The attribute Prefix'Old can be used within a subprogram to refer to the value of the prefix on entry. So for example if you have an argument of a record type X called Arg1, you can refer to Arg1.Field'Old which yields the value of Arg1.Field on entry. The implementation simply involves generating an object declaration which captures the value on entry. Any prefix is allowed except one of a limited type (since limited types cannot be copied to capture their values) or a local variable (since it does not exist at subprogram entry time).
The following example shows the use of 'Old to implement a test of a postcondition:
with Old_Pkg; procedure Old is begin Old_Pkg.Incr; end Old; package Old_Pkg is procedure Incr; end Old_Pkg; package body Old_Pkg is Count : Natural := 0; procedure Incr is begin ... code manipulating the value of Count pragma Assert (Count = Count'Old + 1); end Incr; end Old_Pkg;
Note that it is allowed to apply 'Old to a constant entity, but this will result in a warning, since the old and new values will always be the same.