Operator overloading

Operator overloading refers to re-defining already defined operators with new definitions. The term is – although imprecisely – used for operator definitions, that have not yet been defined, too.

overloadable operators

Almost all operators can be (re-)defined:

• assignment operators
  • :=
  • the special operator explicit (referring to explicit typecasts)
• arithmetic operators
  • + (only binary operation)
  • - (both unary and binary)
  • *
  • **
  • div
  • mod
  • /
  • inc
  • dec
• comparison operators
  • <
  • >
  • >=
  • <=
  • =
  • <>
  • ><
  • in
• logical operators
  • and
  • or
  • not
  • xor
  • shl
  • shr
• and the special operator enumerator.

The only operators that can not be overloaded are as and is.

definition

An operator is declared as if it was a function, with a few differences:

• Instead of the word function it starts with operator.
• The function's identifier is always one of the available operators e.g. ><, although this would not constitute a valid identifier anywhere else.
• The parameter list has to name exactly one or two parameters, depending on the operator.
• A result identifier can be specified in front of the colon separating the result type, but can be omitted where the special identifier result is available.

The following shows a valid operator declaration.

operator := (x: myNewType) y: someOtherType;

Operators are defined the same way as any other function, by following the signature with a block.

Some operator overloads are not allowed:

• overloading shortstring assignments with lengths other than 255
• As of 2019-01-20 in the future: + in conjunction with dynamic arrays
• + in conjunction with enumerated types

routing

Which operator overload definition is chose differs in many aspects how a function is chosen.

• The assignment operator := is used for implicit typecasts.
• Unlike regular function overloads, assignment operator overloads are chosen by their result type.
• Operator overloads can not be chosen explicitly by their scope they are defined in. Something like unitDefiningOverloads.+ is not possible. The last operator definition always wins and this can not be changed.

Operator overloads should be used with caution. They potentially make it harder to identify problems, since it is not necessarily obvious a operator overload applies.

see also

• chapter “Operator overloading” in the “Free Pascal Reference Guide”