Operator overloading in C++

Question

If you overload - like operator-(), it is to be used to the left of the object, however overloading () like operator()() it is used to the right of the object. How do we know which operator is to be used on the left and which ones to be used on the right?

• Chris Lutz: This has a simple answer, but from the perspective of someone new to programming I can imagine this could be a source of much headache. +1

Answer 1

Look at the operator precedence chart. This will tell you the direction the operator associates (binds). Note that some operators have multiple forms with different meanings, such as binary and unary -. In such cases, you may have multiple overloads, e.g.:

T operator-()

and:

T operator-(const T &o)

The compiler chooses the right one based on the syntactical interpretation of the operator.

See also this useful set of guidelines.

• Jonathan Leffler: Does 'operator-()' really return nothing - shouldn't it return a value of the appropriate type that is either the negation of the value or the difference between the values?
• Matthew Flaschen: Thanks for catching that, Jonathan. Fixed now.

Answer 2

Most unary operators can only be placed at a specified side of their operand. For the two special cases, ++ and --, see this FAQ.

• Donald Duck is with Ukraine: While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes.