I put a vector<T> within the definition of T?

Question

For one of my projects, what I really wanted to do was this (simplifying it to the bare minimum);

struct Move {     int src;     int dst; };  struct MoveTree {     Move move;     std::vector<MoveTree> variation; }; 

I must admit that I assumed that it wouldn't be possible to do this directly, I thought a vector of MoveTree s within a MoveTree would be verboten. But I tried it anyway, and it works beautifully. I am using Microsoft Visual Studio 2010 Express.

Is this portable ? Is it good practise ? Do I have anything to worry about ?

Edit: I've asked a second question hoping to find a good way of doing this.

Answer 1

The C++ Standard (2003) clearly says that instantiating a standard container with an incomplete type invokes undefined-behavior.

The spec says in §17.4.3.6/2,

In particular, the effects are undefined in the following cases:

__ [..]
— if an incomplete type (3.9) is used as a template argument when instantiating a template component.
__ [..]

Things have changed with the C++17 standard, which explicitely allows this types of recursion for std::list, std::vector and std::forward_list. For reference, see http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4510.html and this answer: How can I declare a member vector of the same class?

Answer 2

MoveTree is an incomplete type inside its definition. The standard does not guarantee instantiation of STL templates with incomplete types.