why two aliases to "array of string" treated differently? - delphi

In Pascal there are two kinds of type declarations:
type aliases: type NewName = OldType
type creation: type NewType = type OldType
The former is just creating convenient shorthand, like typedef in C. The aliases are compatible one to another and to their original type. The created types are intentionally incompatible and cannot be mixed without explicit and unsafe by definition typecast.
var
nn: NewName; nt: NewType; ot: OldType;
...
nn := ot; // should work
nt := ot; // should break with type safety violation error.
nt := NewType(ot); // Disabling type safety. Should work even if
// it has no sense semantically and types really ARE incompatible.
Those are Pascal basics as i understand them.
Now let's look at one certain type and two its aliases:
System.Types.TStringDynArray = array of string;
System.TArray<T> = array of T;
in particular that means TArray<string> = array of string; by definition.
Now let's take function returning the former type alias and feed its result to the function expecting the latter one:
uses Classes, IOUtils;
TStringList.Create.AddStrings(
TDirectory.GetFiles('c:\', '*.dll') );
TStringList.Create.AddStrings(
TArray<string>( // this is required by compiler - but why ???
TDirectory.GetFiles('c:\', '*.dll') ) );
1st snippet would not compile due to types violation.
2nd one happily compiles and works, but is fragile towards future type changes and is redundant.
QC tells that compiler is right and the RTL design is wrong.
http://qc.embarcadero.com/wc/qcmain.aspx?d=106246
WHY compiler is right here ?
Why those aliases are incompatible ?
Even the very manner RTL was designed suggests that they were deemed compatible!
PS. David suggested even simplier example, without using TArray<T>
type T1 = array of string; T2 = array of string;
procedure TForm1.FormCreate(Sender: TObject);
function Generator: T1;
begin Result := T1.Create('xxx', 'yyy', 'zzz'); end;
procedure Consumer (const data: T2);
begin
with TStringList.Create do
try
AddStrings(data);
Self.Caption := CommaText;
finally
Free;
end;
end;
begin
Consumer(Generator);
end;
Same gotcha without explanation...
PPS. There are a number of doc refs now. I want to stress one thing: while this restriction might be indirectly inherited from Pascal Report of 1949, today is 2012 and Delphi used very differently from school labs of half-century ago.
I named few BAD effects of keeping this restrictions, and yet did not saw any good one.
Ironic thing, that this restricion may be lifted without breaking rules of Pascal: in Pascal there is no such non-strict beast as Open Arrays and Dynamic Arrays. So let those original fixed arrays be restricted as they wish, but Open Arrays and Dynamic Arrays are not Pascal citizens and are not obliged to be limited by its codebook!
Please, communicate Emba in QC or maybe even here, but if u just pass by without expressing your opinion - nothing would change!

The key to understanding this issue is the Type Compatibility and Identity topic in the language guide. I suggest you have a good read of that topic.
It is also helpful to simplify the example. The inclusion of generics in the example serves mainly to complicate and confuse matters.
program TypeCompatibilityAndIdentity;
{$APPTYPE CONSOLE}
type
TInteger1 = Integer;
TInteger2 = Integer;
TArray1 = array of Integer;
TArray2 = array of Integer;
TArray3 = TArray1;
var
Integer1: TInteger1;
Integer2: TInteger2;
Array1: TArray1;
Array2: TArray2;
Array3: TArray3;
begin
Integer1 := Integer2; // no error here
Array1 := Array2; // E2010 Incompatible types: 'TArray1' and 'TArray2'
Array1 := Array3; // no error here
end.
From the documentation:
When one type identifier is declared using another type identifier, without qualification, they denote the same type.
This means that TInteger1 and TInteger2 are the same type, and are indeed the same type as Integer.
A little further on in the documentation is this:
Language constructions that function as type names denote a different type each time they occur.
The declarations of TArray1 and TArray2 fall into this category. And that means that these two identifiers denote different types.
Now we need to look at the section discussing compatibility. This gives a set of rules to follow to determine whether or not two types are compatible or assignment compatible. We can in fact shortcut that discussion by referring to another help topic: Structured Types, Array Types and Assignments which states clearly:
Arrays are assignment-compatible only if they are of the same type.
This makes it clear why the assignment Array1 := Array2 results in a compiler error.
Your code looked at passing parameters, but mine focused on assignment. The issues are the same because, as the Calling Procedures and Functions help topic explains:
When calling a routine, remember that:
expressions used to pass typed const and value parameters must be assignment-compatible with the corresponding formal parameters.
.......

Delphi is a strongly typed language. That means that identical (in this case I mean their definitions look exactly the same) types are not assignment compatible.
When you write array of <type> you are defining a type and not an alias. As David already said in his comment the two identical types like
type
T1 = array of string;
T2 = array of string;
are not assignment compatible.
Same goes for
type
TStringDynArray = array of string;
TArray<T> = array of string;
Often people forget about the incompatibility of identical types and my guess would be that they did when they introduced IOUtils for example. Theoretically the definition of TStringDynArray should have been changed to TStringDynArray = TArray<string> but I guess that could have raised other problems (not saying bugs with generics...).

I also had the same problem with Delphi, where I wanted to pass values from one identical array to another. Not only did I have "incompatibility" problems with two like array assignments, but I also could not use the "Copy()" procedure. To get around this problem, I found that I could use a pointer to an type array of array of string, instead.
For example:
type RecArry = array of array of string
end;
var TArryPtr : ^RecArry;
Now, I can pass the values from any fixed array to another identical array without any compatibility or function problems. For example:
TArryPtr := #RecArry.LstArray //This works!
TArryPtr := #LstArray //This also works!
With this created array assignment template, I can now work with all two dimensional arrays without any problems. However, it should be understood, that when accessing this type of string array pointer, an extra element is created, so that when we would expect this type of array 2D array below, for example:
Two_Dimensional_Fixed_Array[10][0]
We now get an extra element adjusted array as seen here:
New_Two_Dimensional_Fixed_Array[10][1]
This means that we have to use some slightly tricky code to access the pointer array, because all the populated elements in Two_Dimensional_Fixed_Array[10][0] have moved down, so that they are offset by 1, as in New_Two_Dimensional_Fixed_Array[10][1].
Therefore where we would normally find the value 'X' in Two_Dimensional_Fixed_Array[1][0], it will now be found here in TArryPtr[0][1].
Its a trade off we all have to live with!
Another important note to bear in mind is the definition of a pointer array when it is declared. When a pointer array is type declared, the Borland compiler will not allow the Pointer array to have the same element size as the array to which it is pointing too. For example, if an array is declared as:
Orig_Arry : array [1..50,1] of string;
The pointer array which should point to it would be declared in the following fashion:
Type Pntr_Arry : array [1..50,2] of string;
Did you notice the the extra element? I am guessing the the Borland compiler has to widen the array pointer to allow for the pointer address.

Related

Delphi SetLength for array of Integer

I want to write a procedure that appends to an array of Integer, but Delphi IDE gives me compile-time error 'Incompatyble types'. This is my code :
procedure appendToIntegerArray(var intArr : array of Integer; valueToAppend : Integer);
var
counter : Integer;
begin
counter := Length(intArr);
SetLength(intArr, counter + 1); // This is where it gives me the compile-time error
intArr[counter] := valueToAppend;
end;
Can anyone help me fix my procedure ?
You are facing the error because SetLength operates on dynamic arrays and that is not a dynamic array. That is an open array parameter.
You'll need to use a dynamic array instead:
procedure appendToIntegerArray(var intArr: TArray<Integer>; valueToAppend: Integer);
....
If you use an older Delphi that does not support generic arrays, you'll need to declare a type for the array:
type
TIntegerArray = array of Integer;
procedure appendToIntegerArray(var intArr: TIntegerArray; valueToAppend: Integer);
....
Or you could use the type declared in the RTL, TIntegerDynArray. This type is declared in the Types unit.
One of the irritations of Delphi's type system is that a type like TIntegerArray is not assignment compatible with, say, TIntegerDynArray because they are distinct types. That makes it harder than it should be to work with code that uses distinct array types. One of the great benefits of generic arrays is that the rules for type compatibility of generics are a little more forgiving. So, if you can use TArray<T>, do so.
If you are appending item by item, then I would generally suggest that a list class would be better. In this case you would use TList<Integer> and simply call its Add method.
It's one of the quirks of Delphi's syntax: declaring an array of whatever as a function parameter does not define it as an array like you might think, but as an open array, a "magic" type that can accept any array of the correct base type. If you want to use a specific type of array, you need a named type.
This is the sort of thing that the generic TArray<T> was designed for. Make your parameter a TArray<integer> if you can. If not, consider updating to a newer version of Delphi (you're missing out on a whole lot if you're still on an older version!) and in the meantime, declare an array type like so:
type
TMyIntegerArray = array of integer;
And use that type as your parameter type, rather than declaring array of integer there.

Why are two seemingly identical dynamic array types deemed not assignment compatible?

Just a little question, I'm not finding a specific answer so i guessed it might be faster to ask here.
The compiler rejects the code below with the following error:
incompatible types 'dynamic array' and 'array of string'
TMailInfo = record
FileName,
MailAdresse,
MailBCC,
MailCC,
MailBetreff: string;
MailText,
Anhang: array of string;
MailAcknowledge,
MailTXT: Boolean
end;
class function TEMail.SendOutlookCOMMail(aFileName, aMailAdresse,
aMailBCC, aMailCC, aMailBetreff: string;
aMailText, aAnhang: array of string;
const aMailAcknowledge, aMailTXT: Boolean): Boolean;
var
mailInfo: TMailInfo;
begin
...
mailInfo.MailBetreff := aMailBetreff; // these two lines cause the error
mailInfo.MailText := aMailText;
...
end;
What am I doing wrong? Both are arrays of string, so I don't get why one seems to be dynamic.
You cannot readily assign to MailText and Anhang because you cannot declare another object with a compatible type. That's because you used a dynamic array type inline in your record declaration. You really need to use a type that can be named. To illustrate a bit better, consider this:
X: array of Integer;
Y: array of Integer;
Now X and Y are of different types and X := Y does not compile.
The other problem is your open array parameter. An open array parameter is assignment compatible with nothing. You can copy element by element, but you cannot assign the array in one go.
The best way out of this is to declare the field like this:
MailText,
Anhang: TArray<string>;
And change the open array parameters in the function to also be of that type.
Then you need to decide whether you want to copy the reference or the array:
MailText := aMailText; // copy reference, or
MailText := Copy(aMailText); // copy array
You have two problems. First, as David says, two declarations of a type that look the same, but are declared separately, make them different, assignment incompatible types.
Weirdly enough, this is not so for generic types, like TArray<string>, so it makes sense to use that, if you version of Delphi supports it.
But the second problem is that you are confusing an open array parameter like aMailText with a dynamic array like mailInfo.MailText. aMailText, the parameter, is not necessarily a dynamic array at all, it can be any kind of array.
Take a look at the documentation: Open Arrays
Another explanation: Open array parameters and array of const

Is there any way to get RTTI hints for a real48 and shortstring variable in a structure where FieldType is nil in TRttiField?

I have discovered what I think is an odd oversight (probably intentional) on the part of the Extended RTTI feature in Delphi.
I would like to dump all the fields in an record type that has about 1500 different fields in it. Yes, seriously.
Some of them are of type real48 and some are shortstring, for those two, it appears that FieldType is nil for these types at runtime:
function TRttiField.GetValue(Instance: Pointer): TValue;
var
ft: TRttiType;
begin
ft := FieldType;
if ft = nil then
raise InsufficientRtti; // This fires!
TValue.Make(PByte(Instance) + Offset, ft.Handle, Result);
end;
If I was willing to assume that all nil-fieldtype fields are in fact real48's, I could simply use the offset and (if the field width is 6) grab a real48 value.
However the second complication is that all shortstring (ie string[30]) types are similarly afflicted.
Has anybody got these two Ancient Pascal Types to work with modern Extended RTTI?
Right now I'm using a best-guess approach, and where that fails I am hardcoding rules by name of the field, but if there was some technique I could use that would get me there without having to write a lot of code to extract information from all these old pascal file-of-records that I am modernizing, I would appreciate a better idea.
Unfortunately Real48 does not have any type info.
You can see that when you try compile this:
program Project1;
begin
TypeInfo(Real48);
end.
The same goes for the string[n] syntax. But there you could probably fix it by defining your own string types like:
type
string30 = string[30];
That alone would not include the rtti for the record field so you need to hack/fix the rtti as I showed here: https://stackoverflow.com/a/12687747/587106

What are the reasons to use TArray<T> instead of Array of T?

I'm migrating a legacy Delphi application to Delphi-XE2, and I'm wondering if there's a good reason to replace the arrays defined as Array of MyType to TArray<MyType>. So the question is what are the pros and cons of TArray<T> usage instead of Array of MyType?
The main advantage is less onerous type identity rules. Consider:
a: array of Integer;
b: array of Integer;
These two variables are not assignment compatible. It is a compiler error to write:
a := b;
On the other hand if you use the generic syntax:
a: TArray<Integer>;
b: TArray<Integer>;
then these two variables are assignment compatible.
Sure, you can write
type
TIntegerArray = array of Integer;
But all parties need to agree on the same type. It's fine if all code is in your control, but when using code from a variety of sources, the advent of generic dynamic arrays makes a huge difference.
The other advantage that springs to mind, in similar vein, is that you can readily use the generic array type as the return type of a generic method.
Without the generic array you are compelled to declare a type of this form:
TArrayOfT = array of T
in your generic class, which is rather messy. And if you are writing a generic method in a non-generic class, then you've no way to make that declaration. Again the generic array solves the problem.
TMyClass = class
class function Foo<T>: TArray<T>; static;
end;
This all follows on from type compatibility rules described in the documentation like this:
Type Compatibility
Two non-instantiated generics are considered assignment
compatible only if they are identical or are aliases to a
common type.
Two instantiated generics are considered assignment
compatible if the base types are identical (or are aliases to a
common type) and the type arguments are identical.
You can initialize TArray<T> with values with one construct:
var
LArray: TArray<Integer>;
begin
LArray := TArray<Integer>.Create(1, 2, 3, 4);
For array of Integer you would need to write much more code:
var
LArray: array of Integer;
begin
SetLength(LArray, 4);
LArray[0] := 1;
LArray[1] := 2;
LArray[2] := 3;
LArray[3] := 4;
It comes in handy for function results.
Example:
The following is not allowed in Delphi. You need to declare a separate type here. What a waste of time.
function MyFunc:array of integer;
begin
end;
Wait, generics to the resque:
function MyFunc:TArray<integer>;
begin
end;

RTTI properties not returned for fixed enumerations: is it a bug?

I need to browse all published properties of some classes.
Properties where the type is an enumeration with fixed values are not listed.
See example below:
TMyEnum = (meBlue, meRed, meGreen);
TMyEnumWithVals = (mevBlue=1, mevRed=2, mevGreen=3);
TMyClass =
...
published
property Color: TMyEnum read FColor write SetColor; // This one is found
property ColorVal: TMyEnumWithVals read FColorVal write SetColorVal; // This one is never found
end;
I need fixed values because these properties are stored in a database and I need to ensure that allocated values will always be the same, regardless of Delphi compiler choices in next versions, and prevent any misplaced insert of future values in the enumeration list.
I tried with both new Delphi 2010 RTTI (with .GetDeclaredProperties) and "old" RTTI (with GetPropInfos): all properties are found except the above type of property.
The same behaviour is seen on all classes. I also reproduced this in a sample project.
Tried with and without various RTTI directives without change.
Is this a bug, a known limitation?
Is there a workaround (except removing fixed values of the enumeration)?
Using Delphi2010 Ent+Update5
[Edit] The answer below provides the workaround: The first value of the enumeration must be set to 0 instead of 1, and values contiguous. Tested and working solution.
Thanks,
Barry Kelly says:
Discontiguous enumerations and enumerations which don't start at zero don't have typeinfo. For typeinfo to be implemented, it would need to be in a different format from the existing tkEnumeration, owing to backward compatibility issues.
I considered implementing a tkDiscontiguousEnumeration (or possibly better named member) for Delphi 2010, but the benefit seemed small considering their relative scarcity and the difficulties in enumeration - how do you encode the ranges efficiently? Some encodings are better for some scenarios, worse for others.
So it's not a bug, but a known limitation.
This is a limitation. Enumerated types with explicit ordinality can not have RTTI. Documented here.
Possible solution is to declare Reserved values within your enumerated type, eg:
TMyEnum = (meReserved1, meBlue, meRed, meGreen, meReserved2, meWhite); // and so on
...
Assert(MyEnum in [meBlue..meGreen, meWhite], 'sanity check failed')
Practical enums tends to occupy contiguous ranges, so you probably will not end up in having declaring gazillions of reserved values. In case you are interoperating with C style bitfields you will have to split value to distinct enums and sets.
To add some more background information, Enums with values, are treated as subrange types with predefined constants.
For example:
type
TMyEnumWithVals = (mevBlue=1, mevRed=2, mevGreen=3);
Is treated as:
type
TMyEnumWithVals = 1..3;
const
mevBlue : TMyEnumWithVals = 1;
mevRed : TMyEnumWithVals = 2;
mevGreen : TMyEnumWithVals = 3;
However, you can't combine these with integers without casting.
var
enum : TMyEnumWithVals;
ival : Integer;
begin
enum := mevBlue;
ival := Integer(mevRed);
To make matters worse, if you use:
type
TTypeId = (tidPrimary = 100, tidSecundary = 200, tidTertiary = 300);
TTypeArray = array [TTypeID] of string;
You get an array of 201 elements, not of 3. You can use the values in between using:
var
enum : TTypeId ;
ival : Integer;
begin
enum := TTypeId(150); // Is valid.
This information can be found in the Delphi language guide, with the enum type section.

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