Re: Load an object from a file

[Olivier Scalbert <olivier.scalbert@algosyn.com>]

Hello,

I have an endianness problem. In the file, The u2 data are stored in 
big-endian order. As I am working on a Pentium, the reading is done in 
little-endian order. Is it possible to fix it with the representation 
stuff ?

Thanks,

Olivier

Ludovic Brenta wrote:
> 
> I would create a record type with one discriminant for each array,
> like so:
> 
> type Constant_Pool_Array is array (Positive range <>) of cp_info;
> type Interfaces_Array is array (Positive range <>) of u2;
> -- etc.
> 
> type Class_File
>   (constant_pool_count,
>    interfaces_count,
>    fields_count,
>    methods_count,
>    attributes_count : u2)
> is record
>    ...
>    constant_pool : Constant_Pool_Array (2 .. constant_pool_count);
>    ...
>    interfaces : Interfaces_Array (1 .. interfaces_count);
>    ... etc.
> end record;
> 
>> Also how can I fill this array ?
> 
> You would normally simply call the predefined Class_File'Read but this
> wouldn't work since the order of the components in type Class_File
> does not match the order in the file. So, you'd specify your own Read:
> 
> function Input(
>    Stream : not null access Ada.Streams.Root_Stream_Type'Class)
>    return Class_File); -- see RM 13.13(22) and following
> for Class_File'Input use Input; -- as per RM 13.13(38/2)
> 
> function Input(
>    Stream : not null access Ada.Streams.Root_Stream_Type'Class)
>    return Class_File)
> is
>    Constant_Pool_Count : u2;
>    type Constant_Pool_Array_Access is access Constant_Pool_Array;
>    procedure Free is new Ada.Unchecked_Deallocation
>      (Constant_Pool_Array, Constant_Pool_Array_Access);
>    Constant_Pool : Constant_Pool_Array_Access;
> begin
>    ...
>    u2'Read (Stream, Constant_Pool_Count);
>    Constant_Pool := new Constant_Pool_Array (1 .. Constant_Pool - 1);
>    Constant_Pool_Array'Read (Stream, Constant_Pool.all);
>    ...
> 
> After reading all members, construct the result:
> 
> declare
>    Result : Class_File
>   (constant_pool_count => Constant_Pool_Count,
>    interfaces_count => Interfaces_Count,
>    fields_count => Fields_Count,
>    methods_count => Methods_Count,
>    attributes_count => Attricutes_Count);
> begin
>    Result.Constant_Pool := Constant_Pool.all;
>    ...
>    Free (Constant_Pool);
>    ...
>    return Result;
> end;
> end Input;
> 
> You can also eliminate the use of access types and dynamic allocation
> and deallocation by nesting declare blocks, e.g.
> 
> u2'Read (Stream, Constant_Pool_Count);
> declare
>    Constant_Pool : Constant_Pool_Array (1 .. Constant_Pool_Count);
> begin
>    Constant_Pool_Array'Read (Stream, Constant_Pool);
>    ...
>    u2'Read (Stream, Interfaces_Count);
>    declare
>       Interfaces : Interfaces_Count_Array (1 .. Interfaces_Count);
>    begin
>       ...
>    end;
> end;
> 
> HTH
> 
> --
> Ludovic Brenta.