In almost all versions of GNAT, System.Address is a private type in accordance with the implementation advice in the RM. This means that integer values, in particular integer literals, are not allowed as address values. If the configuration pragma Allow_Integer_Address is given, then integer expressions may be used anywhere a value of type System.Address is required. The effect is to introduce an implicit unchecked conversion from the integer value to type System.Address. The reverse case of using an address where an integer type is required is handled analogously. The following example compiles without errors:
pragma Allow_Integer_Address; with System; use System; package AddrAsInt is X : Integer; Y : Integer; for X'Address use 16#1240#; for Y use at 16#3230#; m : Address := 16#4000#; n : constant Address := 4000; p : constant Address := Address (X + Y); v : Integer := y'Address; w : constant Integer := Integer (Y'Address); type R is new integer; RR : R := 1000; Z : Integer; for Z'Address use RR; end AddrAsInt;
Note that pragma Allow_Integer_Address is ignored if System.Address is not a private type. In implementations of GNAT where System.Address is a visible integer type, this pragma serves no purpose but is ignored rather than rejected to allow common sets of sources to be used in the two situations.