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// P0847R7
// { dg-do run { target c++23 } }
// an adaptation of one of the examples in P0847R7
struct Leaf { };
struct Node;
struct Tree {
enum class stored {leaf, node};
stored _discriminator;
union {
Leaf _leaf;
Node* _node;
};
Tree(Leaf) : _discriminator(stored::leaf), _leaf() {}
Tree(Node& node) : _discriminator(stored::node), _node(&node) {}
};
struct Node {
Tree left;
Tree right;
};
template<typename Visitor>
auto visit_tree(Visitor&& visitor, Tree const& tree)
{
switch (tree._discriminator)
{
case Tree::stored::leaf:
return visitor (tree._leaf);
case Tree::stored::node:
return visitor (tree._node);
default:
__builtin_abort ();
}
}
template<typename... Ts>
struct overload : Ts... { using Ts::operator()...; };
int main()
{
static constexpr int true_num_leaves = 8;
Node branch0{.left = Leaf{}, .right = Leaf{}};
Node branch1{.left = Leaf{}, .right = branch0};
Node branch2{.left = Leaf{}, .right = Leaf{}};
Node branch3{.left = branch1, .right = branch2};
Node branch4{.left = branch3, .right = Leaf{}};
Node branch5{.left = Leaf{}, .right = Leaf{}};
Node branch6{.left = branch4, .right = branch5};
Tree root (branch6);
int num_leaves = visit_tree (overload{
[](Leaf const&) { return 1; },
[](this auto const& self, Node* n) -> int {
return visit_tree (self, n->left) + visit_tree (self, n->right);
}},
root);
if (num_leaves != true_num_leaves)
__builtin_abort ();
}
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