singleton.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_TF_SINGLETON_H
#define PXR_BASE_TF_SINGLETON_H


#include "pxr/pxr.h"
#include "pxr/base/arch/pragmas.h"

#include <atomic>

PXR_NAMESPACE_OPEN_SCOPE

template <class T>
class TfSingleton {
public:
    inline static T& GetInstance() {
        // Suppress undefined-var-template warnings from clang; _instance
        // is expected to be instantiated in another translation unit via
        // the TF_INSTANTIATE_SINGLETON macro.
        ARCH_PRAGMA_PUSH
        ARCH_PRAGMA_UNDEFINED_VAR_TEMPLATE
        T *p = _instance.load();
        if (!p) {
            p = _CreateInstance(_instance);
        }
        ARCH_PRAGMA_POP
        return *p;
    }

    inline static bool CurrentlyExists() {
        // Suppress undefined-var-template warnings from clang; _instance
        // is expected to be instantiated in another translation unit via
        // the TF_INSTANTIATE_SINGLETON macro.
        ARCH_PRAGMA_PUSH
        ARCH_PRAGMA_UNDEFINED_VAR_TEMPLATE
        return static_cast<bool>(_instance.load());
        ARCH_PRAGMA_POP
    }

    inline static void SetInstanceConstructed(T& instance);
     
    inline static void DeleteInstance();
    
private:
    static T *_CreateInstance(std::atomic<T *> &instance);
    
    static std::atomic<T *> _instance;
};

PXR_NAMESPACE_CLOSE_SCOPE

#endif