{.emit.} for types is too late: Nim uses them before their declaration

[mratsim]

I'm trying to wrap a C++ library and I need to resort to {.emit.} to workaround the absence of default field value for Nim and the C++ backend (#4687)

Unfortunately Nim generates types before they were emitted.

Test case:

{.push header: "<memory>".}
type
  CppSharedPtr* {.importcpp: "std::shared_ptr", bycopy.} [T] = object

func make_shared*(T: typedesc): CppSharedPtr[T] {.importcpp: "std::make_shared<'*0>()".}
func deref*[T](p: CppSharedPtr[T]): var T {.importcpp: "(* #)".}
{.pop.}

{.passC: "-Wfatal-errors".} # Only block at the first issue


{.emit: ["""
#include <memory>

struct Base {
  Base() = delete;
};

struct Derived: public Base {
  std::shared_ptr<Base> f1{nullptr};
  std::shared_ptr<Base> f2{nullptr};
};

"""].}

type
  Base {.pure, inheritable, importcpp.} = object
  Derived {.inheritable, importcpp.} = object of Base
    f1: CppSharedPtr[Base]
    f2: CppSharedPtr[Base]

proc main() =
  let derived = make_shared(Derived)

main()

The code generated (with danger) is:

/* section: NIM_merge_HEADERS */

#include "nimbase.h"
#include <memory>
#undef LANGUAGE_C
#undef MIPSEB
#undef MIPSEL
#undef PPC
#undef R3000
#undef R4000
#undef i386
#undef linux
#undef mips
#undef near
#undef far
#undef powerpc
#undef unix

/* section: NIM_merge_FRAME_DEFINES */
#define nimfr_(x, y)
#define nimln_(x, y)

/* section: NIM_merge_TYPES */
typedef std::shared_ptr<Derived>  TY__r4mQCt9bhZT6EjBvshZiuCw; // <-------- Generated before the emitted type !!!

/* section: NIM_merge_PROC_HEADERS */
#include <memory>

struct Base {
  Base() = delete;
};

struct Derived: public Base {
  std::shared_ptr<Base> f1{nullptr};
  std::shared_ptr<Base> f2{nullptr};
};


N_LIB_PRIVATE N_NIMCALL(void, main__8FadbF9aXJUyuO9aYZ4NjXHQ)(void);
static N_INLINE(void, initStackBottomWith)(void* locals);
N_LIB_PRIVATE N_NOINLINE(void, nimGC_setStackBottom)(void* theStackBottom);
N_LIB_PRIVATE N_NIMCALL(void, systemDatInit000)(void);
N_LIB_PRIVATE N_NIMCALL(void, systemInit000)(void);
N_LIB_PRIVATE N_NIMCALL(void, NimMainModule)(void);

Note that if you remove the importcpp (in the same code), a forward declaration section appears NIM_merge_FORWARD_TYPES

/* section: NIM_merge_HEADERS */

#include "nimbase.h"
#include <memory>
#undef LANGUAGE_C
#undef MIPSEB
#undef MIPSEL
#undef PPC
#undef R3000
#undef R4000
#undef i386
#undef linux
#undef mips
#undef near
#undef far
#undef powerpc
#undef unix

/* section: NIM_merge_FRAME_DEFINES */
#define nimfr_(x, y)
#define nimln_(x, y)

/* section: NIM_merge_FORWARD_TYPES */ # <--------------- This is new !
struct tyObject_Derived__Xjo9b4GYxjCDKcoIkW13VSg;
struct tyObject_Base__lHJ509cStVLqAdHwmmNfOsw;

/* section: NIM_merge_TYPES */
struct tyObject_Base__lHJ509cStVLqAdHwmmNfOsw {
char dummy;
};
typedef std::shared_ptr<tyObject_Base__lHJ509cStVLqAdHwmmNfOsw>  TY__Ib9bHVVFSEeJD7L58LFE9aDw;
struct tyObject_Derived__Xjo9b4GYxjCDKcoIkW13VSg : public tyObject_Base__lHJ509cStVLqAdHwmmNfOsw {
TY__Ib9bHVVFSEeJD7L58LFE9aDw f1;
TY__Ib9bHVVFSEeJD7L58LFE9aDw f2;
};
typedef std::shared_ptr<tyObject_Derived__Xjo9b4GYxjCDKcoIkW13VSg>  TY__r4mQCt9bhZT6EjBvshZiuCw;

/* section: NIM_merge_PROC_HEADERS */
#include <memory>

struct Base {
  Base() = delete;
};

struct Derived: public Base {
  std::shared_ptr<Base> f1{nullptr};
  std::shared_ptr<Base> f2{nullptr};
};

Is there a way to notify importcpp that a type is defined in the same module and so need forward declaration?

[alaviss]

Isn't there a special emit comment /** TYPESECTION **/ to do that or something?

[alaviss]

See this for an example in the testsuite:

Nim/tests/statictypes/tstaticimportcpp.nim

Lines 6 to 23 in 662c508

	{.emit: """/*TYPESECTION*/
	template <int N, class T>
	struct GenericIntType {
	T data[N];
	};
	template <class T>
	struct GenericTType {
	T field;
	};
	struct SimpleStruct {
	int field;
	};
	""" .}

[mratsim]

Indeed, apparently it hooks there https://github.com/nim-lang/Nim/blob/acf3715/compiler/ccgstmts.nim#L1489-L1495
so there is also includes and var.

Is there a plan to document that or maybe have specialised emitVar, emitInclude, emitType templates?

[cooldome]

It is already documented https://nim-lang.org/docs/manual.html#implementation-specific-pragmas-emit-pragma

[timotheecour]

see also #13953