3.3. C++ ABI and type factory infrastructure

3.3.1. Cast

When C++ interfaces with daScript, the cast ABI is followed.

• Value types are converted to and from vec4f, in specific memory layout

• Reference types have their address converted to and from vec4f

It is expected that vec4f * can be pruned to a by value type by simple pointer cast becase the daScript interpreter will in certain cases access pre-cast data via the v_ldu intrinsic:

template <typename TT>
TT get_data ( vec4f * dasData ) {           // default version
    return cast<TT>::to(v_ldu((float *)dasData));
}

int32_t get_data ( vec4f * dasData ) {      // legally optimized version
    return * (int32_t *) dasData;
}

ABI infrastructure is implemented via the C++ cast template, which serves two primary functions:

• Casting from C++ to daScript

• Casting to C++ from daScript

The from function expects a daScript type as an input, and outputs a vec4f.

The to function expects a vec4f, and outputs a daScript type.

Let’s review the following example:

template <>
struct cast <int32_t> {
    static __forceinline int32_t to ( vec4f x )            { return v_extract_xi(v_cast_vec4i(x)); }
    static __forceinline vec4f from ( int32_t x )          { return v_cast_vec4f(v_splatsi(x)); }
};

It implements the ABI for the int32_t, which packs an int32_t value at the beginning of the vec4f using multiplatform intrinsics.

Let’s review another example, which implements default packing of a reference type:

template <typename TT>
struct cast <TT &> {
    static __forceinline TT & to ( vec4f a )               { return *(TT *) v_extract_ptr(v_cast_vec4i((a))); }
    static __forceinline vec4f from ( const TT & p )       { return v_cast_vec4f(v_splats_ptr((const void *)&p)); }
};

Here, a pointer to the data is packed in a vec4f using multiplatform intrinsics.

3.3.2. Type factory

When C++ types are exposed to daScript, type factory infrastructure is employed.

To expose any custom C++ type, use the MAKE_TYPE_FACTORY macro, or the MAKE_EXTERNAL_TYPE_FACTORY and IMPLEMENT_EXTERNAL_TYPE_FACTORY macro pair:

MAKE_TYPE_FACTORY(clock, das::Time)

The example above tells daScript that the C++ type das::Time will be exposed to daScript with the name clock.

Let’s look at the implementation of the MAKE_TYPE_FACTORY macro:

#define MAKE_TYPE_FACTORY(TYPE,CTYPE) \
    namespace das { \
    template <> \
    struct typeFactory<CTYPE> { \
        static TypeDeclPtr make(const ModuleLibrary & library ) { \
            return makeHandleType(library,#TYPE); \
        } \
    }; \
    template <> \
    struct typeName<CTYPE> { \
        constexpr static const char * name() { return #TYPE; } \
    }; \
    };

What happens in the example above is that two templated policies are exposed to C++.

The typeName policy has a single static function name, which returns the string name of the type.

The typeFactory policy creates a smart pointer to daScript the das::TypeDecl type, which corresponds to C++ type. It expects to find the type somewhere in the provided ModuleLibrary (see Modules).

3.3.3. Type aliases

A custom type factory is the preferable way to create aliases:

struct Point3 { float x, y, z; };

template <>
struct typeFactory<Point3> {
    static TypeDeclPtr make(const ModuleLibrary &) {
        auto t = make_smart<TypeDecl>(Type::tFloat3);
        t->alias = "Point3";
        t->aotAlias = true;
        return t;
    }
};

template <> struct typeName<Point3>   { constexpr static const char * name() { return "Point3"; } };

In the example above, the C++ application already has a Point3 type, which is very similar to daScript’s float3. Exposing C++ functions which operate on Point3 is preferable, so the implementation creates an alias named Point3 which corresponds to the das Type::tFloat3.

Sometimes, a custom implementation of typeFactory is required to expose C++ to a daScript type in a more native fashion. Let’s review the following example:

  struct SampleVariant {
      int32_t _variant;
      union {
          int32_t     i_value;
          float       f_value;
          char *      s_value;
      };
  };

template <>
struct typeFactory<SampleVariant> {
    static TypeDeclPtr make(const ModuleLibrary & library ) {
        auto vtype = make_smart<TypeDecl>(Type::tVariant);
        vtype->alias = "SampleVariant";
        vtype->aotAlias = true;
        vtype->addVariant("i_value", typeFactory<decltype(SampleVariant::i_value)>::make(library));
        vtype->addVariant("f_value", typeFactory<decltype(SampleVariant::f_value)>::make(library));
        vtype->addVariant("s_value", typeFactory<decltype(SampleVariant::s_value)>::make(library));
        // optional validation
        DAS_ASSERT(sizeof(SampleVariant) == vtype->getSizeOf());
        DAS_ASSERT(alignof(SampleVariant) == vtype->getAlignOf());
        DAS_ASSERT(offsetof(SampleVariant, i_value) == vtype->getVariantFieldOffset(0));
        DAS_ASSERT(offsetof(SampleVariant, f_value) == vtype->getVariantFieldOffset(1));
        DAS_ASSERT(offsetof(SampleVariant, s_value) == vtype->getVariantFieldOffset(2));
        return vtype;
    }
};

Here, C++ type SomeVariant matches the daScript variant type with its memory layout. The code above exposes a C++ type alias and creates a corresponding TypeDecl.