visitValue.h

//
// Copyright 2022 Pixar
//
// Licensed under the terms set forth in the LICENSE.txt file available at
// https://openusd.org/license.
//
#ifndef PXR_BASE_VT_VISIT_VALUE_H
#define PXR_BASE_VT_VISIT_VALUE_H
 
#include "pxr/pxr.h"
 
#include "pxr/base/vt/value.h"
 
PXR_NAMESPACE_OPEN_SCOPE
 
namespace Vt_ValueVisitDetail {
 
// These two overloads do SFINAE to detect whether the visitor can be invoked
// with the given held type T.  If the visitor cannot be invoked with T, it is
// instead invoked with the VtValue itself.  We use the int/long trick and pass
// 0 in the caller to disambiguate in case both are viable.
template <class T, class Visitor, class ...Args>
auto
Visit(VtValue const &val, Visitor &&visitor, int, Args &&...args) ->
    decltype(std::forward<Visitor>(visitor)(
                 val.UncheckedGet<T>(), std::forward<Args>(args)...))
{
    return std::forward<Visitor>(visitor)(
        val.UncheckedGet<T>(), std::forward<Args>(args)...);
}
 
template <class T, class Visitor, class ...Args>
auto
Visit(VtValue const &val, Visitor &&visitor, long, Args &&...args) {
    return std::forward<Visitor>(visitor)(val, std::forward<Args>(args)...);
}
 
} // Vt_ValueVisitDetail
 
 
template <class Visitor, class ...Args>
auto VtVisitValue(VtValue const &value, Visitor &&visitor, Args&&...args)
{
    // This generally gets the compiler to emit a jump table to dispatch
    // directly to the code for each known value type.
    switch (value.GetKnownValueTypeIndex()) {
 
// Cases for known types.
#define VT_CASE_FOR_TYPE_INDEX(unused, elem)                                   \
        case VtGetKnownValueTypeIndex<VT_TYPE(elem)>():                        \
            return Vt_ValueVisitDetail::Visit<VT_TYPE(elem)>(                  \
                value, std::forward<Visitor>(visitor), 0,                      \
                std::forward<Args>(args)...);                                  \
            break;
VT_FOR_EACH_VALUE_TYPE(VT_CASE_FOR_TYPE_INDEX)
#undef VT_CASE_FOR_TYPE_INDEX
 
        default:
            // Invoke visitor with value itself.
            return Vt_ValueVisitDetail::Visit<VtValue>(
                value, std::forward<Visitor>(visitor), 0,
                std::forward<Args>(args)...);
            break;
    };
}
 
template <
    template <class T, class ...> class Visitor,
    typename ...TypeArgs,
    typename ...FnArgs
    >
auto VtVisitValueType(VtValue const &value, FnArgs&&...args)
{
    // This generally gets the compiler to emit a jump table to dispatch
    // directly to the code for each known value type.
    switch (value.GetKnownValueTypeIndex()) {
 
// Cases for known types.
#define VT_CASE_FOR_TYPE_INDEX(unused, elem)                                   \
        case VtGetKnownValueTypeIndex<VT_TYPE(elem)>():                        \
            return Visitor<VT_TYPE(elem), TypeArgs...>::Visit(                 \
                std::forward<FnArgs>(args)...);                                \
            break;
VT_FOR_EACH_VALUE_TYPE(VT_CASE_FOR_TYPE_INDEX)
#undef VT_CASE_FOR_TYPE_INDEX
 
        default:
            // Invoke visitor with VtValue itself.
            return Visitor<VtValue, TypeArgs...>::Visit(
                std::forward<FnArgs>(args)...);
    };
}
 
template <
    template <class T, template <class...> class, class ...> class Visitor,
    template <class...> class Tmpl,
    typename ...TypeArgs,
    typename ...FnArgs
    >
auto VtVisitValueType(VtValue const &value, FnArgs&&...args)
{
    // This generally gets the compiler to emit a jump table to dispatch
    // directly to the code for each known value type.
    switch (value.GetKnownValueTypeIndex()) {
 
// Cases for known types.
#define VT_CASE_FOR_TYPE_INDEX(unused, elem)                                   \
        case VtGetKnownValueTypeIndex<VT_TYPE(elem)>():                        \
            return Visitor<VT_TYPE(elem), Tmpl, TypeArgs...>::Visit(           \
                std::forward<FnArgs>(args)...);                                \
            break;
VT_FOR_EACH_VALUE_TYPE(VT_CASE_FOR_TYPE_INDEX)
#undef VT_CASE_FOR_TYPE_INDEX
 
        default:
            // Invoke visitor with VtValue itself.
            return Visitor<VtValue, Tmpl, TypeArgs...>::Visit(
                std::forward<FnArgs>(args)...);
    };
}
 
PXR_NAMESPACE_CLOSE_SCOPE
 
#endif // PXR_BASE_VT_VISIT_VALUE_H