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| Also, the implementation of the SGPropertyNode bindings in [https://gitorious.org/fg/flightgear/blobs/next/src/Scripting/nasal-props.cxx nasal-props.cxx] contains additional examples. | | Also, the implementation of the SGPropertyNode bindings in [https://gitorious.org/fg/flightgear/blobs/next/src/Scripting/nasal-props.cxx nasal-props.cxx] contains additional examples. |
| --> | | --> |
| == A standard interface for mapping C++ classes to Nasal objects (Status 2013: implemented in cppbind) ==
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| There's a plan to eventually generalize and unify all those helpers in FGPositioned.cxx and provide them as part of the FGNasalSys class (or some custom abstract base class that C++ classes should implement to expose objects to Nasal), so that they automatically become available to all users of the Nasal subsystem, just by implementing a certain interface. By exposing objects with methods, rather than plain variables, it will be possible to compute results lazily.
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|
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| Such an abstract base class should feature:
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| * virtual constructor to set up: ghost dtor, ghost name, ghost getter
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| * virtual destructor function to delete Nasal ghosts
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| * mapping helper to map C++ functions with SG/STL return types to Nasal data structures (std::string, std::vector, std::map)
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| * lazy getter
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| * helper: hashset()
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| * helper: stringToNasal
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| * helper: to set up valid parents vector for inheritance to work properly
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|
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|
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| At the moment, the following discussions focuses on the FGPositioned/NasalPositioned implementation, however this will be further generalized during the next months.
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| Looking at the code in [http://gitorious.org/fg/flightgear/blobs/78afdb3c2227e75fc1542b3992dcea26181d98cf/src/Scripting/NasalPositioned.cxx#line139 NasalPositioned.cxx]:
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|
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| The function bodies for these helpers are largely identical:
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|
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| * naRef ghostForPositioned(naContext c, const FGPositioned* pos);
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| * naRef ghostForAirport(naContext c, const FGAirport* apt)
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| * naRef ghostForNavaid(naContext c, const FGNavRecord* n)
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| * naRef ghostForRunway(naContext c, const FGRunway* r)
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| * naRef ghostForWaypt(naContext c, const flightgear::Waypt* wpt)
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|
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| So they could be replaced by a single template:
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|
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| <syntaxhighlight lang="cpp">
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| template <class T>
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| static T*
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| getGhost(naRef r)
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| {
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| if( naGhost_type(r) == &T::GhostType)
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| return (T*) naGhost_ptr(r);
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| return 0;
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| }
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| </syntaxhighlight>
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|
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| This would only require adding a new '''naGhostType''' member to the FGPositioned class (and automatically to FGAirport, FGNavRecord, FGRunway because they are derived from FGPositioned).
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|
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| So it would make sense to simply use a new "FGNasalWrapper" class, add it to the top of NasalSys.hxx, i.e. for C++ classes that are intended to be exposed to Nasal ?
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|
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| <syntaxhighlight lang="cpp">
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| class FGNasalWrapper {
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| public:
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| protected:
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| naGhostType GhostType;
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| private:
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| };
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| </syntaxhighlight>
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|
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| This would provide an opportunity to come up with an interface that is required to expose C++ classes as Nasal ghosts.
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|
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| And then, by changing positioned.hxx such that it derives from the new "FGNasalWrapper" class:
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|
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| <syntaxhighlight lang="cpp">
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| class FGPositioned : public SGReferenced, public FGNasalWrapper {
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| // ...
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| };
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| </syntaxhighlight>
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|
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| Once each instance/subclass of FGPositioned, has such a "GhostType" field, the getMember() wrappers you created [http://gitorious.org/fg/flightgear/blobs/78afdb3c2227e75fc1542b3992dcea26181d98cf/src/Scripting/NasalPositioned.cxx#line55 at the top of NasalPositioned.hxx], could be moved to the new FGNasalWrapper class, so that the GhostType field could be initialized by the ctor, using its own getMember() method:
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|
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| <syntaxhighlight lang="cpp">
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| class FGNasalWrapper {
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| public:
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| FGNasalWrapper(void *ghost_destructor, const char* ghost_name
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| const char* getMember(naContext c, void* g, naRef field, naRef* out);
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| protected:
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| naGhostType GhostType;
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| private:
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| };
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| </syntaxhighlight>
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|
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| The ghost_destructor could also be implemented as a method of FGNasalWrapper (rather than being a global void* function), so that each C++ class that is to be exposed to Nasal, would only need to implement the class-specific methods.
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|
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| Ultimately, this would also make it possible to reimplement the ghostFor* helpers as a template: http://gitorious.org/fg/flightgear/blobs/78afdb3c2227e75fc1542b3992dcea26181d98cf/src/Scripting/NasalPositioned.cxx#line139
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|
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| * ghostForPositioned();
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| * ghostForAirport();
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| * ghostForNavaid();
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| * ghostForRunway();
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| * ghostForWaypt();
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|
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| So that the "getGhost" method could be a part of the NasalWrapper class:
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|
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| <syntaxhighlight lang="cpp">
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| template <class T>
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| naRef getGhost(naContext c, const T* pos)
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| {
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| if (!pos) {
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| return naNil();
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| }
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| T::get(pos); // take a ref
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| return naNewGhost2(c, &T::GhostType, (void*) pos);
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| }
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| </syntaxhighlight>
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|
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| = Passing Pointers to Nasal scripts = | | = Passing Pointers to Nasal scripts = |
| Things get more complicated if you need to pass a handle to a C/C++ object into a Nasal script. There, you need to use a wrapped handle type called a ghost ("Garbage-collectable Handle to an OutSide Thing"), which has a callback that you need to implement to deal with what happens when the Nasal interpreter garbage collects your object. | | Things get more complicated if you need to pass a handle to a C/C++ object into a Nasal script. There, you need to use a wrapped handle type called a ghost ("Garbage-collectable Handle to an OutSide Thing"), which has a callback that you need to implement to deal with what happens when the Nasal interpreter garbage collects your object. |