singleton.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_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
 
struct Tf_SingletonInitState;
 
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 = _CreateOrWaitForInstance(_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 *_CreateOrWaitForInstance(std::atomic<T *> &instance);
    
    static std::atomic<T *> _instance;
    static std::atomic<Tf_SingletonInitState *> _initState;
};
 
PXR_NAMESPACE_CLOSE_SCOPE
 
#endif