I am making a program in Ada for Data Structures and Algorithms class.
My current problem is the error 'actual for "this" must be a variable'
I did some looking around and I read about it being because of in out mode, but I'm not entirely grasping why it's happening to me I guess.
The Examples I seen made sense but I guess since it's my coding I'm just not seeing it?
Procedure AddUnmarked(g:Grid; this:Linked_List_Coord.List_Type; c:Cell) is
cNorth : Cell := getCell(g, North(c));
cEast : Cell := getCell(g, East(c));
cSouth : Cell := getCell(g, South(c));
cWest : Cell := getCell(g, West(c));
Begin
if CellExists(g, cNorth) and not cNorth.IsMarked then
Linked_List_Coord.Append(this, cNorth.Coords);
elsif CellExists(g, cEast) and not cEast.IsMarked then
Linked_List_Coord.Append(this, cEast.Coords);
elsif CellExists(g, cSouth) and not cSouth.IsMarked then
Linked_List_Coord.Append(this, cSouth.Coords);
elsif CellExists(g, cWest) and not cWest.IsMarked then
Linked_List_Coord.Append(this, cWest.Coords);
end if;
End AddUnmarked;
before "this" is passed to the function it is a Linked_List of my self defined type Coord (2 integers). It is initialized and has had a Coordinate pair added to it before the list is passed to the function above in my code.
What it means is that the list cannot be modified unless you are passing it as a modifiable parameter, that is, in out.
To elaborate, think of LIST_TYPE as being a handle to a tagged-type object; in order to ensure that LIST_TYPE is valid you need to pass it in via an in parameter (or create/manipulate a local object), but to pass out your results you need an out parameter.
So, in order to do your operations on an already-existing object {and get the results back} you need in out.
In Ada, subroutine parameters all have a usage mode associated with them. The available modes are in, out, and in out*. If you don't specify a mode, (like you didn't in your code), then it defaults to in only.
The modes specify what you can do with that parameter on the inside of the subprogram. If you want to read a value passed in from outside the routine, it must have in on it. If you want to write to the parameter (and/or possibly have it read outside the routine), then it must have out on it.
Since none of your parameters have out on them, you cannot write to any of them.
(* - There's another possible mode: access, but that's an advanced topic).
Related
Assume having the following setup:
type My is new Integer;
type My_Acc is access My;
procedure Replace(Self : in out My_Acc; New_Int : Integer)
with Pre => New_Int /= Self.all, Post => Self'Old.all /= Self.all;
Note: Code above might not be fully valid, but I hope the concept is understandable.
Now what happens if Unchecked_Deallocation() is used on Self inside Replace
and a new Integer is allocated and set to Self (This should result in Self'Old pointing to a now invalid memory location)?
Does Ada keep kind of a snapshot where Self'Old points to the previous memory location, but before Unchecked_Deallocation() is executed?
If Self'Old would get invalid for use in the Post contract, how could you still access the previous value? Is it possible to create a manual snapshot in the Pre contract that can then be used in Post? Maybe it can be achieved using Ghost_Code?
I want to make everything in Spark, in case that changes something.
Edit: Fixed Self to in out as mentioned by Simon Wright.
Edit: Fixed type of Self to allow null
It may be that the latest versions of SPARK support access types; it used not to, at all.
First, your Not_Null_My_Acc needs to be a subtype of My_Acc, unless you meant it to be a type in its own right.
Second, you can’t deallocate Self inside Replace and allocate a new value; Self is in-mode, & hence not writable.
Third, you can’t apply ’Old to Self, because
warning: attribute "Old" applied to constant has no effect
What you can say is
Post => Self.all'Old /= Self.all;
In ARM 6.1.1(26ff) it says
Each X'Old in a postcondition expression that is enabled denotes a constant that is implicitly declared at the beginning of the subprogram body, entry body, or accept statement.
The implicitly declared entity denoted by each occurrence of X'Old is declared as follows:
...
X'Old : constant S := X;
... in other words, nothing fancy is expected, just a straight copy of (in this case) Self: not Self.all.
So, if your Replace deallocates Self, then Self’Old is a dangling reference, and erroneous.
I suggested previously that changing the postcondition to
Post => Self.all'Old /= Self.all;
would be safe; why wouldn’t that meet your requirements? is there something going on you haven’t told us about?
Note the subtle difference between Self’Old.all and Self.all’Old. The first one takes a copy of Self as it was before the call, which gets dereferenced after the call (by which time it’s pointing into hyperspace), while the second one dereferences the prior Self and copies the integer value it finds there; on return that’s still valid.
I am refactoring some code originally written using access types, but not yet tested. I found access types to be troublesome in Ada because they can only refer to dynamically allocated items, and referring to items defined at compile time is apparently not allowed. (This is Ada83.)
But now I come to a function like this one:
function Get_Next(stateInfo : State_Info_Access_Type) return integer;
I know that I can easily pass parameter "contents" of an access type rather than the access pointer itself, so I am considering writing this as
function Get_Next(stateInfoPtr : State_Info_Type) return integer;
where State_Info_Type is the type that State_Info_Access_Type refers to.
With this refactor, for all intents and purposes I think I'm still really passing what amounts to an implicit pointer back to the contents (using the .all) syntax).
I want to refactor and test starting with the lowest level functions, working my way up the call chains. My goal is to push the access types out of the code as I go.
Am I understanding this correctly or am I missing something?
I think original author(s), and possibly OP are missing a point, that is, how Ada parameter modes work.
To quote #T.E.D
Every Ada compiler I know of under the hood passes objects larger than fit in a machine register by reference. It is the compiler, not the details of your parameter passing mechanisim, that enforces not writing data back out of the routine.
Ada does this automatically, and leaves the parameter modes as a way of describing the flow of information (Its NOT the C style reference / value conundrum). See the useful wikibook.
What worries me is that the code you have inherited looks like the author has used the explicit access parameter type as a way of getting functions to have side effects (usually considered a bad thing in Ada - World).
My recommendation is to change your functions to:
function Get_Next(State_Info : in State_Info_Type) return Integer;
and see if the compiler tells you if you are trying to modify State_Info. If so, you may need to change your functions to procedures like this:
procedure Get_Next(State_Info : in out State_Info_Type;
Result : out Integer);
This explicitly shows the flow of information without needing to know the register size or the size of State_Info_Type.
As an aside Ada 2012 Will allow you to have functions that have in out parameters.
To quote #T.E.D,
Every Ada compiler I know of under the hood passes objects larger than fit in a machine register by reference. It is the compiler, not the details of your parameter passing mechanisim, that enforces not writing data back out of the routine.
Since this code hasn’t yet been tested, I think you are completely right to rework what looks like code written with a C mindset. But, you oughtn’t to mention pointers at all; you suggested
function Get_Next(stateInfoPtr : State_Info_Type) return integer;
but this would be better as
function Get_Next(stateInfo : State_Info_Type) return integer;
or even (IMO) as
function Get_Next(State_Info : State_Info_Type) return Integer;
to use more standard Ada styling! My editor (Emacs, but GPS can do this too) will change state_info into State_Info on the fly.
As an afterthought, you might be able to get rid of State_Info_Type_Pointer altogether. You mention .all, so I guess you’ve got
SITP : State_Info_Type_Pointer := new State_Info_Type;
... set up components
Next := Get_Next (SITP.all);
but what about
SIT : State_Info_Type;
... set up components
Next := Get_Next (SIT);
I wouldn't recommend this, but you can get pointers to variables in Ada 83 by using 'Address.
You can then use overlays (again this is all Ada83 stuff) to achieve access...
function something(int_address : Address) return integer
is
x : integer;
for x'address use int_address;
begin
-- play with x as you will.
I have a complicated situation so I hope I can explain it properly.
I am using unchecked_access in combination with Ada.Finalization.Controlled types to pass references to vectors to an out of scope protected hashed map type on the Initialize procedure and removing references in the protected map on the finalize procedure. This way I thought I could assure that it would be impossible for any tasks to see out of scope references in the map.
However, the way I have the protected map organized snap shots are taken at each instance that is it used (effectively making it a normal hashed_map) which during the course of it being used vector references could go out of scope and the snap shot would still have the reference and attempt to access it.
I could see this creating 2 problems, either I am creating dangling pointers and trying to reference freed memory or my snapshot of the references is keeping the memory alive and I am leaving garbage around. I made a test to see what would happen and it seems the memory is still alive but is this really the case? Are there any other problems with my organization?
with Ada.Containers; use Ada.Containers;
with Ada.Containers.Vectors;
with Ada.Text_IO; use Ada.Text_IO;
procedure Test is
package V_Integer is new Ada.Containers.Vectors(Positive, Integer);
use V_Integer;
type V_Access is access all Vector;
Bar : V_Access;
begin
declare
Foo : aliased Vector;
begin
Bar := Foo'unrestricted_Access;
Foo.Append(3);
Foo.Append(5);
Put_Line("In scope: " & count_type'image(Length(Bar.all)));
end;
-- Will this reference always exist? Does it need to be freed?
Put_Line("Out of scope: " & count_type'image(Length(Bar.all)));
end Test;
begin
declare
Foo : aliased Vector;
begin
Bar := Foo'unrestricted_Access;
Foo.Append(3);
Foo.Append(5);
Put_Line("In scope: " & count_type'image(Length(Bar.all)));
end;
-- Will this reference always exist? Does it need to be freed?
Put_Line("Out of scope: " & count_type'image(Length(Bar.all)));
end Test;
Foo will be an object of type Vector, and it will probably exist on the stack. This object is itself a relatively small record, maybe about 6 32-bit words in GNAT (give or take a few, I haven't checked). It contains an access component that is used to get at all the vector's elements, and it contains some other housekeeping information. Bar will point to that small 6-word record. Essentially it will contain the address of something on the stack.
Since the small record Foo exists on the stack, when it goes out of scope, the stack space could be used for something else. Whether it will happen in this case, I don't know. But if you have another declare...begin...end block after the one in the example, local variables used by the new block could reuse the same stack space. Or if a procedure is called, that will put different stuff on the stack. In either case, the stack space previously used by Foo will be overwritten. And Bar will still point to the same address, but the area it points to will have been overrun with other data. So the result is likely to be wrong and could be a disaster.
I don't know if this is exactly how the implementation will work, but no matter how it's implemented, this code is potentially disastrous. The main things to keep in mind are: (1) If a variable goes out of scope, the space used for that variable may be reused at will; (2) if you use 'Unchecked_Access (or 'Unrestricted_Access) to set up a pointer to that variable, then after the variable goes out of scope, accessing data pointed to by the pointer could get you garbage or very bad behavior; (3) variables, whether aliased or not, are not (usually) dynamically allocated and the program does not need to (and cannot) explicitly free them, even if you've created a reference to them with 'Unchecked_Access.
I have a specification of a function that acts like a constructor. The specification of the function is
function Create_Controller return Type_Controller;
Also, in the specification file, I have the Type_Controller type, which is an access. I copy the relevant fragment:
type Type_Controller_Implementation;
type Type_Controller is access Type_Controller_Implementation;
So, this is what I've attempted:
function Create_Controller return Type_Controller
is
My_Controller : aliased Type_Controller_Implementation;
begin
return My_Controller'Access;
end Create_Controller;
I tried to compile the program without the aliased keyword, but then, the compiler says:
prefix of "Access" attribute must be aliased
So, I put the aliased keyword and the compiler now suggests that I should change the specification:
result must be general access type
add "all" to type "Controlador_De_Impresion" defined at controller.ads
The problem is that I'm not allowed to change the specification. I've read the chapter about access types in the Ada Programming Wikibook, but I still don't understand why my code doesn't work. What am I doing wrong?
The implementation of the Create_Controller function body is incorrect. Were it to work as coded, you'd be returning a pointer to a variable local to that function body's scope...which would be immediately lost upon returning from the function, leaving you with an invalid pointer.
No, an instance of the type needs to be allocated and returned. If there's no explicit initialization that needs to occur you can simply do:
return new Type_Controller_Implementation;
If there is some initialization/construction that has to occur, then:
function Create_Controller return Type_Controller
is
My_Controller : Type_Controller := new Type_Controller_Implementation;
begin
-- Set fields of My_Controller
...
return My_Controller;
end Create_Controller;
When you declare an access type as access T, you're saying that "this is a pointer to a T and it must point to objects of type T allocated from a pool". (That is, allocated with a new keyword.) When you declare an access type as access all T, you're saying that it can point either to a T allocated from a pool, or to an aliased variable of type T.
If the type is declared as access T and you can't change it, then all access values of the type have to point to something allocated with new. You can't make it point to a variable (even to a "global" variable that isn't located on the stack).
The reasons for this are historical, I think. The first version of Ada (Ada 83) only had "pool-specific types." You couldn't make an access value point to some other variable at all, without trickery. This meant that a compiler could implement access values as indexes into some storage block, or as some other value, instead of making them the actual address of an object. This could save space (an access value could be smaller than an address) or allow more flexibility in how pool memory was managed. Allowing access values to point directly to objects takes away some of that flexibility. I think that's why they decided to keep the old meaning, for backward compatibility, and require an all keyword to indicate the new kind of access.
I have a variable in a package (rec in this case) that needs to be set when called from package 3, but it's private. Previously the function set_true only set rec to true, so it wasn't a big deal. But I have another package that does the same processing (I'm giving a simple example, but my literal case is more complex), so I thought, well I could pass in the variable I want modified, and let it get changed. Is the only way to set rec in the below layout, to create a second function in package one, that calls set_true with rec as the parameter? I would like to avoid having to keep creating additional functions to handle the local variables. I can't move the variable to public (spec) as I am trying to follow convention and this "type" of variable isn't public anywhere else, and I don't want anyone to be able to just set it on their own (I want functions to have to set). I don't want to have to create a second function named for example set_local_true, and creating an overloaded function set_true, with no parameters, that calls set_true(value => rec) just seems deceptive, does anyone have any better suggestions with the limitations I have?
My two requirements:
Can't make the local variable public.
Be able to use the function to calculate something both externally and internally.
package one is
procedure set_true(value : out Boolean);
end one;
package body one is
rec : Boolean;
begin
procedure set_true(value : out Boolean)
begin
value := true;
end set_true;
end one;
package body two is
local_rec : Boolean;
begin
procedure call_function is
begin
one.set_true(value => local_rec);
end call_function;
end two;
package body three is
begin
procedure call_function is
begin
one.set_true(value => <PACKAGE ONE'S REC))
end call_function;
end three;
EDIT: Or perhaps, what would be a better naming convention for the functions to specify that they are modifying the variable that is local to that package? Set_Local_True again is deceptive cause if you call it from package 3, you're not setting your local true, you're setting package one's local to true....
First off, this is very silly code. I'll assume it is shorthand for something else. But as presented, I can assure you that your clients can set their own booleans themselves without you writing a routine to do it for them. In fact, they can do it better. For the remainder of this answer, I'll assume you aren't acutally writing variables to set booleans for people, but rather doing something of actual use. If not, ignore the rest of this answer and just delete your silly routines.
Secondly, if you are creating a routine with a single out parameter, then unless the object happens to be very large, you should probably make it a function instead. That will allow your clients to use functional programming if they chose. The way you have it, the poor coder has to stop and create a special variable just to call your routine, even if they only want to do it once.
Thirdly, rather than using a unique set routine for each state, I generally prefer to pass in the requested state.
function Set_Frobnost (New_State : boolean := true) return boolean;
If the state is really and truly boolean (no possible third state in the future), then it is debateable. However, it can be a big advantage to your client if they might already have to store the state in a variable (or loop through it).
Your edit about naming shows me you are on the right track.
You should do one of two things here.
Find the higher-level concept controlled by that variable, and name the "setter" routine after that.
Get the hell out of the way and put the flag variable in the pacakge spec.
If you have to access private variables, you might to do it in a child package.
package One is
procedure Foo (X : Boolean);
private
One_Private : Boolean;
end One;
and then
package body One.Two is
procedure Bar is
One.Foo (One.One_Private);
end Bar;
end One.Two;
Elements in the "private" part of a package are like "protected" entities in C++/Java. Truly private variables (only in package body) are not accessible from anywhere else.