ClickHouse/base/poco/Foundation/include/Poco/ThreadLocal.h

//
// ThreadLocal.h
//
// Library: Foundation
// Package: Threading
// Module:  Thread
//
// Definition of the ThreadLocal template and related classes.
//
// Copyright (c) 2004-2006, Applied Informatics Software Engineering GmbH.
// and Contributors.
//
// SPDX-License-Identifier:	BSL-1.0
//


#ifndef Foundation_ThreadLocal_INCLUDED
#define Foundation_ThreadLocal_INCLUDED


#include <map>
#include "Poco/Foundation.h"


namespace Poco
{


class Foundation_API TLSAbstractSlot
/// This is the base class for all objects
/// that the ThreadLocalStorage class manages.
{
public:
    TLSAbstractSlot();
    virtual ~TLSAbstractSlot();
};


template <class C>
class TLSSlot : public TLSAbstractSlot
/// The Slot template wraps another class
/// so that it can be stored in a ThreadLocalStorage
/// object. This class is used internally, and you
/// must not create instances of it yourself.
{
public:
    TLSSlot() : _value() { }

    ~TLSSlot() { }

    C & value() { return _value; }

private:
    TLSSlot(const TLSSlot &);
    TLSSlot & operator=(const TLSSlot &);

    C _value;
};


class Foundation_API ThreadLocalStorage
/// This class manages the local storage for each thread.
/// Never use this class directly, always use the
/// ThreadLocal template for managing thread local storage.
{
public:
    ThreadLocalStorage();
    /// Creates the TLS.

    ~ThreadLocalStorage();
    /// Deletes the TLS.

    TLSAbstractSlot *& get(const void * key);
    /// Returns the slot for the given key.

    static ThreadLocalStorage & current();
    /// Returns the TLS object for the current thread
    /// (which may also be the main thread).

    static void clear();
    /// Clears the current thread's TLS object.
    /// Does nothing in the main thread.

private:
    typedef std::map<const void *, TLSAbstractSlot *> TLSMap;

    TLSMap _map;

    friend class Thread;
};


template <class C>
class ThreadLocal
/// This template is used to declare type safe thread
/// local variables. It can basically be used like
/// a smart pointer class with the special feature
/// that it references a different object
/// in every thread. The underlying object will
/// be created when it is referenced for the first
/// time.
/// See the NestedDiagnosticContext class for an
/// example how to use this template.
/// Every thread only has access to its own
/// thread local data. There is no way for a thread
/// to access another thread's local data.
{
    typedef TLSSlot<C> Slot;

public:
    ThreadLocal() { }

    ~ThreadLocal() { }

    C * operator->() { return &get(); }

    C & operator*()
    /// "Dereferences" the smart pointer and returns a reference
    /// to the underlying data object. The reference can be used
    /// to modify the object.
    {
        return get();
    }

    C & get()
    /// Returns a reference to the underlying data object.
    /// The reference can be used to modify the object.
    {
        TLSAbstractSlot *& p = ThreadLocalStorage::current().get(this);
        if (!p)
            p = new Slot;
        return static_cast<Slot *>(p)->value();
    }

private:
    ThreadLocal(const ThreadLocal &);
    ThreadLocal & operator=(const ThreadLocal &);
};


} // namespace Poco


#endif // Foundation_ThreadLocal_INCLUDED