ObjectFieldUtilities.h File Reference

#include "Stroika/Foundation/StroikaPreComp.h"
#include <cstddef>
#include "ObjectFieldUtilities.inl"

Go to the source code of this file.

Namespaces
namespace	Stroika::Foundation

Functions
template<typename APPARENT_MEMBER_TYPE , typename OUTER_OBJECT , typename AGGREGATED_OBJECT_TYPE >
OUTER_OBJECT *	Stroika::Foundation::Memory::GetObjectOwningField (APPARENT_MEMBER_TYPE *aggregatedMember, AGGREGATED_OBJECT_TYPE(OUTER_OBJECT::*aggregatedPtrToMember))

Detailed Description

Note

Code-Status: Beta

Definition in file ObjectFieldUtilities.h.

Function Documentation

GetObjectOwningField()

template<typename APPARENT_MEMBER_TYPE , typename OUTER_OBJECT , typename AGGREGATED_OBJECT_TYPE >

OUTER_OBJECT * Stroika::Foundation::Memory::GetObjectOwningField	(	APPARENT_MEMBER_TYPE *	aggregatedMember,
		AGGREGATED_OBJECT_TYPE OUTER_OBJECT::*	aggregatedPtrToMember
	)

Given an objects field (given by pointer to member) and that owned object, run 'offsetof' backwards to get the original owning object.

Example Usage

void test ();
struct X1 {
    int a;
    int b;
};
struct X2 {
public:
    int a;
private:
    int b;
private:
    friend void test();
};
void DoTest ()
{
    {
        X1 t;
        static_assert (is_standard_layout_v<X1>);
        void* aAddr = &t.a;
        void* bAddr = &t.b;
        assert (GetObjectOwningField (aAddr, &X1::a) == &t);
        assert (GetObjectOwningField (bAddr, &X1::b) == &t);
    }
    {
        // Check and warning but since X2 is not standard layout, this isn't guaranteed to work
        X2 t;
        static_assert (not is_standard_layout_v<X2>);
        void* aAddr = &t.a;
        void* bAddr = &t.b;
        assert (GetObjectOwningField (aAddr, &X2::a) == &t);
        assert (GetObjectOwningField (bAddr, &X2::b) == &t);
    }
}

Example Usage

struct Header {
    ReadOnlyProperty<int> p {[] (const auto* property) {
        const Header* h = GetObjectOwningField(property, &Header::p);
        // This should recover the pointer to the object Header from the pointer to its field 'p';
    }};

Note

- for this to work - OUTER_OBJECT (in the above example: Header) must be of standard_layout type according to c++ standard, but I'm not sure what todo otherwise, it it seems to work otherwise. Posted to stackoverflow to find a better, more portable way: https://stackoverflow.com/questions/65940393/c-why-the-restriction-on-offsetof-for-non-standard-layout-objects-or-how-t

See Docs on Memory::OffsetOf() - experiments to find a way to do this portably.

Definition at line 16 of file ObjectFieldUtilities.inl.