staticInterface.h

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//
// Copyright 2016 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_PLUG_STATIC_INTERFACE_H
#define PXR_BASE_PLUG_STATIC_INTERFACE_H


#include "pxr/pxr.h"
#include "pxr/base/plug/api.h"

#include <atomic>
#include <type_traits>
#include <typeinfo>

PXR_NAMESPACE_OPEN_SCOPE

// Base class for common stuff.
class Plug_StaticInterfaceBase {
public:
    bool IsInitialized() const
    {
        return _initialized;
    }

#if !defined(doxygen)
    typedef void* Plug_StaticInterfaceBase::*UnspecifiedBoolType;
#endif

protected:
    PLUG_API
    void _LoadAndInstantiate(const std::type_info& type) const;

protected:
    // POD types only!
    mutable std::atomic<bool> _initialized;
    mutable void* _ptr;
};

template <class Interface>
class PlugStaticInterface : private Plug_StaticInterfaceBase {
public:
    static_assert(std::is_abstract<Interface>::value,
                  "Interface type must be abstract.");

    typedef PlugStaticInterface<Interface> This;

    using Plug_StaticInterfaceBase::IsInitialized;

    operator UnspecifiedBoolType() const
    {
        return _GetPtr() ? &This::_ptr : nullptr;
    }

    bool operator!() const
    {
        return !*this;
    }

    Interface* Get() const
    {
        return _GetPtr();
    }

    Interface* operator->() const
    {
        return _GetPtr();
    }

    Interface& operator*() const
    {
        return *_GetPtr();
    }

private:
    Interface* _GetPtr() const
    {
        if (!_initialized) {
            _LoadAndInstantiate(typeid(Interface));
        }

        // XXX: We must assume _ptr has the right type since we have
        //      no common base class to dynamic_cast<> from.
        return static_cast<Interface*>(_ptr);
    }
};

PXR_NAMESPACE_CLOSE_SCOPE

#endif // PXR_BASE_PLUG_STATIC_INTERFACE_H