What is the 'Address of an Ada subprogram passed as an access parameter? -

compiling following program without optimization, , running it, see program_error,

raised program_error : addreq.adb:16 explicit raise 

or, updating in view of simon wright's answer,

raised program_error : using address 

this happens when using gnat gpl 2014 on either mac os x or on gnu/linux x84_64, on linux, strangely, program. other versions of gnat produce code doesn't raise, older compilers not accept (access-to-function parameters being more recent, i'm not surprised). since program needs identify addresses of subprograms, hoping definitive statement in rm; 13.3 (11) has ramifications grow waivers, iinm. so, referring program below, would

yes_no.all'address = no'adress 

be true statement if interpreted lrm? legit? yes_no.all proper way refer function no (if taking 'address)? there indirection through different pointer types, 1 having deeper accessibility, change picture? thinking yes_no.all should yield same 'address no, apparently not, compilers.

with system;  procedure addreq     function no (ignored : integer) return boolean    begin       return false;    end no;     procedure taking      (yes_no : access function (n : integer) return boolean)          use type system.address;    begin       if yes_no.all'address /= no'address          raise program_error;       end if;    end taking;  begin    taking (no'access); end addreq; 

one more update: if make addreq package , have subprogram call taking, thus

with addreq;  --  package procedure driver    use addreq; begin    taking (no'access); end driver; 

then no exception raised.

if yes_no.all'address not equal no'address, yes_no.all'address address of kind of wrapper code.

no function nested inside procedure. if no'access, compiler cannot create one-word pointer value address of no. reason when code makes indirect call through access value, code has special no can access local variables belonging addreq, somewhere on stack. example, 1 way provide access pass static link parameter no; pointer points addreq's stack frame, contain local variables (or along lines). thus, when indirect call made through access, caller has know static link is. 1 solution make nested access-to-function types dope vectors, contain function address , static link. generate wrapper code. wrapper code responsible calling called subprogram static link parameter, , access value one-word pointer, address of wrapper code. believe gnat takes approach. advantage makes possible pass my_function'access parameter c function, use callback. when c code calls through function pointer, calls wrapper function calls nested function correct static link. there significant amount of public ada code depends on mechanism. (gtkada relies heavily on it.)

however, if access value points wrapper, instead of actual function, the_access.all'address won't return think should. when code executes the_access.all'address, if the_access single word address in it, that's attribute can return--the address in pointer.

more: don't know whether original code part of larger example or test see compiler does. comparing 'address values see if subprogram-access parameter refers specific subprogram strikes me poor design, , comparing 'access no better. avoid doing in c. there's more object-oriented solution problem (note can use tagged types cause indirect subprogram calls take place, because tagged type operations dispatching). e.g.

type boolean_function_object abstract tagged null record; function apply (obj : boolean_function_object; n : integer) return boolean; function is_constant_false (obj : boolean_function_object) return boolean;  type no_function new boolean_function_object null record; overriding function apply (obj : no_function; n : integer) return boolean (false); overriding function is_constant_false (obj : no_function) return boolean (true);  procedure taking (func : boolean_function_object) begin     if not func.is_constant_false         raise program_error;     end if; end taking; 

might not best design in cases, should considered if there seems need check subprogram address particular subprogram. 1 thing, more flexible; programmer can define derived type apply return false else, such writing argument log file.