Object-oriented Software in Ada 95

12 Defining new operators
This chapter shows how the predefined operators in Ada can be overloaded with a new meaning.
12.1 Defining operators in Ada
The existing operators in Ada can be overloaded with a new meaning. These new operators have the same
precedence as their existing counterparts. For example, to trace every executed integer + operation in a program,
the operator + can be overloaded by a function that writes trace information to the terminal before delivering the
normal integer addition. This is implemented by the following function:
function "+" ( F:in Integer; S:in Integer ) return Integer is
begin
Put("[Performing "); Put(F, Width=>1 );
Put(" + "); Put(S, Width=>1 ); Put("]");
return Standard."+"( F, S );
end "+";
Note: To perform the inbuilt + the functional notation for the plus operation must be used. This is written as
Standard."+"( f, s ). In Ada, the inbuilt operators are considered to belong to the package
Standard which is automatically made visible to all program units.
Section C.4, Appendix C gives the specification for the package standard.
The above function can be used to trace the use of + in the following program:
with Ada.Text_Io, Ada.Integer_Text_Io;
use Ada.Text_Io, Ada.Integer_Text_Io;
procedure Main is
function "+" ( F:in Integer; S:in Integer ) return Integer is
begin
Put("[Performing "); Put(F, Width=>1 );
Put(" + "); Put(S, Width=>1 ); Put("]");
return Standard."+"( F, S );
end "+";
begin
Put("The sum of 1 + 2 is: "); Put ( 1+2 ); New_Line;
Put("The sum of 1 + 2 is: ");
Put( Standard."+"(1,2), Width=>1 ); New_Line;
Put("The sum of 1 + 2 is: ");
Put( "+"(1,2), Width=>1 ); New_Line;
end Main;
Note:
As the package Standard is considered to be included with all program units. To achieve the effect
of tracing each use of +, the overloaded function "+" has to be a nested function of the program unit.
The sum of 1 + 2 is: [Performing 1 + 2]3
The sum of 1 + 2 is: 3
The sum of 1 + 2 is: [Performing 1 + 2]3
Note: The way of directly using the operator + defined in the package Standard
'Standard."+"(1,2)’.
The function notation for the use of the operator + "+"(1,2).
© M A Smith - May not be reproduced without permission