Unchecked_Conversion in Ada

Question

Can anyone please make me clear about the use of unchecked conversion in the Ada language.I had tried the pdf and net but all doesn't give me a clear picture to me.

Now I have a small piece of code shown below:

subtype Element4_Range is integer range 1..4;
subtype Element3_Range is integer range 1..3;
subtype Myarr_Range is integer range 1..10;
type Myarr3_Type is array (Myarr_Range) of Element3_Range;
type Myarr4_Type is array (Myarr_Range) of Element4_Range;
Myarr3 : Myarr3_Type;
Myarr4 : Myarr4_Type := (1, 2, 3, 3, 1, 3, 2, 1, 2, 1);
Count_1 : Integer := 0;
Count_2 : Integer := 0;
Count_3 : Integer := 0;
*function To_Myarr3 is new Unchecked_Conversion(Myarr4_type, Myarr3_type);*

Now my doubt here is what does the function Myarr3 exactly do?

Answer 1

An instantiation of Unchecked_Conversion copies the bytes of the source value to the target without checking whether this is sensible. Some compilers will warn (depending maybe on compilation options) if the values are of different sizes.

Element3_Range and Element4_Range are both based on Integer and will use the same number of bytes; so both of your array variables (Myarr3, Myarr4) will need the same number of bytes (40 typically).

You could write

Myarr3 := To_Myarr3 (Myarr4);

As it stands, nothing bad would happen because all the values you've used to initialise Myarr4 are legal as values of Element3_Range.

However, if you had

Myarr3 := To_Myarr3 (Myarr4'(1, 2, 3, 4, others => 1));

you'd end up with Myarr3(4) containing a value outside the legal range of Element3_Range, and with the compiler having no reason to believe that it might not be valid. This may well lead to Constraint_Errors down the line.

You could force a check yourself:

if not Myarr3 (4)'Valid then
  -- handle the error case

Answer 2

I had an coworker once who instisted that unchecked_conversion should have been named "Unchcked_Copy" instead. All it does is copy an object of one type into an object of another type.

So your To_Myarr3 routine will accept as a parameter an array of type Myarr4, pretend it is an array of type Myarr3, and then copy every element in it into the left-hand side of your expression.

If you want to change your view of an object from one type to another without copying the whole darn thing around, you can instead use an Unchecked_Conversion on access types for them (so you are only copying the pointer to them). Another method is using for object_name'address use at to overlay one on the other (however, they may both get initialized, which can be bad). But really the best way is to design your system's types so that you never have to use Unchecked_Conversion.