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.

Answer 3

Use a pointer to the type in the Vector, this will be portable.

struct Move
    {
        int src;
        int dst;
    };

struct MoveTree;

struct MoveTree
    {
        Move move;
        std::vector<MoveTree*> variation;
    };

Answer 4

The MoveTree elements in std::vector are in an allocated (as in new [] ) array. Only the control information (the pointer to the array, the size, etc) are stored within the std::vector within MoveTree .

Answer 5

No, it's not portable. codepad.org does not compile it.

t.cpp:14:   instantiated from here
Line 215: error: '__gnu_cxx::_SGIAssignableConcept<_Tp>::__a' has incomplete type
compilation terminated due to -Wfatal-errors.

Answer 6

You should define copy constructors and assignment operators for both Move and MoveTree when using vector, otherwise it will use the compiler generated ones, which may cause problems.