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digitalmars.D.bugs - [Issue 8616] New: Make pointers dereference with UFCS like they do with member functions

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8616

           Summary: Make pointers dereference with UFCS like they do with
                    member functions
           Product: D
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: jmdavisProg gmx.com



PDT ---
As discussed here

http://forum.dlang.org/post/spjfsyaqrcpboypgfrsd forum.dlang.org

pointers to structs and UFCS don't get along all that well at this point. UFCS
dictates that the first function parameter be the type that the function is
being called on, which means that for UFCS to work with a pointer, the function
must take a pointer.

struct S
{
    auto func(int i) {...}
}

auto foo(S* s, int i) { ... }
auto bar(S s, int i) { ... }

S* s;

s.foo(5); //compiles
s.bar(7); //doesn't compile
(*s).bar(8); //compiles

The problem with this is that while it's consistent with UFCS in general, it's
inconsistent with how member functions get called on pointers to structs.

You can do

S* s;
s.func(12);

as long as func is a member function, but if it's a free function, that only
works if the free function takes an S* rather than an S. So, while calling a
member function on a struct is the same whether you're dealing with a pointer
or not

S* s;
S t;
s.func(12);
t.func(14);

it's different when dealing with a free function and UFCS.

S* s;
S t;
(*s).bar();
t.bar();

What this enhancement request proposes is that functions which take the struct
as their first parameter rather than a pointer to the struct be considered in
UFCS for pointers to that struct as long as there's no ambiguity. So, you could
do

S* s;
S t;
s.bar();
t.bar();

even though bar takes an S, not an S*. Presumably, if there's a conflict

auto fiddly(S* s, float f) {...}
auto fiddly(S s, float f) {...}

S* s;
S t;
s.bar(); //fails to compile due to ambiguity
t.bar(); //presumably compiles, since it would never use the S* version.

then you'll get a compilation error, but as long as there isn't one, allowing
for an S* to be used with UFCS just like S is would be useful and make it so
that UFCS works with pointers to structs like member functions work with
pointers to structs, which is the main reason for UFCS in the first place.

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next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8616


Maxim Fomin <maxim maxim-fomin.ru> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |maxim maxim-fomin.ru
         Resolution|                            |DUPLICATE



---
This is duplication of 8603.

The problem is that with this approach pointers are implicitly dereferenced.
Consider function:

void foo (T type) { }

which now can be called:

T var;
T *ptr;
type.foo();
ptr.foo();

*** This issue has been marked as a duplicate of issue 8603 ***

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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PDT ---
 The problem is that with this approach pointers are implicitly dereference
Which is what happens when calling member functions on pointers to structs (unless no member variables are ever used within the function, so no dereferencing is necessary), so I don't see that as a problem. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
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prev sibling next sibling parent d-bugmail puremagic.com writes:
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---

 The problem is that with this approach pointers are implicitly dereference
Which is what happens when calling member functions on pointers to structs (unless no member variables are ever used within the function, so no dereferencing is necessary), so I don't see that as a problem.
Well, if pointers are dereferenced in case of accessing members, then the problem already exists and can't be reason for not accepting enhancement proposal. However, there is still a question what to do when both function are available: foo(T* ptr, ...); foo(T val, ...); I prefer to see the former to have priority over the latter. It may be useful in cases when the first one was written intentionally for e.x. to check for null pointer. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Sep 03 2012
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8616




PDT ---
 However, there is still a question what to do when both function are available
I'd definitely choose to go with making the code not compile. Otherwise, it risks function hijacking. You could have auto foo(S s) {...} in my.pack which S* s; s.foo(); is happily calling, and then when you change the code to import your.pack with auto foo(S* s) {...} the code would silently change which function was being called. In general, we try and make all such situations compilation errors in D. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Sep 03 2012
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8616


Jonathan M Davis <jmdavisProg gmx.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|DUPLICATE                   |



PDT ---

is much better written (if nothing else, because it frames it as an enhancement
request rather than a bug like 8603 does).

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Sep 04 2012
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8616


Jonathan M Davis <jmdavisProg gmx.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tommitissari hotmail.com



PDT ---
*** Issue 8603 has been marked as a duplicate of this issue. ***

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prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8616




Bug 8490 (already marked as "resolved invalid") is related.

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