Re: Typing in Ada

["Peter C. Chapin" <pchapin@sover.net>]

Jeffrey Carter <spam@spam.com> wrote in news:mxJuc.18113$be.10152
@newsread2.news.pas.earthlink.net:

> procedure Strongly_Typed is
>     type I1 is new Integer;
>     type I2 is new Integer;
>     type I3 is range -100 .. 100;
[snip]
> 
> int main () { /* I cannot call this Strongly_Typed */
>     typedef int   I1;
>     typedef int   I2;
>     typedef short I3;

This isn't an entirely fair comparison because in C, typedef doesn't 
introduce a new type it simply creates a new name for an existing type. In 
Ada, it would be more similar to using a subtype, perhaps. Something like

     subtype I1 is Integer;

In any event to create a new type in C you need to introduce a structure. In 
fact, different structures do have different types:

typedef struct {
  int x;
} A;

typedef struct {
  int x;
} B;

int main()
{
  A a;
  B b;

  a = b;  // Error. A and B are different types.

  ...

I'm not sure if there is a formal definition of strong typing or not. I've 
always thought that it had to do with the property of every expression and 
variable having a well defined type. If so, that is as true of C as it is of 
Ada (in some ways its even more true of C because in C literal numbers have 
specific types... there is no "universal integer" type used, for example, for 
integer literals). The real difference is that C does a bunch of automatic 
type conversions and, furthermore, normally performs these conversions 
without checking if the converted value will fit into the target type. I'm 
not sure that's a strong typing issue, however.

Peter