观察者模式（sigslot in C++）

        大家，我是东风，今天抽点时间整理一下我很久前关注的一个不错的库，可以支持我们在使用标准C++的时候使用信号槽机制进行观察者模式设计，sigslot 官网： http://sigslot.sourceforge.net/  

        本文较为详尽探讨了一种观察者模式的精妙实现方式，即2002年由Sarah Thompson匠心设计的sigslot库。该库以其高效、灵活的特点，在信号与槽的连接管理上展现出卓越性能，成为事件驱动编程领域中的一大亮点。 

1.sigslot 类结构

        其中，以 _signal_base_interface 为基类的派生系列主要实现信号的链接与触发机制；以 has_slots_interface 为基类的派生系列主要实现槽对象，提供信号操作的对象；mt_policy、lock_block 则主要实现类似std::lock_guard<>的锁机制，确保线程安全。此外，很重要的一个类 _opaque_connection，它是槽对象的逻辑存储节点，在一定程度上解耦了信号与槽的直接联系。 

2.源码疑问注解

2.1.信号与槽

//template <class mt_policy>
//class _signal_base
typedef std::list<_opaque_connection> connections_list;connections_list m_connected_slots; //template <class mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
//class has_slots
typedef std::set<_signal_base_interface*> sender_set;sender_set m_senders; 

可以这样理解，一个信号可以触发多个槽回调，同样，一个槽对象可以被多个信号触发。 

2.2.函数指针 cast 

        肯定不少人会对_opaque_connection的union实现函数指针 cast 表示疑问

template <typename FromT, typename ToT>
union union_caster {FromT from;ToT to;
}; 

        我先给出自己的理解：union_caster 实现将 FromT 类型转成 ToT。其中本质是利用以下两个性质：

    （1） union 同一时刻只能存储一个值，要么是 from，要么是 to

    （2）函数指针在同一平台上的字节大小是一样的

 基于这两点，那么执行下列代码时，会有以下效果（见代码） 

template <typename FromT, typename ToT>
union union_caster {FromT from;ToT to;
};void fun_t(int a)
{std::cout << "fun_t " << a << std::endl;
}void fun_f(int a,int b)
{std::cout << "fun_f " << a << "," << b << std::endl;
}int main()
{typedef void (*emit_t)(int a);typedef void (*em_t)(int a, int b);union_caster<em_t, emit_t> caster2;   // <em_t, emit_t>caster2.from = fun_f;//此时，只看数值，caster2.to == caster2.from == fun_femit_t pemit = caster2.to;union_caster<emit_t,em_t> caster;   // <emit_t,em_t> 巧妙完成类型转换caster.from = pemit;   //caster的from是caster2的to(caster.to)(30, 20);   //caster的to是caster2的from，因此这里等价 fun_f(30, 20)//验证想法std::cout << "sizeof fun_t(): " << sizeof(&fun_t) << "  sizeof fun_t(): " << sizeof(&fun_f) << std::endl;std::cout << "caster2.from: " << caster2.from << "  caster2.to: " << caster2.to << std::endl;std::cout << "caster.from: " << caster.from << "  caster.to: " << caster.to << std::endl;getchar();return 0;
}

3.库使用

这里我们先给一个常规的应用，可以参照作者示例

#include "sigslot.h"
#include <iostream>using namespace sigslot;class Switch
{
public:Switch(){}~Switch(){}public:signal1<int> ToggleLight;};class Light : public has_slots<>
{
public:Light() {};~Light() {};public:void TurnState(int a){std::cout << "Light on/off " << a << std::endl;}};int main()
{Switch s;Light l;s.ToggleLight.connect(&l,&Light::TurnState);//l.signal_disconnect(&(s.ToggleLight));//l.disconnect_all();//s.ToggleLight.disconnect(&l);s.ToggleLight.emit(1000);//s.ToggleLight.emit(1000);getchar();return 0;
}

        我认为只需搞明白connect()即可，其他行为就显而易见了。通过调试跟踪，我们可以发现，connect() 大概流程为：

        很显然，connect其实就是完成 m_senders 集合和 m_connected_slots 链表的更新，后续 emit() 、 disconnect()等等操作都是基于这两个数据结构来开展的。举个例子，当信号对象执行emit()时，不用查阅代码我们都可以知道，流程大概是：信号对象遍历 m_connected_slots ，针对每一个 _opaque_connection，回调它事先存储的槽对象成员方法。

TIPS:

      （1） has_slots<> 的 signal_connect()、signal_disconnect()成员只是操作了 m_senders，并没有更新 m_connected_slots；disconnect_all() 成员更新了 m_senders 和 m_connected_slots；

      （2）signal 的 connect()、disconnect() 等基本所有类似成员方法里都同时更新了 m_senders和 m_connected_slots。 

若有不对，提醒我啦！

附录：sigslot 源码

//头文件：sigslot.h
//
// sigslot.h: Signal/Slot classes
//
// Written by Sarah Thompson (sarah@telergy.com) 2002.
//
// License: Public domain. You are free to use this code however you like, with
// the proviso that the author takes on no responsibility or liability for any
// use.
//
// QUICK DOCUMENTATION
//
//        (see also the full documentation at http://sigslot.sourceforge.net/)
//
//    #define switches
//      SIGSLOT_PURE_ISO:
//        Define this to force ISO C++ compliance. This also disables all of
//        the thread safety support on platforms where it is available.
//
//      SIGSLOT_USE_POSIX_THREADS:
//        Force use of Posix threads when using a C++ compiler other than gcc
//        on a platform that supports Posix threads. (When using gcc, this is
//        the default - use SIGSLOT_PURE_ISO to disable this if necessary)
//
//      SIGSLOT_DEFAULT_MT_POLICY:
//        Where thread support is enabled, this defaults to
//        multi_threaded_global. Otherwise, the default is single_threaded.
//        #define this yourself to override the default. In pure ISO mode,
//        anything other than single_threaded will cause a compiler error.
//
//    PLATFORM NOTES
//
//      Win32:
//        On Win32, the WEBRTC_WIN symbol must be #defined. Most mainstream
//        compilers do this by default, but you may need to define it yourself
//        if your build environment is less standard. This causes the Win32
//        thread support to be compiled in and used automatically.
//
//      Unix/Linux/BSD, etc.:
//        If you're using gcc, it is assumed that you have Posix threads
//        available, so they are used automatically. You can override this (as
//        under Windows) with the SIGSLOT_PURE_ISO switch. If you're using
//        something other than gcc but still want to use Posix threads, you
//        need to #define SIGSLOT_USE_POSIX_THREADS.
//
//      ISO C++:
//        If none of the supported platforms are detected, or if
//        SIGSLOT_PURE_ISO is defined, all multithreading support is turned
//        off, along with any code that might cause a pure ISO C++ environment
//        to complain. Before you ask, gcc -ansi -pedantic won't compile this
//        library, but gcc -ansi is fine. Pedantic mode seems to throw a lot of
//        errors that aren't really there. If you feel like investigating this,
//        please contact the author.
//
//
//    THREADING MODES
//
//      single_threaded:
//        Your program is assumed to be single threaded from the point of view
//        of signal/slot usage (i.e. all objects using signals and slots are
//        created and destroyed from a single thread). Behaviour if objects are
//        destroyed concurrently is undefined (i.e. you'll get the occasional
//        segmentation fault/memory exception).
//
//      multi_threaded_global:
//        Your program is assumed to be multi threaded. Objects using signals
//        and slots can be safely created and destroyed from any thread, even
//        when connections exist. In multi_threaded_global mode, this is
//        achieved by a single global mutex (actually a critical section on
//        Windows because they are faster). This option uses less OS resources,
//        but results in more opportunities for contention, possibly resulting
//        in more context switches than are strictly necessary.
//
//      multi_threaded_local:
//        Behaviour in this mode is essentially the same as
//        multi_threaded_global, except that each signal, and each object that
//        inherits has_slots, all have their own mutex/critical section. In
//        practice, this means that mutex collisions (and hence context
//        switches) only happen if they are absolutely essential. However, on
//        some platforms, creating a lot of mutexes can slow down the whole OS,
//        so use this option with care.
//
//    USING THE LIBRARY
//
//      See the full documentation at http://sigslot.sourceforge.net/
//
// Libjingle specific:
//
// This file has been modified such that has_slots and signalx do not have to be
// using the same threading requirements. E.g. it is possible to connect a
// has_slots<single_threaded> and signal0<multi_threaded_local> or
// has_slots<multi_threaded_local> and signal0<single_threaded>.
// If has_slots is single threaded the user must ensure that it is not trying
// to connect or disconnect to signalx concurrently or data race may occur.
// If signalx is single threaded the user must ensure that disconnect, connect
// or signal is not happening concurrently or data race may occur.#ifndef RTC_BASE_THIRD_PARTY_SIGSLOT_SIGSLOT_H_
#define RTC_BASE_THIRD_PARTY_SIGSLOT_SIGSLOT_H_#include <cstring>
#include <list>
#include <set>// On our copy of sigslot.h, we set single threading as default.
#define SIGSLOT_DEFAULT_MT_POLICY single_threaded#if defined(SIGSLOT_PURE_ISO) ||                   \(!defined(WEBRTC_WIN) && !defined(__GNUG__) && \!defined(SIGSLOT_USE_POSIX_THREADS))
#define _SIGSLOT_SINGLE_THREADED
#elif defined(WEBRTC_WIN)
#define _SIGSLOT_HAS_WIN32_THREADS
#include "windows.h"
#elif defined(__GNUG__) || defined(SIGSLOT_USE_POSIX_THREADS)
#define _SIGSLOT_HAS_POSIX_THREADS
#include <pthread.h>
#else
#define _SIGSLOT_SINGLE_THREADED
#endif#ifndef SIGSLOT_DEFAULT_MT_POLICY
#ifdef _SIGSLOT_SINGLE_THREADED
#define SIGSLOT_DEFAULT_MT_POLICY single_threaded
#else
#define SIGSLOT_DEFAULT_MT_POLICY multi_threaded_local
#endif
#endif// 
namespace sigslot {class single_threaded {public:void lock() {}void unlock() {}
};#ifdef _SIGSLOT_HAS_WIN32_THREADS
// The multi threading policies only get compiled in if they are enabled.
class multi_threaded_global {public:multi_threaded_global() {static bool isinitialised = false;if (!isinitialised) {InitializeCriticalSection(get_critsec());isinitialised = true;}}void lock() { EnterCriticalSection(get_critsec()); }void unlock() { LeaveCriticalSection(get_critsec()); }private:CRITICAL_SECTION* get_critsec() {static CRITICAL_SECTION g_critsec;return &g_critsec;}
};class multi_threaded_local {public:multi_threaded_local() { InitializeCriticalSection(&m_critsec); }multi_threaded_local(const multi_threaded_local&) {InitializeCriticalSection(&m_critsec);}~multi_threaded_local() { DeleteCriticalSection(&m_critsec); }void lock() { EnterCriticalSection(&m_critsec); }void unlock() { LeaveCriticalSection(&m_critsec); }private:CRITICAL_SECTION m_critsec;
};
#endif  // _SIGSLOT_HAS_WIN32_THREADS#ifdef _SIGSLOT_HAS_POSIX_THREADS
// The multi threading policies only get compiled in if they are enabled.
class multi_threaded_global {public:void lock() { pthread_mutex_lock(get_mutex()); }void unlock() { pthread_mutex_unlock(get_mutex()); }private:static pthread_mutex_t* get_mutex();
};class multi_threaded_local {public:multi_threaded_local() { pthread_mutex_init(&m_mutex, nullptr); }multi_threaded_local(const multi_threaded_local&) {pthread_mutex_init(&m_mutex, nullptr);}~multi_threaded_local() { pthread_mutex_destroy(&m_mutex); }void lock() { pthread_mutex_lock(&m_mutex); }void unlock() { pthread_mutex_unlock(&m_mutex); }private:pthread_mutex_t m_mutex;
};
#endif  // _SIGSLOT_HAS_POSIX_THREADStemplate <class mt_policy>
class lock_block {public:mt_policy* m_mutex;lock_block(mt_policy* mtx) : m_mutex(mtx) { m_mutex->lock(); }~lock_block() { m_mutex->unlock(); }
};class _signal_base_interface;class has_slots_interface {private:typedef void (*signal_connect_t)(has_slots_interface* self,_signal_base_interface* sender);typedef void (*signal_disconnect_t)(has_slots_interface* self,_signal_base_interface* sender);typedef void (*disconnect_all_t)(has_slots_interface* self);const signal_connect_t m_signal_connect;const signal_disconnect_t m_signal_disconnect;const disconnect_all_t m_disconnect_all;protected:has_slots_interface(signal_connect_t conn,signal_disconnect_t disc,disconnect_all_t disc_all): m_signal_connect(conn),m_signal_disconnect(disc),m_disconnect_all(disc_all) {}// Doesn't really need to be virtual, but is for backwards compatibility// (it was virtual in a previous version of sigslot).virtual ~has_slots_interface() {}public:void signal_connect(_signal_base_interface* sender) {m_signal_connect(this, sender);}void signal_disconnect(_signal_base_interface* sender) {m_signal_disconnect(this, sender);}void disconnect_all() { m_disconnect_all(this); }
};class _signal_base_interface {private:typedef void (*slot_disconnect_t)(_signal_base_interface* self,has_slots_interface* pslot);typedef void (*slot_duplicate_t)(_signal_base_interface* self,const has_slots_interface* poldslot,has_slots_interface* pnewslot);const slot_disconnect_t m_slot_disconnect;const slot_duplicate_t m_slot_duplicate;protected:_signal_base_interface(slot_disconnect_t disc, slot_duplicate_t dupl): m_slot_disconnect(disc), m_slot_duplicate(dupl) {}~_signal_base_interface() {}public:void slot_disconnect(has_slots_interface* pslot) {m_slot_disconnect(this, pslot);}void slot_duplicate(const has_slots_interface* poldslot,has_slots_interface* pnewslot) {m_slot_duplicate(this, poldslot, pnewslot);}
};class _opaque_connection {private:typedef void (*emit_t)(const _opaque_connection*);template <typename FromT, typename ToT>union union_caster {FromT from;ToT to;};emit_t pemit;has_slots_interface* pdest;// Pointers to member functions may be up to 16 bytes (24 bytes for MSVC)// for virtual classes, so make sure we have enough space to store it.
#if defined(_MSC_VER) && !defined(__clang__)unsigned char pmethod[24];
#elseunsigned char pmethod[16];
#endifpublic:template <typename DestT, typename... Args>_opaque_connection(DestT* pd, void (DestT::*pm)(Args...)) : pdest(pd) {typedef void (DestT::*pm_t)(Args...);static_assert(sizeof(pm_t) <= sizeof(pmethod),"Size of slot function pointer too large.");std::memcpy(pmethod, &pm, sizeof(pm_t));typedef void (*em_t)(const _opaque_connection* self, Args...);union_caster<em_t, emit_t> caster2;caster2.from = &_opaque_connection::emitter<DestT, Args...>;pemit = caster2.to;}has_slots_interface* getdest() const { return pdest; }_opaque_connection duplicate(has_slots_interface* newtarget) const {_opaque_connection res = *this;res.pdest = newtarget;return res;}// Just calls the stored "emitter" function pointer stored at construction// time.template <typename... Args>void emit(Args... args) const {typedef void (*em_t)(const _opaque_connection*, Args...);union_caster<emit_t, em_t> caster;caster.from = pemit;(caster.to)(this, args...);}private:template <typename DestT, typename... Args>static void emitter(const _opaque_connection* self, Args... args) {typedef void (DestT::*pm_t)(Args...);pm_t pm;static_assert(sizeof(pm_t) <= sizeof(pmethod),"Size of slot function pointer too large.");std::memcpy(&pm, self->pmethod, sizeof(pm_t));(static_cast<DestT*>(self->pdest)->*(pm))(args...);}
};template <class mt_policy>
class _signal_base : public _signal_base_interface, public mt_policy {protected:typedef std::list<_opaque_connection> connections_list;_signal_base(): _signal_base_interface(&_signal_base::do_slot_disconnect,&_signal_base::do_slot_duplicate),m_current_iterator(m_connected_slots.end()) {}~_signal_base() { disconnect_all(); }private:_signal_base& operator=(_signal_base const& that);public:_signal_base(const _signal_base& o): _signal_base_interface(&_signal_base::do_slot_disconnect,&_signal_base::do_slot_duplicate),m_current_iterator(m_connected_slots.end()) {lock_block<mt_policy> lock(this);for (const auto& connection : o.m_connected_slots) {connection.getdest()->signal_connect(this);m_connected_slots.push_back(connection);}}bool is_empty() {lock_block<mt_policy> lock(this);return m_connected_slots.empty();}void disconnect_all() {lock_block<mt_policy> lock(this);while (!m_connected_slots.empty()) {has_slots_interface* pdest = m_connected_slots.front().getdest();m_connected_slots.pop_front();pdest->signal_disconnect(static_cast<_signal_base_interface*>(this));}// If disconnect_all is called while the signal is firing, advance the// current slot iterator to the end to avoid an invalidated iterator from// being dereferenced.m_current_iterator = m_connected_slots.end();}#if !defined(NDEBUG)bool connected(has_slots_interface* pclass) {lock_block<mt_policy> lock(this);connections_list::const_iterator it = m_connected_slots.begin();connections_list::const_iterator itEnd = m_connected_slots.end();while (it != itEnd) {if (it->getdest() == pclass)return true;++it;}return false;}
#endifvoid disconnect(has_slots_interface* pclass) {lock_block<mt_policy> lock(this);connections_list::iterator it = m_connected_slots.begin();connections_list::iterator itEnd = m_connected_slots.end();while (it != itEnd) {if (it->getdest() == pclass) {// If we're currently using this iterator because the signal is firing,// advance it to avoid it being invalidated.if (m_current_iterator == it) {m_current_iterator = m_connected_slots.erase(it);} else {m_connected_slots.erase(it);}pclass->signal_disconnect(static_cast<_signal_base_interface*>(this));return;}++it;}}private:static void do_slot_disconnect(_signal_base_interface* p,has_slots_interface* pslot) {_signal_base* const self = static_cast<_signal_base*>(p);lock_block<mt_policy> lock(self);connections_list::iterator it = self->m_connected_slots.begin();connections_list::iterator itEnd = self->m_connected_slots.end();while (it != itEnd) {connections_list::iterator itNext = it;++itNext;if (it->getdest() == pslot) {// If we're currently using this iterator because the signal is firing,// advance it to avoid it being invalidated.if (self->m_current_iterator == it) {self->m_current_iterator = self->m_connected_slots.erase(it);} else {self->m_connected_slots.erase(it);}}it = itNext;}}static void do_slot_duplicate(_signal_base_interface* p,const has_slots_interface* oldtarget,has_slots_interface* newtarget) {_signal_base* const self = static_cast<_signal_base*>(p);lock_block<mt_policy> lock(self);connections_list::iterator it = self->m_connected_slots.begin();connections_list::iterator itEnd = self->m_connected_slots.end();while (it != itEnd) {if (it->getdest() == oldtarget) {self->m_connected_slots.push_back(it->duplicate(newtarget));}++it;}}protected:connections_list m_connected_slots;// Used to handle a slot being disconnected while a signal is// firing (iterating m_connected_slots).connections_list::iterator m_current_iterator;bool m_erase_current_iterator = false;
};template <class mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
class has_slots : public has_slots_interface, public mt_policy {private:typedef std::set<_signal_base_interface*> sender_set;typedef sender_set::const_iterator const_iterator;public:has_slots(): has_slots_interface(&has_slots::do_signal_connect,&has_slots::do_signal_disconnect,&has_slots::do_disconnect_all) {}has_slots(has_slots const& o): has_slots_interface(&has_slots::do_signal_connect,&has_slots::do_signal_disconnect,&has_slots::do_disconnect_all) {lock_block<mt_policy> lock(this);for (auto* sender : o.m_senders) {sender->slot_duplicate(&o, this);m_senders.insert(sender);}}~has_slots() { this->disconnect_all(); }private:has_slots& operator=(has_slots const&);static void do_signal_connect(has_slots_interface* p,_signal_base_interface* sender) {has_slots* const self = static_cast<has_slots*>(p);lock_block<mt_policy> lock(self);self->m_senders.insert(sender);}static void do_signal_disconnect(has_slots_interface* p,_signal_base_interface* sender) {has_slots* const self = static_cast<has_slots*>(p);lock_block<mt_policy> lock(self);self->m_senders.erase(sender);}static void do_disconnect_all(has_slots_interface* p) {has_slots* const self = static_cast<has_slots*>(p);lock_block<mt_policy> lock(self);while (!self->m_senders.empty()) {std::set<_signal_base_interface*> senders;senders.swap(self->m_senders);const_iterator it = senders.begin();const_iterator itEnd = senders.end();while (it != itEnd) {_signal_base_interface* s = *it;++it;s->slot_disconnect(p);}}}private:sender_set m_senders;
};template <class mt_policy, typename... Args>
class signal_with_thread_policy : public _signal_base<mt_policy> {private:typedef _signal_base<mt_policy> base;protected:typedef typename base::connections_list connections_list;public:signal_with_thread_policy() {}template <class desttype>void connect(desttype* pclass, void (desttype::*pmemfun)(Args...)) {lock_block<mt_policy> lock(this);this->m_connected_slots.push_back(_opaque_connection(pclass, pmemfun));pclass->signal_connect(static_cast<_signal_base_interface*>(this));}void emit(Args... args) {lock_block<mt_policy> lock(this);this->m_current_iterator = this->m_connected_slots.begin();while (this->m_current_iterator != this->m_connected_slots.end()) {_opaque_connection const& conn = *this->m_current_iterator;++(this->m_current_iterator);conn.emit<Args...>(args...);}}void operator()(Args... args) { emit(args...); }
};// Alias with default thread policy. Needed because both default arguments
// and variadic template arguments must go at the end of the list, so we
// can't have both at once.
template <typename... Args>
using signal = signal_with_thread_policy<SIGSLOT_DEFAULT_MT_POLICY, Args...>;// The previous verion of sigslot didn't use variadic templates, so you would
// need to write "sigslot::signal2<Arg1, Arg2>", for example.
// Now you can just write "sigslot::signal<Arg1, Arg2>", but these aliases
// exist for backwards compatibility.
template <typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
using signal0 = signal_with_thread_policy<mt_policy>;template <typename A1, typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
using signal1 = signal_with_thread_policy<mt_policy, A1>;template <typename A1,typename A2,typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
using signal2 = signal_with_thread_policy<mt_policy, A1, A2>;template <typename A1,typename A2,typename A3,typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
using signal3 = signal_with_thread_policy<mt_policy, A1, A2, A3>;template <typename A1,typename A2,typename A3,typename A4,typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
using signal4 = signal_with_thread_policy<mt_policy, A1, A2, A3, A4>;template <typename A1,typename A2,typename A3,typename A4,typename A5,typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
using signal5 = signal_with_thread_policy<mt_policy, A1, A2, A3, A4, A5>;template <typename A1,typename A2,typename A3,typename A4,typename A5,typename A6,typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
using signal6 = signal_with_thread_policy<mt_policy, A1, A2, A3, A4, A5, A6>;template <typename A1,typename A2,typename A3,typename A4,typename A5,typename A6,typename A7,typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
using signal7 =signal_with_thread_policy<mt_policy, A1, A2, A3, A4, A5, A6, A7>;template <typename A1,typename A2,typename A3,typename A4,typename A5,typename A6,typename A7,typename A8,typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
using signal8 =signal_with_thread_policy<mt_policy, A1, A2, A3, A4, A5, A6, A7, A8>;}  // namespace sigslot#endif /* RTC_BASE_THIRD_PARTY_SIGSLOT_SIGSLOT_H_ */

//实现文件：sigslot.cc
//
// sigslot.h: Signal/Slot classes
//
// Written by Sarah Thompson (sarah@telergy.com) 2002.
//
// License: Public domain. You are free to use this code however you like, with
// the proviso that the author takes on no responsibility or liability for any
// use.#include "sigslot.h"namespace sigslot {#ifdef _SIGSLOT_HAS_POSIX_THREADSpthread_mutex_t* multi_threaded_global::get_mutex() {static pthread_mutex_t g_mutex = PTHREAD_MUTEX_INITIALIZER;return &g_mutex;
}#endif  // _SIGSLOT_HAS_POSIX_THREADS}  // namespace sigslot