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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c++
Subject: Re: Can someone please verify the execution trace of this?
Date: Mon, 20 May 2024 19:31:37 -0700
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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
> wij <wyniijj5@gmail.com> writes:
> [...]
>> typedef int (*ptr)();  // ptr is pointer to int function
>> int H(ptr x, ptr y);
>> int D(ptr x)
>> {
>>   int Halt_Status = H(x, x);
>>   if (Halt_Status)
>>     HERE: goto HERE;
>>   return Halt_Status;
>> }
>>
>> int main()
>> { 
>>   H(D,D);
>>   return 0;
>> }
>>
>> The code above does not compile:
>
> Yes, it does (as you acknowledge in a later post).
>
> This:
>     typedef int (*ptr)();
> defines "ptr" as an alias for a type that can be described in English
> as "pointer to function returning int".  The empty parentheses
> indicate that the function takes an unspecified but fixed number
> and type(s) of arguments; this is an old-style declaration.
>
> (The C23 standard, not yet released, changes the semantics of empty
> parentheses, causing the code to be invalid, but let's assume C17.)
>
> The function H is declared but not defined.  That doesn't prevent
> the code from being *compiled*, but it does prevent it from being
> *linked* to produce an executable program.  Perhaps a definition
> of H has been presented in some other article, but I will not waste
> my time hunting for it.
>
> Ignoring variadic functions like printf, every function call
> must pass the appropriate number and types of arguments, matching
> the parameters defined by the corresponding function definition.
> (In some cases there can be implicit type conversions, but there
> are no implicit conversions for arguments or parameters of function
> pointer type.)  If a correct *prototype* (a function declaration that
> specifies the types of all parameters) is visible, this is enforced
> at compile time; failing to do so is a *constraint violation*,
> requiring a compile-time diagnostic.  If no such prototype is
> visible, violations of this rule need not be diagnosed, but result
> in *undefined behavior*; the C standard says nothing about what
> the program will do.
>
> I'll note that the code (declares and) defines the function D,
> but never calls it.  The address of D is passed to H, but without
> a definition of H we can't guess what it does with that address.
>
> It's possible to rewrite the code to (a) avoid the use of old-style
> function declarations and (b) avoids any undefined behavior --
> but without knowing or being able to guess just what the program
> is supposed to do, I see no point in doing so.
>
> The main point is this: The function H is declared but never
> defined, so it's impossible to create a running program from this
> code, and impossible to guess what it's intended to do without more
> information.  I will not make any assumptions about how H is meant
> to be defined or consult other posts to find a definition.  I may or
> may not follow this thread to see if any clarifications are posted.
>
> The code as presented is a valid C *translation unit*, but it is
> not a valid *program*, and it has no behavior.

My apologies, I did not notice this had been posted to comp.lang.c++
until just as I posted my followup.  If I had noticed that, I probably
wouldn't have posted at all.  I'll try to pay better attention.

C++ does not have old-style function declaration.  Empty parentheses
in a C++ function declaration or definition indicate that the
function has no parameters.  In C++, a pointer of type "ptr" can
only point to a function with no parameters.  The call "H(D, D)"
passes the address of the function D (which has one parameter) to
a parameter of type "ptr" (which requires a pointer to a function
with no parameters).  That's an error.

The code is ill-formed in C++.  And of course the lack of any
definition of H is another problem that makes it difficult or
impossible to correct the C++ errors and produce a valid program.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */