Re: Class hierarchy of exceptions (Ada, C++)

[Tapio Kelloniemi <spam17@thack.org>]

Ioannis Vranos <ivr@remove.this.grad.com> wrote:
>Ioannis Vranos wrote:
>
>> Actually most compilers provide warnings for lots of stuff and provide
>> an option to display all warnings (e.g. /Wall), even comparison between
>> signed and unsigned integers generates a warning. And they can also
>> treat all warnings as *errors* if you specify so.
>>
>> However the default is to display warnings for anything that is not
>> required by the standard to be treated as an error, and compile it.
>>
>>
>> In most cases, one fixes *all* warnings. However there *are* some cases
>> where the programmer knows better.
>>
>> C++ is enabling by default, which I suppose is the opposite direction of
>> Ada, and that's why I think we can't understand one another. :-)
>>
>>
>> C++ being less restrictive as the default, doesn't mean a compiler does
>> not generate lots of warnings!

The question is not only about compiler warnings or errors. Ada (as a
language) has been designed so that it is possible for the
compiler to check many mistakes which may cause bad results at run time.
Ada also makes it easier for the user to notice this kind of errors.
For example:

procedure X is

   type Metres is new Natural;
   type Seconds is new Natural;

   M : Metrses := 0;
   S : Seconds := 10;
begin
   if M < S then --  Error, < is not defined for these types
      ...
   end if;
end X;

This is a bit more verbose than using pure int instead of Metres and
Seconds, but if I wanted a C++ compiler to check this kind of error, I'm
afread that the resulting C++ code would be much more verbose.

Such mistakes as using a pointer to nothing and writing past the array
bounds don't often happen in Ada.

>An example. First compile with the default behaviour, then with all warnings tu
>rned on:
>
>int main()
>{
>     int i=0;
>
>     unsigned j=4;
>
>     j<i;
>}

procedure Temp is
   I : Integer := 0;
   J : Natural := 4;
begin
   I < J;
end Temp;

Without any warnings:
# gnatmake temp
gcc -c temp.adb
temp.adb:5:05: missing ":="
gnatmake: "temp.adb" compilation error

Notice how the language prevents doing useless things.

If a replace I < J; with null; the result is:
gcc -c -gnatg temp.adb
temp.adb:2:04: warning: "I" is not modified, could be declared constant
temp.adb:2:04: warning: variable "I" is not referenced
temp.adb:3:04: warning: "J" is not modified, could be declared constant
temp.adb:3:04: warning: variable "J" is not referenced
gnatmake: "temp.adb" compilation error

-- 
Tapio