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Subject: Re: [std-proposals] Unify '.' and '->' operator behavior
From: Arthur O'Dwyer (arthur.j.odwyer_at_[hidden])
Date: 2020-02-15 21:06:50


Hi Jim,

Sadly your idea doesn't work in C++, because C++ has *member functions*.
That is, we can write things like this:

struct Widget {
    void reset(); // reset the widget
};
[...]
std::unique_ptr<Widget> p = std::make_unique<Widget>();
p->reset(); // reset the pointed-to widget by calling Widget::reset()
member function
p.reset(); // reset the pointer's own value to nullptr

So -> and . literally have different meanings, and you can't combine them
in the way you're trying to.

(This would be a good example to show to your C colleagues when they talk
about . versus -> being a "stumbling block.")

HTH,
Arthur

On Sat, Feb 15, 2020 at 3:35 PM J Decker via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> I would like to see the operators '.' and '->' inspect their left hand
> value's type and generate the appropriate base address from it added to C.
> Since I use C++ compiler often to compile C code as a strongly typed C
> compiler, this should be suggested here too.
>
> See end for additional C++ comments
> ---
>
> My proposal is to make the operator '.' consider the type of the left hand
> value and behave as '.' or '->' appropriately. I could say, make '.' and
> '->' equivalent, however, the desire would be to migrate to just using '.'
> operators and dropping the -> altogether(sometime, in the far future,
> removal/deprecation is not part of this proposal), since it would become
> redundant. Maybe it becomes a styling mechanism.
>
> This change could be shared with C++ to make sure that c2x(and later) code
> could still be compiled with C++. C++ Does have operator overloading, but
> the compiler should be able to determine if the compiler is operating on a
> structure/union/(class) or a pointer to(array of) struct/union/(class);(see
> end)
>
> In C, in order for either '.' or '->' to be used, the left hand value must
> be a completely defined structure or union, or a pointer to a structure or
> union. A pointer to a pointer cannot be used with either of those,
> requiring some other dereference operation to get down to 1(->) or 0(.)
> levels of indirection.
>
> In either case, the compiler (GCC for instance) has the complete type
> information of the left hand value, and can inspect whether the value is a
> reference to a instance of a structure or union, or a pointer to a
> structure or union. In the first case(instance) the address comes from the
> local function address space (stack pointer) or some other memory location
> computed by the compiler, in the second case, the address is given by the
> value of the left hand value instead of computed by the compiler/linker,
> and in both cases, the right hand value is a name of a field in a
> structure/union, which is the additional offset to apply to the base
> address.
>
> The modification to the GCC C compiler (9.x) is nearly trivial. The
> modification to make '.' and '->' work the same, and to inspect the type of
> the left hand value and use the 'get variable location' or 'get value of
> pointer' appropriately. Because of the requirement of a complete type
> (incomplete types used with '.' or '->' generate an error in all C
> compilers), every compiler must know the type and it's definition of the
> left value to use the right hand name in. (the right hand value must be a
> name within a(the?) structure/union, and cannot be a computed value, ie
> `ptr->(1+3)` is not valid)
>
> This change would have no impact on existing code. Everything that used
> to compile with a C compiler would continue to compile with a C compiler.
>
> Code written against the new standard would potentially not be compatible
> with previous standards.
>
> ---
> My reasoning for approaching this, or even thinking about it, was from
> having taken C code which I ported to JS by replacing `TYPE name` with `val
> name` and '->' to '.', and it was 95% done; other than the function
> signatures. Taking this code and bringing it back to C becomes and issue
> because of the loss of -> operators; so I began considering why the implied
> type information form Javascript wouldn't already exist in the C compiler,
> since everything must be 'pre-typed', or defined ahead of time with its
> full type information, that the compiler couldn't also do the 'smart' thing
> based on the type of the value of the left hand expression.
>
> I asked on IRC in ##C and ##programming about this idea, and someone
> mentioned that the usage of '.' and '->' to programmers just learning the
> language is very often a stumbling block... since to they do 'the same
> thing' it's hard to understand why '.' and '->' are even different...
> Eventually, you understand that they are different because you declared
> either a 'static' instance of a thing ( FILE a ) vs creating a pointer to a
> thing ( FILE* pa ).
>
> There is the `*` that can dereference a pointer, so you can still use '.'
> everywhere and never use `->', `(*thing).` ; or `thing[0].` ; however that
> coding style doesn't improve the ability to back-port C ported to JS back
> to C. Those mechanisms would still work, since they result in a reference
> to a structure or union instance instead of the pointer to the same
> instance.
>
> Conversely, it was suggested that losing the distinction of '.' or '->'
> operators loses information about the types of a thing you are working on.
> 'pointer->struc.member.something->name' you just 'know' that there is a
> pointer to a structure that is a single (large) of other structures that
> has a pointer to a name... so you 'know' it's only 2 memory locations
> you're dealing with.
>
> ---
> ===
> C++ notes.
> This would only apply for the builtin/default handlers for the '.' and
> '->' operators. Those operator overrides that have been defined by a
> class/struct should still expect to do their same function. The function
> of '.' and '->' should still be overloadable with separate handlers;
> although the C++ code doesn't interact at a compiler level here, and would
> have no way (?) of generating, 'use base address of structure instance' or
> 'read value from variable and use as base address of structure'.
>
> I've not followed the development and growth of C++ with all the safe
> pointer extensions... I am far from a deep C++ guru.
>
> Jim B
> https://github.com/d3x0r ( it's pronouced Decker)
>
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>



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