Member pointers functionality #77

denzor200 · 2021-04-25T20:11:44Z

Hello Anton. I have some idea and i want to suggest it for you. This patch will add pretty useful functionality for extracting member pointer of some structure. For example:

struct Foo
{
	char ch;
	short id;
	short opt;
	int value;
};

auto ch_memptr = boost::pfr::get_memptr<0>(Foo{});
auto id_memptr = boost::pfr::get_memptr<1>(Foo{});
auto opt_memptr = boost::pfr::get_memptr<2>(Foo{});
auto value_memptr = boost::pfr::get_memptr<3>(Foo{});

auto obj = Foo{};

obj.*ch_memptr = 'c';
obj.*id_memptr = 100;
obj.*opt_memptr = 200;
obj.*value_memptr = 3000;

std::cout << obj.ch << " " << obj.id << " " << obj.opt << " " << obj.value << std::endl; ///< will print "c 100 200 3000"

Its just a draft - not a final code. If you are interested - i will continue for it.

denzor200 · 2021-04-25T20:14:43Z

This may help to resolve this issue:
#62
BUT i unfortunately do not know how to make get_memptr constexpr((

Update core_memptr.hpp

apolukhin · 2021-05-06T16:08:13Z

Good idea!

I''ve simplified it a little bit https://godbolt.org/z/e8rKWrrEs

In my implementation problems are the same:

pointer arithmetic is not a constant expression (bit_cast refuses to convert pointers to integers)
Something must be done with the default constructor

denzor200 · 2021-06-17T18:21:24Z

Something must be done with the default constructor

template<typename T> struct declval_helper { static T value; };
But it not for clang and msvc
https://godbolt.org/z/nYz6M61YT
What if we pass a NULL reference into boost::pfr::get? This is works good, but i cant anticipate the consequences

denzor200 · 2021-06-19T12:59:17Z

Something must be done with the default constructor

Why don't we use the already checked offset_based_getter? In my opinion it solves this problem good.
https://godbolt.org/z/hfxbnvqrE
This works on MSVC too

schaumb · 2022-02-23T21:07:21Z

I tried to construct constexpr member pointer getter:

GCC and msvc are promising:
https://godbolt.org/z/hPa9718aY
https://godbolt.org/z/Th8KTKxns

Caveats:

msvc static assert: same member pointer at runtime, but not recognized as equal at compile time.
only for not final classes
clang not works with this technique

denzor200 · 2022-02-23T22:34:00Z

@schaumb interesting. Unfortunatelly, refl cannot be passed as template parameter, only orig can.

schaumb · 2022-02-24T17:41:36Z

@denzor200

I created a gcc ticket, because it is a compiler bug (probably?).

msvc constexpr member pointers can be used as template parameter. But:

template<auto> struct X {};
static_assert(std::is_same_v<X<refl>, X<orig>>);  // ERROR


template <auto V> decltype(V) wash() { return V; };
static_assert(&wash<refl> == &wash<orig>);  // OK
static_assert(wash<refl>() == wash<orig>()); // OK
// compiler save first call function instance template argument (currently the refl), so:
static_assert(wash<refl>() == refl); // OK
static_assert(wash<orig>() == orig); // ERROR

Not equality problem occures only if the original member pointer is used. But wash can be used as equality checker.

denzor200 · 2022-07-15T17:33:33Z

@schaumb
I created clang ticket here: llvm/llvm-project#56541

Let's see where the answer will be faster :)

apolukhin

Looks good! Here's a few nitpicks

apolukhin · 2023-05-21T07:11:44Z

include/boost/pfr/core_memptr.hpp

+///     s.*memptr = 0;
+/// \endcode
+template<std::size_t I, class T>
+inline auto get_memptr(boost::pfr::type_identity<T>) noexcept


Let's change it to

template<std::size_t I, class T> inline auto get_memptr() noexcept

In that case the type_identity is not required and should be removed. The

template<std::size_t I, class T> inline auto get_memptr(const T&) noexcept

overload is not required either

apolukhin · 2023-05-21T07:13:35Z

include/boost/pfr/detail/core_memptr.hpp

+    std::size_t offset = 0;
+    std::size_t space = sizeof(T);
+
+    return tie_as_offsets_tuple_impl<T>(detail::make_index_sequence<detail::fields_count<T>()>{}, offset, space);


add a namespace to tie_as_offsets_tuple_impl to avoid ADL

apolukhin · 2023-05-21T07:14:01Z

include/boost/pfr/detail/core_memptr.hpp

+        "====================> Boost.PFR: For safety reasons it is forbidden to reflect unions. See `Reflection of unions` section in the docs for more info."
+    );
+
+    return tie_as_memptrs_tuple_impl<T>(detail::make_index_sequence<detail::fields_count<T>()>{});


add a namespace to tie_as_memptrs_tuple_impl to avoid ADL

apolukhin · 2023-05-21T07:15:41Z

include/boost/pfr/detail/memptr_cast.hpp

+            "====================> Boost.PFR: Internal error while casting offset to member pointer: overflow was detected"
+        );
+
+        union {


That's an UB. Just use std::memcpy. The efficiency would remain the same - compilers know how to optimize std::memcpy

apolukhin · 2023-05-21T07:16:28Z

include/boost/pfr/detail/align.hpp

+    {
+        //BOOST_ASSERT(boost::alignment::detail::is_alignment(alignment)); TODO enable
+        if (size <= space) {
+            std::size_t p = ~(alignment - 1) & (offset + alignment - 1);


provide a more clear names for local variables instead of p and n

apolukhin · 2023-05-21T07:17:26Z

test/run/memptr.cpp

+    short id;
+    short opt;
+    int value;
+};


add a non-trivial type to an aggregate, std:;string for example

denzor200 · 2023-08-13T00:01:55Z

test/run/memptr.cpp

@@ -0,0 +1,42 @@
+// Copyright (c) 2021 Denis Mikhailov


Move this file to the test/core_memptr dir

All new test filed should be moved to test/core_memptr

Conflicts: include/boost/pfr.hpp test/core/run/memptr.cpp

denzor200 · 2023-11-18T15:55:14Z

test/core/compile-fail/packed.cpp

+};
+
+int main() {
+	(void)boost::pfr::get_memptr<3>(boost::pfr::type_identity<Foo>{}); // Must be a compile time error


denzor200 · 2023-11-18T16:01:19Z

test/core/compile-fail/packed.cpp

+#include <boost/pfr/core_memptr.hpp>
+
+#pragma pack(1)
+struct Foo


Provide the same test but with a structure like this:

#pragma pack(1) struct sample { int ch; int id; int opt; int value; };

In order to test the case when settled alignment doesn't affect the size.

BTW default align is not always sizeof(int). You should provide some logic for select appropriate type between int, long, etc and for do nothing when nothing was selected for this platform.

Draft for boost::pfr::get_memptr

b353849

denzor200 marked this pull request as draft April 25, 2021 20:12

denzor200 added 8 commits April 27, 2021 00:01

Update core_memptr.hpp

3329156

Update core_memptr.hpp

a330973

Update core_memptr.hpp

b48f596

Update memptr_cast.hpp

e69f265

Merge pull request #1 from denzor200/denzor200-patch-1

ef0015f

Update core_memptr.hpp

fix

0ff1475

fix

89b9aa4

Update memptr_cast.hpp

724775f

apolukhin requested changes May 21, 2023

View reviewed changes

denzor200 mentioned this pull request Aug 12, 2023

Field's names functionality #129

Merged

denzor200 commented Aug 13, 2023

View reviewed changes

denzor200 added 3 commits September 23, 2023 00:45

Merge remote-tracking branch 'origin/develop' into feature/get_memptr

76ccbc1

Conflicts: include/boost/pfr.hpp test/core/run/memptr.cpp

Update copyrights

2636b2e

Create packed.cpp

7dcd890

denzor200 commented Nov 18, 2023

View reviewed changes

schaumb mentioned this pull request Nov 29, 2023

GIve away the details of get_name #150

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Member pointers functionality #77

Member pointers functionality #77

denzor200 commented Apr 25, 2021

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Member pointers functionality #77

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Member pointers functionality #77

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denzor200 commented Apr 25, 2021

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denzor200 commented Jun 19, 2021 •

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