12.2 Operator declarations

To define the action of an operator is much like defining a function:

Operator definitions

 --              -        ---                       ---
   operator definition operator  -assignment operator definition-|
                            - arithmetic operator definition-|
----            --  -        comparision operator definition-----------
   -result-identifier--|:  result type ;  subroutine block

--assignment operator definition-:= - (- value parameter-)----------------

--arithmetic operator definition|+ ---( -parameter list )----------------
                          - *--|
                          - / -|

--comparision operator definition|-= --( -parameter list-)---------------
                           --< -|
                           - > -|

The parameter list for a comparision operator or an arithmetic operator must always contain 2 parameters, with the exception of the unary minus, where only 1 parameters is needed. The result type of the comparision operator must be Boolean.

Remark: When compiling in Delphi mode or Objfpc mode, the result identifier may be dropped. The result can then be accessed through the standard Result symbol.

If the result identifier is dropped and the compiler is not in one of these modes, a syntax error will occur.

The statement block contains the necessary statements to determine the result of the operation. It can contain arbitrary large pieces of code; it is executed whenever the operation is encountered in some expression. The result of the statement block must always be defined; error conditions are not checked by the compiler, and the code must take care of all possible cases, throwing a run-time error if some error condition is encountered.

In the following, the three types of operator definitions will be examined. As an example, throughout this chapter the following type will be used to define overloaded operators on :

  complex = record  
    re : real;  
    im : real;  

This type will be used in all examples.

The sources of the Run-Time Library contain 2 units that heavily use operator overloading:

This unit contains a complete calculus for complex numbers.
This unit contains a complete calculus for matrices.