The behaviors of new operator and operator new is different.

If we don’t have a good reason for overloading an operator, don’t overload it. In the case of &&, ||, and ,, it’s difficult to have a good reason: we can’t make them behave the way they’re supposed to.

The prefix and postfix forms of increment and decrement operators return different types: prefix forms return a reference, while postfix forms return a const object. For efficiency, prefer prefix forms unless the behavior of postfix ones is necessary. To guarantee consistency, implement postfix operators in terms of the prefix operators.

Implicit type conversions usually lead to more harm than good, so don’t provide conversion functions unless we’re sure we want them.

Including meaningless default constructors affects the efficiency of classes, so avoiding them in classes guarantees fully-initialized objects, with the cost of some limits on how such classes can be used.