Reuse C memory for D struct?

Hi! I'm wondering if it's possible to have a struct in D which uses the same pointer and memory as returned by the extern C function. This would allow me to manipulate and use the C struct directly in D code. I'm not sure how to better explain this, hopefully the following pseudocode clarifies my question struct Tree { enum treeSize = 40; ubyte[treeSize] _this; this(int apples) { // use the C provided pointer somehow? // neither of the following seem to do the trick //this = *cppNew(apples); //_this = *cast(ubyte*)cppNew(apples) } Tree* cppNew(int apples) { // calls extern C constructor which returns a pointer to a Tree of size treeSize } ~this() { cppDelete(); } void cppDelete() { // this needs the pointer returned by cppNew to do an extern C delete } } void main() { Tree tree = Tree(12); }

On Thursday, 21 November 2013 at 17:12:26 UTC, evilrat wrote: > On Thursday, 21 November 2013 at 15:22:10 UTC, Lemonfiend wrote: >> Hi! >> >> I'm wondering if it's possible to have a struct in D which uses the same pointer and memory as returned by the extern C function. >> This would allow me to manipulate and use the C struct directly in D code. > > so what really stops you from accessing that pointer in D? Well right now, in the code above, the original C pointer is lost, only the D pointer remains. > if you fear for data layout just wrap this pointer in D struct and give properties to accessing stuff from pointer(weird) Yes a solution would be to simply store the C pointer in the D struct, but then the C and D structs are no longer the same.

On 11/21/2013 07:22 AM, Lemonfiend wrote: > I'm wondering if it's possible to have a struct in D which uses the same > pointer and memory as returned by the extern C function. > This would allow me to manipulate and use the C struct directly in D code. > > I'm not sure how to better explain this, hopefully the following > pseudocode clarifies my question > > struct Tree > { > enum treeSize = 40; > > ubyte[treeSize] _this; > > this(int apples) > { > // use the C provided pointer somehow? > // neither of the following seem to do the trick > //this = *cppNew(apples); > //_this = *cast(ubyte*)cppNew(apples) If you instead maintain a slice of Apples and assuming that 'apples' is the number of apples, then you can use the following syntax Apple[] _this; // ... _this = cppNew[0 .. apples]; I have some information about that syntax under the "Producing a slice from a pointer" section here: http://ddili.org/ders/d.en/pointers.html One thing that is not mentioned in there is that you are still responsible for the Apples that are returned by the C library. Ali

On 11/22/2013 12:22 AM, Lemonfiend wrote: > Hi! > > I'm wondering if it's possible to have a struct in D which uses the same > pointer and memory as returned by the extern C function. > This would allow me to manipulate and use the C struct directly in D code. I'm trying to understand why you would want to this, but I just can't see a reason. Why not just define a struct in D that matches the C definition and then use the pointer directly? Example: // foo.h typedef struct { int x; } Foo; extern Foo* newFoo( void ); // foo.d struct Foo { int x; } extern( C ) Foo* newFoo(); auto f = newFoo(); writeln( f.x ); Of course, if Foo is an opaque struct on the C side, you'll be playing with fire if you define it in D and don't have control of the C library. In that case, you should make it opaque in D as well.

On Thursday, 21 November 2013 at 15:22:10 UTC, Lemonfiend wrote: > Hi! > > I'm wondering if it's possible to have a struct in D which uses the same pointer and memory as returned by the extern C function. > This would allow me to manipulate and use the C struct directly in D code. > > I'm not sure how to better explain this, hopefully the following pseudocode clarifies my question > > struct Tree > { > enum treeSize = 40; > > ubyte[treeSize] _this; > > this(int apples) > { > // use the C provided pointer somehow? > // neither of the following seem to do the trick > //this = *cppNew(apples); > //_this = *cast(ubyte*)cppNew(apples) > } > > Tree* cppNew(int apples) > { > // calls extern C constructor which returns a pointer to a Tree of size treeSize > } > > ~this() > { > cppDelete(); > } > > void cppDelete() > { > // this needs the pointer returned by cppNew to do an extern C delete > } > } > > void main() > { > Tree tree = Tree(12); > } if your D struct is POD and alocated on the heap, it's the same, as long as data type are the same and not plateform-dependent.I'm mean that that you just cast from the initial pointer...

On Saturday, 23 November 2013 at 01:39:00 UTC, Baz wrote: > > if your D struct is POD and alocated on the heap, it's the same, as long as data type are the same and not plateform-dependent.I'm mean that that you just cast from the initial pointer... and because of D UFCS it is possible to have methods for this struct lying in module scope, not struct :)