How to code a union using a struct as template, while having that union as an element

Question

1. I am trying to construct a union that will replace the pair of pointers (left & right) by a union which is accessible as an array. Originally this is working code for a Binary Search Tree (BST)

I want to be able to do this:

p = p->pLR.array[value>insertValue];

in addition to the old branched

if(value>insertValue) p = p->right;
else  p = p->left;

1b. This is not exactly to avoid costly branching misprediction, but simply to be able to learn to implement [something] like this.

2. Mostly the problem that BinaryNode is templated in the union, and the union is an element of the BinaryNode!

Here is my non-compiling code-in-progress:

template    <class dataType, class BinaryNode<dataType> >
union BinaryNodePointerPair {
struct {
        BinaryNode<dataType> *left;
        BinaryNode<dataType> *right;
    };
    BinaryNode<dataType> *array[2]; 
};  
template    <class dataType>
struct BinaryNode {
    dataType    value;
    BinaryNodePointerPair<dataType,BinaryNode<dataType> > pLR;
};
[Error] 'BinaryNode' is not a template

refactored from:

template    <class dataType>
struct BinaryNode {
    dataType    value;
    BinaryNode  *left;
    BinaryNode  *right;
};

3. What I have tried so far:

3a. According to 'X is not a template' error the template should be

template    <class dataType, class <dataType>BinaryNode >
[Error] expected identifier before '<' token
[Error] expected '>' before '<' token
[Error] type/value mismatch at argument 2 in template parameter list for 'template<class dataType, int <anonymous> > union BinaryNodePointerPair'

3b. So i change it to

template    <class dataType, class BinaryNode<dataType> >

But this gives the errors, plus the original.

[Error] 'BinaryNode' is not a class template
[Error] 'class BinaryNode' is not a valid type for a template non-type parameter
[Error] 'BinaryNode' is not a template

3c. So i again change it to

template    <class dataType, class BinaryNode<dataType> >

but this gives even more errors

[Error] 'BinaryNode' is not a class template
[Error] 'class BinaryNode' is not a valid type for a template non-type parameter
[Error] 'BinaryNode' is not a template
[Error] 'BinaryNode' is not a template type

3d. Now, this is eerily close to CRTP (no need to tell me this isnt real CRTP, but it is curiously)

template    <class dataType>
union BinaryNodePointerPair {
struct {
    BinaryNode<dataType> *left;
    BinaryNode<dataType> *right;
};
BinaryNode<dataType> *array[2]; 
};  
template    <class dataType, BinaryNodePointerPair<dataType> >
struct BinaryNode {
    dataType    value;
    BinaryNodePointerPair<dataType> pLR;
};
[Error] 'BinaryNode' is not a template

Only 1 error! WOW! This has got to be a winner!

3e. Now I really really force-pattern it into CRTP... it's even worse.

template    <class dataType, class BinaryNode<dataType> >
union BinaryNodePointerPair {
    struct {
        BinaryNode<dataType> *left;
        BinaryNode<dataType> *right;
    };
    BinaryNode<dataType> *array[2]; 
};  
template    <class dataType>
struct BinaryNode {
    dataType    value;
    BinaryNodePointerPair<dataType, BinaryNode<dataType>> pLR;
};
[Error] 'BinaryNode' is not a class template
[Error] 'class BinaryNode' is not a valid type for a template non-type parameter
[Error] 'BinaryNode' is not a template

It doesn't have to be CRTP. Is there a way to do it?

How do I write a union w/ pointers to a struct having an element of that union?

Answer 1

I don't really get the question, but did you perhaps forget the forward declaration of BinaryNode ?

template <class dataType>
struct BinaryNode;

template <class dataType>
union BinaryNodePointerPair
{
  struct
  {
    BinaryNode<dataType> *left;
    BinaryNode<dataType> *right;
  };
  BinaryNode<dataType> *array[2]; 
};

template <class dataType>
struct BinaryNode
{
  dataType value;
  BinaryNodePointerPair<dataType> pLR;
};