types.h

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//
// Copyright 2016 Pixar
//
// Licensed under the terms set forth in the LICENSE.txt file available at
// https://openusd.org/license.
//
#ifndef PXR_USD_SDF_TYPES_H
#define PXR_USD_SDF_TYPES_H

/// \file sdf/types.h
/// Basic Sdf data types

#include "pxr/pxr.h"
#include "pxr/usd/sdf/api.h"
#include "pxr/usd/sdf/assetPath.h"
#include "pxr/usd/sdf/declareHandles.h"
#include "pxr/usd/sdf/listOp.h"
#include "pxr/usd/sdf/opaqueValue.h"
#include "pxr/usd/sdf/pathExpression.h"
#include "pxr/usd/sdf/timeCode.h"
#include "pxr/usd/sdf/valueTypeName.h"

#include "pxr/base/arch/demangle.h"
#include "pxr/base/arch/inttypes.h"
#include "pxr/base/arch/pragmas.h"
#include "pxr/base/gf/half.h"
#include "pxr/base/gf/matrix2d.h"
#include "pxr/base/gf/matrix3d.h"
#include "pxr/base/gf/matrix4d.h"
#include "pxr/base/gf/quatd.h"
#include "pxr/base/gf/quatf.h"
#include "pxr/base/gf/quath.h"
#include "pxr/base/gf/vec2d.h"
#include "pxr/base/gf/vec2f.h"
#include "pxr/base/gf/vec2h.h"
#include "pxr/base/gf/vec2i.h"
#include "pxr/base/gf/vec3d.h"
#include "pxr/base/gf/vec3f.h"
#include "pxr/base/gf/vec3h.h"
#include "pxr/base/gf/vec3i.h"
#include "pxr/base/gf/vec4d.h"
#include "pxr/base/gf/vec4f.h"
#include "pxr/base/gf/vec4h.h"
#include "pxr/base/gf/vec4i.h"
#include "pxr/base/tf/enum.h"
#include "pxr/base/tf/preprocessorUtilsLite.h"
#include "pxr/base/tf/staticTokens.h"
#include "pxr/base/tf/token.h"
#include "pxr/base/tf/type.h"
#include "pxr/base/vt/array.h"
#include "pxr/base/vt/dictionary.h"
#include "pxr/base/vt/value.h"

#include <iosfwd>
#include <list>
#include <map>
#include <stdint.h>
#include <string>
#include <typeinfo>
#include <vector>

PXR_NAMESPACE_OPEN_SCOPE

class SdfPath;

/// An enum that specifies the type of an object. Objects
/// are entities that have fields and are addressable by path.
enum SdfSpecType {
    // The unknown type has a value of 0 so that SdfSpecType() is unknown.
    SdfSpecTypeUnknown = 0,

    // Real concrete types
    SdfSpecTypeAttribute,
    SdfSpecTypeConnection,
    SdfSpecTypeExpression,
    SdfSpecTypeMapper,
    SdfSpecTypeMapperArg,
    SdfSpecTypePrim,
    SdfSpecTypePseudoRoot,
    SdfSpecTypeRelationship,
    SdfSpecTypeRelationshipTarget,
    SdfSpecTypeVariant,
    SdfSpecTypeVariantSet,

    SdfNumSpecTypes
};

/// An enum that identifies the possible specifiers for an
/// SdfPrimSpec. The SdfSpecifier enum is registered as a TfEnum
/// for converting to and from <c>std::string</c>.
///
/// <b>SdfSpecifier:</b>
/// <ul>
/// <li><b>SdfSpecifierDef.</b> Defines a concrete prim.
/// <li><b>SdfSpecifierOver.</b> Overrides an existing prim.
/// <li><b>SdfSpecifierClass.</b> Defines an abstract prim.
/// <li><b>SdfNumSpecifiers.</b> The number of specifiers.
/// </ul>
///
enum SdfSpecifier {
    SdfSpecifierDef,
    SdfSpecifierOver,
    SdfSpecifierClass,
    SdfNumSpecifiers
};

/// Returns true if the specifier defines a prim.
inline
bool
SdfIsDefiningSpecifier(SdfSpecifier spec)
{
    return (spec != SdfSpecifierOver);
}

/// An enum that defines permission levels.
///
/// Permissions control which layers may refer to or express
/// opinions about a prim.  Opinions expressed about a prim, or
/// relationships to that prim, by layers that are not allowed
/// permission to access the prim will be ignored.
///
/// <b>SdfPermission:</b>
/// <ul>
/// <li><b>SdfPermissionPublic.</b> Public prims can be referred to by
///     anything. (Available to any client.)
/// <li><b>SdfPermissionPrivate.</b> Private prims can be referred to
///     only within the local layer stack, and not across references
///     or inherits. (Not available to clients.)
/// <li><b>SdfNumPermission.</b> Internal sentinel value.
/// </ul>
///
enum SdfPermission {
    SdfPermissionPublic,         
    SdfPermissionPrivate,        

    SdfNumPermissions            
};

/// An enum that identifies variability types for attributes.
/// Variability indicates whether the attribute may vary over time and
/// value coordinates, and if its value comes through authoring or
/// or from its owner.
///
/// <b>SdfVariability:</b>
/// <ul>
///     <li><b>SdfVariabilityVarying.</b> Varying attributes may be directly 
///            authored, animated and affected on by Actions.  They are the 
///            most flexible.
///     <li><b>SdfVariabilityUniform.</b> Uniform attributes may be authored 
///            only with non-animated values (default values).  They cannot 
///            be affected by Actions, but they can be connected to other 
///            Uniform attributes.
///     <li><b>SdNumVariabilities.</b> Internal sentinel value.
/// </ul>
///
enum SdfVariability {
    SdfVariabilityVarying,
    SdfVariabilityUniform,

    SdfNumVariabilities 
};


/// An enum for TfError codes related to authoring operations.
///
/// <b>SdfAuthoringError:</b>
/// <ul>
///     <li><b>SdfAuthoringErrorUnrecognizedFields.</b> This error is raised if
///            SdfLayer::TransferContent, or SdfLayer::SetField API is called
///            for layers of differing schema, and fields from the source layer
///            are not recognized by the target layer's schema.
///     <li><b>SdfAuthoringErrorUnrecognizedSpecType.</b> This error is raised
///            in attempts to create specs on layers with spec types that are
///            not recognized by the layer's schema.
/// </ul>
///
enum SdfAuthoringError
{
    SdfAuthoringErrorUnrecognizedFields,
    SdfAuthoringErrorUnrecognizedSpecType
};

// Each category of compatible units of measurement is defined by a
// preprocessor sequence of tuples.  Each such sequence gives rise to an enum
// representing the corresponding unit category.  All the unit categories are
// listed in _SDF_UNITS where each entry is a two-tuple with the unit category
// name as the first element, and the second element is the units in that
// category.  Each tuple in a unit category sequence corresponds to a unit of
// measurement represented by an enumerant whose name is given by concatenating
// 'Sdf', the unit category name, the word 'Unit' and the first entry in the
// tuple.  (E.g. units of category 'Length' are represented by an enum named
// SdfLengthUnit with enumerants SdfLengthUnitInch, SdfLengthUnitMeter and so
// forth.)  The second element in the tuple is the display name for the unit,
// and the third element is the relative size of the unit compared to the menv
// default unit for the unit category (which has a relative size of 1.0).
// Dimensionless quantities use a special 'Dimensionless' unit category
// represented by the enum SdfDimensionlessUnit.
#define _SDF_LENGTH_UNITS       \
((Millimeter, "mm",  0.001))    \
((Centimeter, "cm",  0.01))     \
((Decimeter,  "dm",  0.1))      \
((Meter,      "m",   1.0))      \
((Kilometer,  "km",  1000.0))   \
((Inch,       "in",  0.0254))   \
((Foot,       "ft",  0.3048))   \
((Yard,       "yd",  0.9144))   \
((Mile,       "mi",  1609.344))

#define _SDF_ANGULAR_UNITS      \
((Degrees, "deg", 1.0))         \
((Radians, "rad", 57.2957795130823208768))

#define _SDF_DIMENSIONLESS_UNITS \
((Percent, "%", 0.01))           \
((Default, "default", 1.0))

#define _SDF_UNITS                          \
((Length, _SDF_LENGTH_UNITS),               \
(Angular, _SDF_ANGULAR_UNITS),              \
(Dimensionless, _SDF_DIMENSIONLESS_UNITS))

#define _SDF_UNIT_TAG(tup) TF_PP_TUPLE_ELEM(0, tup)
#define _SDF_UNIT_NAME(tup) TF_PP_TUPLE_ELEM(1, tup)
#define _SDF_UNIT_SCALE(tup) TF_PP_TUPLE_ELEM(2, tup)

#define _SDF_UNITSLIST_CATEGORY(tup) TF_PP_TUPLE_ELEM(0, tup)
#define _SDF_UNITSLIST_TUPLES(tup) TF_PP_TUPLE_ELEM(1, tup)
#define _SDF_UNITSLIST_ENUM(elem) TF_PP_CAT(TF_PP_CAT(Sdf, \
                                    _SDF_UNITSLIST_CATEGORY(elem)), Unit)

#define _SDF_DECLARE_UNIT_ENUMERANT(tag, elem) \
    TF_PP_CAT(Sdf ## tag ## Unit, _SDF_UNIT_TAG(elem)),

#define _SDF_DECLARE_UNIT_ENUM(elem)                     \
enum _SDF_UNITSLIST_ENUM(elem) {                         \
    TF_PP_SEQ_FOR_EACH(_SDF_DECLARE_UNIT_ENUMERANT,      \
                       _SDF_UNITSLIST_CATEGORY(elem),    \
                       _SDF_UNITSLIST_TUPLES(elem))      \
};

#define _SDF_FOR_EACH_UNITS_IMPL(macro, ...)             \
    TF_PP_FOR_EACH(macro, __VA_ARGS__)
#define _SDF_FOR_EACH_UNITS(macro, args)                 \
    _SDF_FOR_EACH_UNITS_IMPL(macro, TF_PP_EAT_PARENS(args))

// On Windows this call to _SDF_FOR_EACH_UNITS generates a C4003 warning.
// This is harmless, but we disable the warning here so that external
// projects that include this header don't run into it as well.
ARCH_PRAGMA_PUSH
ARCH_PRAGMA_MACRO_TOO_FEW_ARGUMENTS
_SDF_FOR_EACH_UNITS(_SDF_DECLARE_UNIT_ENUM, _SDF_UNITS)
ARCH_PRAGMA_POP

/// A map of mapper parameter names to parameter values.
typedef std::map<std::string, VtValue> SdfMapperParametersMap;

/// A map of reference variant set names to variants in those sets.
typedef std::map<std::string, std::string> SdfVariantSelectionMap;

/// A map of variant set names to list of variants in those sets.
typedef std::map<std::string, std::vector<std::string> > SdfVariantsMap;

/// A map of source SdfPaths to target SdfPaths for relocation.
//  Note: This map needs to be lexicographically sorted for some downstream
//        clients, so SdfPath::FastLessThan is explicitly omitted as
//        the Compare template parameter.
typedef std::map<SdfPath, SdfPath> SdfRelocatesMap;

/// A single relocate specifying a source SdfPath and a target SdfPath for a 
/// relocation.
typedef std::pair<SdfPath, SdfPath> SdfRelocate;

/// A vector of relocation source path to target path pairs.
typedef std::vector<SdfRelocate> SdfRelocates;

/// A map from sample times to sample values.
typedef std::map<double, VtValue> SdfTimeSampleMap;

/// Gets the show default unit for the given /a typeName.
SDF_API TfEnum SdfDefaultUnit( TfToken const &typeName );

/// Gets the show default unit for the given /a unit.
SDF_API const TfEnum &SdfDefaultUnit( const TfEnum &unit );

/// Gets the unit category for a given /a unit.
SDF_API const std::string &SdfUnitCategory( const TfEnum &unit );

/// Converts from one unit of measure to another. The \a fromUnit and \a toUnit
/// units must be of the same type (for example, both of type SdfLengthUnit).
SDF_API double SdfConvertUnit( const TfEnum &fromUnit, const TfEnum &toUnit );

/// Gets the name for a given /a unit.
SDF_API const std::string &SdfGetNameForUnit( const TfEnum &unit );

/// Gets a unit for the given /a name
SDF_API const TfEnum &SdfGetUnitFromName( const std::string &name );

/// Given a value, returns if there is a valid corresponding valueType.
SDF_API bool SdfValueHasValidType(VtValue const& value);

/// Given an sdf valueType name, produce TfType if the type name specifies a
/// valid sdf value type.
SDF_API TfType SdfGetTypeForValueTypeName(TfToken const &name);

/// Given a value, produce the sdf valueType name.  If you provide a value that
/// does not return true for SdfValueHasValidType, the return value is
/// unspecified.
SDF_API SdfValueTypeName SdfGetValueTypeNameForValue(VtValue const &value);

/// Return role name for \p typeName.  Return empty token if \p typeName has no
/// associated role name.
SDF_API TfToken SdfGetRoleNameForValueTypeName(TfToken const &typeName);

// Sdf allows a specific set of types for attribute and metadata values.
// These types and some additional metadata are listed in the preprocessor
// sequence of tuples below. First element is a tag name that is appended to
// 'SdfValueType' to produce the C++ traits type for the value type.
// Second element is the value type name, third element is the corresponding
// C++ type, and the fourth element is the tuple of tuple dimensions.
//
// Libraries may extend this list and define additional value types.
// When doing so, the type must be declared using the SDF_DECLARE_VALUE_TYPE
// macro below. The type must also be registered in the associated schema using
// SdfSchema::_RegisterValueType(s).
#define _SDF_SCALAR_VALUE_TYPES                                \
    ((Bool,           bool,           bool,              () )) \
    ((UChar,          uchar,          unsigned char,     () )) \
    ((Int,            int,            int,               () )) \
    ((UInt,           uint,           unsigned int,      () )) \
    ((Int64,          int64,          int64_t,           () )) \
    ((UInt64,         uint64,         uint64_t,          () )) \
    ((Half,           half,           GfHalf,            () )) \
    ((Float,          float,          float,             () )) \
    ((Double,         double,         double,            () )) \
    ((TimeCode,       timecode,       SdfTimeCode,       () )) \
    ((String,         string,         std::string,       () )) \
    ((Token,          token,          TfToken,           () )) \
    ((Asset,          asset,          SdfAssetPath,      () )) \
    ((Opaque,         opaque,         SdfOpaqueValue,    () )) \
    ((PathExpression, pathExpression, SdfPathExpression, () ))

#define _SDF_DIMENSIONED_VALUE_TYPES                   \
    ((Matrix2d,   matrix2d,   GfMatrix2d,     (2,2) )) \
    ((Matrix3d,   matrix3d,   GfMatrix3d,     (3,3) )) \
    ((Matrix4d,   matrix4d,   GfMatrix4d,     (4,4) )) \
    ((Quath,      quath,      GfQuath,        (4)   )) \
    ((Quatf,      quatf,      GfQuatf,        (4)   )) \
    ((Quatd,      quatd,      GfQuatd,        (4)   )) \
    ((Int2,       int2,       GfVec2i,        (2)   )) \
    ((Half2,      half2,      GfVec2h,        (2)   )) \
    ((Float2,     float2,     GfVec2f,        (2)   )) \
    ((Double2,    double2,    GfVec2d,        (2)   )) \
    ((Int3,       int3,       GfVec3i,        (3)   )) \
    ((Half3,      half3,      GfVec3h,        (3)   )) \
    ((Float3,     float3,     GfVec3f,        (3)   )) \
    ((Double3,    double3,    GfVec3d,        (3)   )) \
    ((Int4,       int4,       GfVec4i,        (4)   )) \
    ((Half4,      half4,      GfVec4h,        (4)   )) \
    ((Float4,     float4,     GfVec4f,        (4)   )) \
    ((Double4,    double4,    GfVec4d,        (4)   ))

#define SDF_VALUE_TYPES _SDF_SCALAR_VALUE_TYPES _SDF_DIMENSIONED_VALUE_TYPES

// Accessors for individual elements in the value types tuples.
#define SDF_VALUE_CPP_TYPE(tup) TF_PP_TUPLE_ELEM(2, tup)
#define SDF_VALUE_CPP_ARRAY_TYPE(tup) VtArray<TF_PP_TUPLE_ELEM(2, tup)>

template <class T>
struct SdfValueTypeTraits {
    static const bool IsValueType = false;
};

// Allow character arrays to be treated as Sdf value types.
// Sdf converts character arrays to strings for scene description.
template <int N>
struct SdfValueTypeTraits<char[N]> {
    static const bool IsValueType = true;
};

#define SDF_DECLARE_VALUE_TYPE_TRAITS(unused, elem)                         \
template <>                                                                 \
struct SdfValueTypeTraits<SDF_VALUE_CPP_TYPE(elem)> {                       \
    static const bool IsValueType = true;                                   \
};                                                                          \
template <>                                                                 \
struct SdfValueTypeTraits<SDF_VALUE_CPP_ARRAY_TYPE(elem)> {                 \
    static const bool IsValueType = true;                                   \
};

TF_PP_SEQ_FOR_EACH(SDF_DECLARE_VALUE_TYPE_TRAITS, ~, SDF_VALUE_TYPES);

/// Convert \p dict to a valid metadata dictionary for scene description.  Valid
/// metadata dictionaries have values that are any of SDF_VALUE_TYPES (or
/// VtArrays of those), plus VtDictionary with values of those types (or
/// similarly nested VtDictionaries).
///
/// Certain conversions are performed in an attempt to produce a valid metadata
/// dictionary.  For example:
///
/// Convert std::vector<VtValue> to VtArray<T> where T is the type of the first
/// element in the vector.  Fail conversion for empty vectors where a concrete
/// type cannot be inferred.
///
/// Convert python sequences to VtArray<T> where T is the type of the first
/// element in the python sequence, when converted to VtValue, if that T is an
/// SDF_VALUE_TYPE).  Fail conversion for empty sequences where a concrete type
/// cannot be inferred.
///
/// If any values cannot be converted to valid SDF_VALUE_TYPES, omit those
/// elements and add a message to \p errMsg indicating which values were
/// omitted.
///
SDF_API
bool
SdfConvertToValidMetadataDictionary(VtDictionary *dict, std::string *errMsg);

#define SDF_VALUE_ROLE_NAME_TOKENS              \
    (Point)                                     \
    (Normal)                                    \
    (Vector)                                    \
    (Color)                                     \
    (Frame)                                     \
    (Transform)                                 \
    (PointIndex)                                \
    (EdgeIndex)                                 \
    (FaceIndex)                                 \
    (Group)                                     \
    (TextureCoordinate)

TF_DECLARE_PUBLIC_TOKENS(SdfValueRoleNames, SDF_API, SDF_VALUE_ROLE_NAME_TOKENS);

SDF_DECLARE_HANDLES(SdfLayer);

SDF_DECLARE_HANDLES(SdfAttributeSpec);
SDF_DECLARE_HANDLES(SdfPrimSpec);
SDF_DECLARE_HANDLES(SdfPropertySpec);
SDF_DECLARE_HANDLES(SdfSpec);
SDF_DECLARE_HANDLES(SdfRelationshipSpec);
SDF_DECLARE_HANDLES(SdfVariantSetSpec);
SDF_DECLARE_HANDLES(SdfVariantSpec);

typedef std::map<std::string, SdfVariantSetSpecHandle> 
    SdfVariantSetSpecHandleMap;

/// Writes the string representation of \c SdfSpecifier to \a out.
SDF_API 
std::ostream & operator<<( std::ostream &out, const SdfSpecifier &spec );

/// Writes the string representation of \c SdfRelocatesMap to \a out.
SDF_API 
std::ostream & operator<<( std::ostream &out,
                           const SdfRelocatesMap &reloMap );

/// Writes the string representation of \c SdfRelocates to \a out.
SDF_API 
std::ostream & operator<<( std::ostream &out,
                           const SdfRelocates &relocates );

/// Writes the string representation of \c SdfTimeSampleMap to \a out.
SDF_API 
std::ostream & operator<<( std::ostream &out,
                           const SdfTimeSampleMap &sampleMap );

SDF_API 
std::ostream &VtStreamOut(const SdfVariantSelectionMap &, std::ostream &);

/// \class SdfUnregisteredValue
/// Stores a representation of the value for an unregistered metadata
/// field encountered during text layer parsing. 
/// 
/// This provides the ability to serialize this data to a layer, as
/// well as limited inspection and editing capabilities (e.g., moving
/// this data to a different spec or field) even when the data type
/// of the value isn't known.
class SdfUnregisteredValue
{
public:
    /// Wraps an empty VtValue
    SDF_API SdfUnregisteredValue();

    /// Wraps a std::string
    SDF_API explicit SdfUnregisteredValue(const std::string &value);

    /// Wraps a VtDictionary
    SDF_API explicit SdfUnregisteredValue(const VtDictionary &value);

    /// Wraps a SdfUnregisteredValueListOp
    SDF_API explicit SdfUnregisteredValue(const SdfUnregisteredValueListOp &value);

    /// Returns the wrapped VtValue specified in the constructor
    const VtValue& GetValue() const {
        return _value;
    }

    /// Hash.
    friend size_t hash_value(const SdfUnregisteredValue &uv) {
        return uv._value.GetHash();
    }

    /// Returns true if the wrapped VtValues are equal
    SDF_API bool operator==(const SdfUnregisteredValue &other) const;

    /// Returns true if the wrapped VtValues are not equal
    SDF_API bool operator!=(const SdfUnregisteredValue &other) const;

private:
    VtValue _value;
};

/// Writes the string representation of \c SdfUnregisteredValue to \a out.
SDF_API std::ostream &operator << (std::ostream &out, const SdfUnregisteredValue &value);

class Sdf_ValueTypeNamesType {
    Sdf_ValueTypeNamesType(const Sdf_ValueTypeNamesType&) = delete;
    Sdf_ValueTypeNamesType& operator=(const Sdf_ValueTypeNamesType&) = delete;
public:
    SdfValueTypeName Bool;
    SdfValueTypeName UChar, Int, UInt, Int64, UInt64;
    SdfValueTypeName Half, Float, Double, TimeCode;
    SdfValueTypeName String, Token, Asset;
    SdfValueTypeName Int2,     Int3,     Int4;
    SdfValueTypeName Half2,    Half3,    Half4;
    SdfValueTypeName Float2,   Float3,   Float4;
    SdfValueTypeName Double2,  Double3,  Double4;
    SdfValueTypeName Point3h,  Point3f,  Point3d;
    SdfValueTypeName Vector3h, Vector3f, Vector3d;
    SdfValueTypeName Normal3h, Normal3f, Normal3d;
    SdfValueTypeName Color3h,  Color3f,  Color3d;
    SdfValueTypeName Color4h,  Color4f,  Color4d;
    SdfValueTypeName Quath,    Quatf,    Quatd;
    SdfValueTypeName Matrix2d, Matrix3d, Matrix4d;
    SdfValueTypeName Frame4d;
    SdfValueTypeName TexCoord2h, TexCoord2f, TexCoord2d;
    SdfValueTypeName TexCoord3h, TexCoord3f, TexCoord3d;
    SdfValueTypeName Opaque;
    SdfValueTypeName Group;
    SdfValueTypeName PathExpression;

    SdfValueTypeName BoolArray;
    SdfValueTypeName UCharArray, IntArray, UIntArray, Int64Array, UInt64Array;
    SdfValueTypeName HalfArray, FloatArray, DoubleArray, TimeCodeArray;
    SdfValueTypeName StringArray, TokenArray, AssetArray;
    SdfValueTypeName Int2Array,     Int3Array,     Int4Array;
    SdfValueTypeName Half2Array,    Half3Array,    Half4Array;
    SdfValueTypeName Float2Array,   Float3Array,   Float4Array;
    SdfValueTypeName Double2Array,  Double3Array,  Double4Array;
    SdfValueTypeName Point3hArray,  Point3fArray,  Point3dArray;
    SdfValueTypeName Vector3hArray, Vector3fArray, Vector3dArray;
    SdfValueTypeName Normal3hArray, Normal3fArray, Normal3dArray;
    SdfValueTypeName Color3hArray,  Color3fArray,  Color3dArray;
    SdfValueTypeName Color4hArray,  Color4fArray,  Color4dArray;
    SdfValueTypeName QuathArray,    QuatfArray,    QuatdArray;
    SdfValueTypeName Matrix2dArray, Matrix3dArray, Matrix4dArray;
    SdfValueTypeName Frame4dArray;
    SdfValueTypeName TexCoord2hArray, TexCoord2fArray, TexCoord2dArray;
    SdfValueTypeName TexCoord3hArray, TexCoord3fArray, TexCoord3dArray;
    SdfValueTypeName PathExpressionArray;

    SDF_API ~Sdf_ValueTypeNamesType();
    struct _Init {
        SDF_API static const Sdf_ValueTypeNamesType* New();
    };

    // For Pixar internal backwards compatibility.
    TfToken GetSerializationName(const SdfValueTypeName&) const;
    TfToken GetSerializationName(const VtValue&) const;
    TfToken GetSerializationName(const TfToken&) const;

private:
    friend const Sdf_ValueTypeNamesType* Sdf_InitializeValueTypeNames();
    Sdf_ValueTypeNamesType();
};

extern SDF_API TfStaticData<const Sdf_ValueTypeNamesType,
    Sdf_ValueTypeNamesType::_Init> SdfValueTypeNames;

/// \class SdfValueBlock
/// A special value type that can be used to explicitly author an
/// opinion for an attribute's default value or time sample value
/// that represents having no value. Note that this is different
/// from not having a value authored.
///
/// One could author such a value in two ways.
/// 
/// \code
/// attribute->SetDefaultValue(VtValue(SdfValueBlock());
/// ...
/// layer->SetTimeSample(attribute->GetPath(), 101, VtValue(SdfValueBlock()));
/// \endcode
///
struct SdfValueBlock { 
    bool operator==(const SdfValueBlock& block) const { return true; }
    bool operator!=(const SdfValueBlock& block) const { return false; }

private:
    friend inline size_t hash_value(const SdfValueBlock &block) { return 0; }
};

// Write out the string representation of a block.
SDF_API std::ostream& operator<<(std::ostream&, SdfValueBlock const&); 

// A class that represents a human-readable value.  This is used for the special
// purpose of producing layers that serialize field values in alternate ways; to
// produce more human-readable output, for example.
struct SdfHumanReadableValue {
    SdfHumanReadableValue() = default;
    explicit SdfHumanReadableValue(std::string const &text) : _text(text) {}

    bool operator==(SdfHumanReadableValue const &other) const {
        return GetText() == other.GetText();
    }
    bool operator!=(SdfHumanReadableValue const &other) const {
        return !(*this == other);
    }

    std::string const &GetText() const { return _text; }
private:
    std::string _text;
};

SDF_API
std::ostream &operator<<(std::ostream &out, const SdfHumanReadableValue &hrval);

SDF_API
size_t hash_value(const SdfHumanReadableValue &hrval);

PXR_NAMESPACE_CLOSE_SCOPE

#endif // PXR_USD_SDF_TYPES_H