Once

A definition of once

A once function is implemented by replacing the do keyword by once e. g.:

foo: INTEGER is
   once
     Result := 1
   end

This has the effect, that code of the feature is only executed once in the system lifetime. If the feature is a function (i. e. has a return type) every call of the feature will return the same Result, be it an expanded value or a reference. For references, the references object may change over time, but the once function will always return the identical object. The routines body is executed only once, even if it is called via different names. I. e.:

foo, bar: INTEGER is
   once
      count := count + 1
      Result := count
   end
   
make is
   do
      assert(foo = 1)
      assert(bar = 1)
   end

count: INTEGER

Further Aspects

• once is not applicable to attributes
• once "manifest string"
• once per object (other eiffel compilers have once ("OBJECT") to execute the feature body once per object) in Liberty there are no plans to implement such a feature