One-line output operator for a binary tree - c++

Question

I wrote a simple binary tree class in C++ and want to add an output operator to it. My first attempt was:
ostream& operator<<(ostream& out, const Tree& tree) {
out << tree.myData;
if (tree.myLeft)
out << "(" << (*tree.myLeft) << ")";
if (tree.myRight)
out << "[" << (*tree.myRight) << "]";
return out;
}
(where myLeft and myRight are pointers to left and right child of the current tree, respectively). This works correctly, however, it is not sufficiently cool, since it spans several lines and requires to write "out << " several times.
As an attempt to create a one-line operator, I wrote this:
ostream& operator<<(ostream& out, const Tree& tree) {
return (out << tree.myData
<< "(" << (tree.myLeft? *tree.myLeft: "") << ")"
<< "[" << (tree.myRight? *tree.myRight: "") << "]");
}
But, this generates an error:
incompatible operand types ('Tree' and 'const char [1]')
So I tried this:
ostream& operator<<(ostream& out, const Tree& tree) {
return (&tree?
out << tree.myData
<< "(" << *(tree.myLeft) << ")"
<< "[" << *(tree.myRight) << "]":
out);
}
This works on my computer, but generates a warning implying that this is undefined behavior:
Reference cannot be bound to dereferenced null pointer in well-defined C++ code; pointer may be assumed to always convert to true [-Wundefined-bool-conversion]
QUESTION: Is there a way to write this output operator in a simple single statement?

Answer 1

A simple and elegant solution is to redesign your tree to work without null pointers. Instead, replace current uses of null pointers with pointers to a sentinel tree node which has behaviour consistent with an empty tree.
Then you can rewrite your output stream operator as follows:
ostream& operator<<(ostream& out, const Tree& tree) {
if (&tree == &Tree::NULL_TREE_SENTINEL) return out;
return out << tree.myData
<< "(" << *tree.myLeft << ")"
<< "[" << *tree.myRight << "]";
}
(This assumes a corresponding static member inside Tree, to which the sentinels are pointers, like singletons.)
Alternatively, the sentinel tree node could be an instance of a subclass of Tree with this behaviour. This is sometimes known as the null object pattern. However, it requires dynamic dispatch (i.e. runtime polymorphism via virtual member functions) to work.
Apart from this you don’t quite diagnose the problem with your second code correctly:
This works on my computer
It appears to work but doesn’t actually. I don’t know under what exact circumstances that code will actually do something nasty. But just to be clear, your code is illegal due to the sub-expressions *(tree.myLeft) and *(tree.myRight): these expressions are dereferencing null pointers, and this is never legal. The warning message you’re receiving about the &tree test is merely a symptom of that prior error.