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.
template <class T, class Visitor,
    class = decltype(std::declval<Visitor>()(std::declval<T>()))>
auto
Visit(VtValue const &val, Visitor &&visitor, int) {
    return std::forward<Visitor>(visitor)(val.UncheckedGet<T>());
}
 
template <class T, class Visitor>
auto
Visit(VtValue const &val, Visitor &&visitor, ...) {
    return std::forward<Visitor>(visitor)(val);
}
 
} // Vt_ValueVisitDetail
 
template <class Visitor>
auto VtVisitValue(VtValue const &value, Visitor &&visitor)
{
    // 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);                     \
            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);
            break;
    };
}
 
PXR_NAMESPACE_CLOSE_SCOPE
 
#endif // PXR_BASE_VT_VISIT_VALUE_H