Re: C bug of the day

["Terje Slettebø" <tslettebo@chello.no.nospam>]

"James Rogers" <jimmaureenrogers@att.net> wrote in message
news:Xns9398C378C97C4jimmaureenrogers@204.127.36.1...
> "Balog Pal" <pasa@lib.hu> wrote in news:3ee8901a@andromeda.datanet.hu:
>
>  > "James Rogers" <jimmaureenrogers@att.net> wrote in message
>  > news:Xns9397C17B49692jimmaureenrogers@204.127.36.1...
>  >
> What C++ allows, and provides for "free" with the STL, is the
> ability to wrap the C style primitives such as arrays in a class,
> with all the bounds checking built into the class.

There's no mandated bounds checking in the standard library (except
where explicitly noted such as std::string's at() function), but an
implementation might provide it, anyway, especially in debug builds.

> This provides safety at the expense of some efficiency. Although
> the STL is part of the language definition, it is not part of the
> language syntax. A C++ compiler cannot optimize out bounds checks
> from STL or other container classes.

It might, if it's able to see that the values are always within the
bounds.

> Ada goes a bit further in this area than most C++ programs.
> Ada allows you to define your own numeric types (as primitives, not
> classes). You are allowed to define several characteristics about
> these types.
>
> type My_Index is range 1..10;
>
> The above line defines an integer type with a valid range of values
> from 1 through 10 inclusive. Now, let's combine that definition with
> an array type definition:
>
> type Nums is array (My_Index) of Float;
>
> This defines an array type indexed by the type My_Index. Each element
> of the array is a Float.
>
> The compiler can now optimize out bounds checking for accessing
> an array of Nums. The only index type allowed is type My_Index.
> Every possible value of that type maps to an array element.

You could also do this in C++, by creating such range types.

In Ada, like in C++, there would likely be bounds checking at a
different place, instead, though: in the range class, itself, ensuring
that operations on it doesn't result in values outside the range. So you
haven't really removed the range checking, just moved it. The array
doesn't need to check, but the range class does.

> Trying to assign an out of range value to a variable of My_Index
> causes the program to raise the exception Constraint_Error at run
time.
>
> Note that it would be illegal to initialize a variable of My_Index to
> 0. 0 is not a valid value for My_Index.

This is what I meant.


Regards,

Terje

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