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From: Kaz Kylheku <643-408-1753@kylheku.com>
Newsgroups: comp.lang.c
Subject: Re: how cast works?
Date: Fri, 9 Aug 2024 21:30:33 -0000 (UTC)
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On 2024-08-09, James Kuyper <jameskuyper@alumni.caltech.edu> wrote:
> On 09/08/2024 12:04, Bart wrote:
>> On 09/08/2024 00:17, Keith Thompson wrote:
> ...
>>> There is no such thing as an "implicit cast" in C.
>>>
>> 
>> Suppose I write this code:
>> 
>>      x = a;                  // implicit 'conversion'
>>      x = (double)a;          // explicit 'conversion'
>> 
>> 
>> My compiler produces these two bits of AST for the RHS of both expressions:
>> 
>> 1 00009 r64---|---2 convert: sfloat_c i32 => r64
>> 1 00009 i32---|---|---1 name: t.main.a.1
>> 
>> 1 00010 r64---|---2 convert: sfloat_c i32 => r64
>> 1 00010 i32---|---|---1 name: t.main.a.1
>
> Of course - an implicit conversion has exactly the same effect as a
> explicit conversion, if the source and destination types are the same.
> That doesn't make it correct to use the term "cast" to describe anything
> other than an explicit conversion.

That's all very neat and clean. However, the problem is that in C,
some of the implicit conversions are unsafe.

Implicit conversions can:

- truncate an integer value to fit a narrower type.

- convert between floating point and integer in a way that the value
  is out of range, with undefined behavior ensuing.

- change the value: e.g -1 becomes UINT_MAX.

- subvert the type system, e.g.  (foo *) -> (void *) -> (bar *).

In computer science, we refer to unsafe conversion as coercion.
THe cast notation is C's coercion operation.

In some languages, some of what C allows to be an implicit conversion
would be classified as requiring a coercion.

It almost makes sense to speak of "implicit cast" (i.e. coercion) in C,
because of what happens implicitly being so unsafe.

-- 
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