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/hints.h"
#include "pxr/base/arch/pragmas.h"
#include "pxr/base/tf/diagnosticLite.h"

#include <mutex>

PXR_NAMESPACE_OPEN_SCOPE

template <class T>
class TfSingleton {
public:
    inline static T& GetInstance() {
        ARCH_PRAGMA_PUSH
        // Suppress warnings from clang. TfSingletons are explicitly
        // instantiated, so the warning around this usage is a false positive.
        ARCH_PRAGMA_UNDEFINED_VAR_TEMPLATE
        return ARCH_LIKELY(_instance) ? *_instance : _CreateInstance();
        ARCH_PRAGMA_POP
    }

    static bool CurrentlyExists() {
        return _instance ? true : false;
    }

    static void SetInstanceConstructed(T& instance) {
        if (_instance)
            TF_FATAL_ERROR("this function may not be called after "
                           "GetInstance() has completed");
        _instance = &instance;
    }
     
    static void DeleteInstance() {
        if (_instance)
            _DestroyInstance();
    }
private:
    static T& _CreateInstance();
    static void _DestroyInstance();
    static T* _instance;
    ARCH_PRAGMA_PUSH
    ARCH_PRAGMA_NEEDS_EXPORT_INTERFACE
    static std::mutex* _mutex;
    ARCH_PRAGMA_POP
};

PXR_NAMESPACE_CLOSE_SCOPE

#endif