visitValue.h

//
// Copyright 2022 Pixar
//
// Licensed under the Apache License, Version 2.0 (the "Apache License")
// with the following modification; you may not use this file except in
// compliance with the Apache License and the following modification to it:
// Section 6. Trademarks. is deleted and replaced with:
//
// 6. Trademarks. This License does not grant permission to use the trade
//    names, trademarks, service marks, or product names of the Licensor
//    and its affiliates, except as required to comply with Section 4(c) of
//    the License and to reproduce the content of the NOTICE file.
//
// You may obtain a copy of the Apache License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the Apache License with the above modification is
// distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied. See the Apache License for the specific
// language governing permissions and limitations under the Apache 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(r, unused, i, elem)                             \
        case i:                                                                \
            return Vt_ValueVisitDetail::Visit<VT_TYPE(elem)>(                  \
                value, std::forward<Visitor>(visitor), 0);                     \
            break;
BOOST_PP_SEQ_FOR_EACH_I(VT_CASE_FOR_TYPE_INDEX, ~, VT_VALUE_TYPES)
#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