C++ Compiler allows circular definition?

Question

I ran into the following oddity when making a mistake writing some code for trees. I\'ve stripped down this example a lot so it is only a Linear Tree.

Basically, in the

Answer 1

That's an unfortunate side-effect of definitions in C++, that declaration and definition is done as separate steps. Because the variables are declared first, they can be used in their own initialization:

std::shared_ptr root = tree.AddLeaf(12, root);
^^^^^^^^^^^^^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^^^^^^^^^
Declaration of the variable  Initialization clause of variable

Once the variable is declared, it can be used in the initialization for the full definition of itself.

It will lead to undefined behavior in AddLeaf if the data of the second argument is used, as the variable is not initialized.