The General Syntax of operator overloading in C++

You cannot change the meaning of operators for built-in types in C++, operators can only be overloaded for user-defined types¹. That is, at least one of the operands has to be of a user-defined type. As with other overloaded functions, operators can be overloaded for a certain set of parameters only once.

Not all operators can be overloaded in C++. Among the operators that cannot be overloaded are: . :: sizeof typeid .* and the only ternary operator in C++, ?:

Among the operators that can be overloaded in C++ are these:

• arithmetic operators: + - * / % and += -= *= /= %= (all binary infix); + - (unary prefix); ++ -- (unary prefix and postfix)
• bit manipulation: & | ^ << >> and &= |= ^= <<= >>= (all binary infix); ~ (unary prefix)
• boolean algebra: == != < > <= >= || && (all binary infix); ! (unary prefix)
• memory management: new new[] delete delete[]
• implicit conversion operators
• miscellany: = [] -> ->* , (all binary infix); * & (all unary prefix) () (function call, n-ary infix)

However, the fact that you can overload all of these does not mean you should do so. See the basic rules of operator overloading.

In C++, operators are overloaded in the form of functions with special names. As with other functions, overloaded operators can generally be implemented either as a member function of their left operand's type or as non-member functions. Whether you are free to choose or bound to use either one depends on several criteria.² A unary operator @³, applied to an object x, is invoked either as operator@(x) or as x.operator@(). A binary infix operator @, applied to the objects x and y, is called either as operator@(x,y) or as x.operator@(y).⁴

Operators that are implemented as non-member functions are sometimes friend of their operand’s type.

¹ _{The term “user-defined” might be slightly misleading. C++ makes the distinction between built-in types and user-defined types. To the former belong for example int, char, and double; to the latter belong all struct, class, union, and enum types, including those from the standard library, even though they are not, as such, defined by users.}

² _{This is covered in a later part of this FAQ.}

³ _{The @ is not a valid operator in C++ which is why I use it as a placeholder.}

⁴ _{The only ternary operator in C++ cannot be overloaded and the only n-ary operator must always be implemented as a member function.}

Continue to The Three Basic Rules of Operator Overloading in C++.