Problem: No type-safe alternatives when polling or needing a reference to a socket

Solution: Introduce a socket_ref that is a non-owning nullable reference to a socket

tests/CMakeLists.txt

@@ -20,6 +20,7 @@ add_executable(
     message.cpp
     context.cpp
     socket.cpp
+    socket_ref.cpp
     poller.cpp
     active_poller.cpp
     multipart.cpp

tests/socket.cpp

@@ -25,9 +25,11 @@ TEST_CASE("socket create assign", "[socket]")
 {
     zmq::context_t context;
     zmq::socket_t socket(context, ZMQ_ROUTER);
-    CHECK(static_cast<void*>(socket));
+    CHECK(static_cast<bool>(socket));
+    CHECK(socket.handle() != nullptr);
     socket = {};
-    CHECK(!static_cast<void*>(socket));
+    CHECK(!static_cast<bool>(socket));
+    CHECK(socket.handle() == nullptr);
 }
 
 TEST_CASE("socket create by enum and destroy", "[socket]")
@@ -75,7 +77,7 @@ TEST_CASE("socket proxy", "[socket]")
         auto s3 = std::move(capture);
         try
         {
-            zmq::proxy(s1, s2, &s3);
+            zmq::proxy(s1, s2, zmq::socket_ref(s3));
         }
         catch (const zmq::error_t& e)
         {
@@ -102,7 +104,7 @@ TEST_CASE("socket proxy steerable", "[socket]")
         auto s3 = std::move(control);
         try
         {
-            zmq::proxy_steerable(s1, s2, ZMQ_NULLPTR, &s3);
+            zmq::proxy_steerable(s1, s2, zmq::socket_ref(), s3);
         }
         catch (const zmq::error_t& e)
         {

tests/socket_ref.cpp

@@ -0,0 +1,116 @@
+#include <catch.hpp>
+#include <zmq.hpp>
+#ifdef ZMQ_CPP11
+
+#ifdef ZMQ_CPP17
+static_assert(std::is_nothrow_swappable_v<zmq::socket_ref>);
+#endif
+static_assert(sizeof(zmq::socket_ref) == sizeof(void *), "size mismatch");
+static_assert(alignof(zmq::socket_ref) == alignof(void *), "alignment mismatch");
+static_assert(std::is_trivially_copyable<zmq::socket_ref>::value,
+              "needs to be trivially copyable");
+
+TEST_CASE("socket_ref default init", "[socket_ref]")
+{
+    zmq::socket_ref sr;
+    CHECK(!sr);
+    CHECK(sr == nullptr);
+    CHECK(nullptr == sr);
+    CHECK(sr.handle() == nullptr);
+}
+
+TEST_CASE("socket_ref create from nullptr", "[socket_ref]")
+{
+    zmq::socket_ref sr = nullptr;
+    CHECK(sr == nullptr);
+    CHECK(sr.handle() == nullptr);
+}
+
+TEST_CASE("socket_ref create from handle", "[socket_ref]")
+{
+    void *np = nullptr;
+    zmq::socket_ref sr{zmq::from_handle, np};
+    CHECK(sr == nullptr);
+    CHECK(sr.handle() == nullptr);
+}
+
+TEST_CASE("socket_ref compare", "[socket_ref]")
+{
+    zmq::socket_ref sr1;
+    zmq::socket_ref sr2;
+    CHECK(sr1 == sr2);
+    CHECK(!(sr1 != sr2));
+}
+
+TEST_CASE("socket_ref compare from socket_t", "[socket_ref]")
+{
+    zmq::context_t context;
+    zmq::socket_t s1(context, zmq::socket_type::router);
+    zmq::socket_t s2(context, zmq::socket_type::dealer);
+    zmq::socket_ref sr1 = s1;
+    zmq::socket_ref sr2 = s2;
+    CHECK(sr1);
+    CHECK(sr2);
+    CHECK(sr1 == s1);
+    CHECK(sr2 == s2);
+    CHECK(sr1.handle() == s1.handle());
+    CHECK(sr1 != sr2);
+    CHECK(sr1.handle() != sr2.handle());
+    CHECK(sr1 != nullptr);
+    CHECK(nullptr != sr1);
+    CHECK(sr2 != nullptr);
+    const bool comp1 = (sr1 < sr2) != (sr1 >= sr2);
+    CHECK(comp1);
+    const bool comp2 = (sr1 > sr2) != (sr1 <= sr2);
+    CHECK(comp2);
+    std::hash<zmq::socket_ref> hash;
+    CHECK(hash(sr1) != hash(sr2));
+    CHECK(hash(sr1) == hash(s1));
+}
+
+TEST_CASE("socket_ref assignment", "[socket_ref]")
+{
+    zmq::context_t context;
+    zmq::socket_t s1(context, zmq::socket_type::router);
+    zmq::socket_t s2(context, zmq::socket_type::dealer);
+    zmq::socket_ref sr1 = s1;
+    zmq::socket_ref sr2 = s2;
+    sr1 = s2;
+    CHECK(sr1 == sr2);
+    CHECK(sr1.handle() == sr2.handle());
+    sr1 = std::move(sr2);
+    CHECK(sr1 == sr2);
+    CHECK(sr1.handle() == sr2.handle());
+    sr2 = nullptr;
+    CHECK(sr1 != sr2);
+    sr1 = nullptr;
+    CHECK(sr1 == sr2);
+}
+
+TEST_CASE("socket_ref swap", "[socket_ref]")
+{
+    zmq::socket_ref sr1;
+    zmq::socket_ref sr2;
+    using std::swap;
+    swap(sr1, sr2);
+}
+
+TEST_CASE("socket_ref reinterpret as void*", "[socket_ref]")
+{
+    struct SVP
+    {
+        void *p;
+    };
+    struct SSR
+    {
+        zmq::socket_ref sr;
+    } ssr;
+
+    zmq::context_t context;
+    zmq::socket_t socket(context, zmq::socket_type::router);
+    CHECK(socket.handle() != nullptr);
+    reinterpret_cast<SVP *>(&ssr)->p = socket.handle();
+    CHECK(ssr.sr == socket);
+}
+
+#endif

zmq.hpp

@@ -56,11 +56,15 @@
 #define ZMQ_EXPLICIT explicit
 #define ZMQ_OVERRIDE override
 #define ZMQ_NULLPTR nullptr
+#define ZMQ_CONSTEXPR_FN constexpr
+#define ZMQ_CONSTEXPR_VAR constexpr
 #else
 #define ZMQ_NOTHROW throw()
 #define ZMQ_EXPLICIT
 #define ZMQ_OVERRIDE
 #define ZMQ_NULLPTR 0
+#define ZMQ_CONSTEXPR_FN
+#define ZMQ_CONSTEXPR_VAR const
 #endif
 
 #include <zmq.h>
@@ -230,8 +234,6 @@ inline std::tuple<int, int, int> version()
 
 class message_t
 {
-    friend class socket_t;
-
   public:
     message_t()
     {
@@ -346,14 +348,14 @@ class message_t
     ZMQ_DEPRECATED("from 4.3.1, use move taking non-const reference instead")
     void move(message_t const *msg_)
     {
-        int rc = zmq_msg_move(&msg, const_cast<zmq_msg_t *>(&(msg_->msg)));
+        int rc = zmq_msg_move(&msg, const_cast<zmq_msg_t *>(msg_->handle()));
         if (rc != 0)
             throw error_t();
     }
 
     void move(message_t &msg_)
     {
-        int rc = zmq_msg_move(&msg, &msg_.msg);
+        int rc = zmq_msg_move(&msg, msg_.handle());
         if (rc != 0)
             throw error_t();
     }
@@ -361,14 +363,14 @@ class message_t
     ZMQ_DEPRECATED("from 4.3.1, use copy taking non-const reference instead")
     void copy(message_t const *msg_)
     {
-        int rc = zmq_msg_copy(&msg, const_cast<zmq_msg_t *>(&(msg_->msg)));
+        int rc = zmq_msg_copy(&msg, const_cast<zmq_msg_t *>(msg_->handle()));
         if (rc != 0)
             throw error_t();
     }
 
     void copy(message_t &msg_)
     {
-        int rc = zmq_msg_copy(&msg, &msg_.msg);
+        int rc = zmq_msg_copy(&msg, msg_.handle());
         if (rc != 0)
             throw error_t();
     }
@@ -511,6 +513,9 @@ class message_t
         std::swap(msg, other.msg);
     }
 
+    ZMQ_NODISCARD zmq_msg_t *handle() ZMQ_NOTHROW { return &msg; }
+    ZMQ_NODISCARD const zmq_msg_t *handle() const ZMQ_NOTHROW { return &msg; }
+
   private:
     //  The underlying message
     zmq_msg_t msg;
@@ -610,6 +615,158 @@ inline void swap(context_t &a, context_t &b) ZMQ_NOTHROW {
     a.swap(b);
 }
 
+namespace detail
+{
+class socket_base
+{
+public:
+    socket_base() ZMQ_NOTHROW : _handle(ZMQ_NULLPTR) {}
+    ZMQ_EXPLICIT socket_base(void *handle) ZMQ_NOTHROW : _handle(handle) {}
+
+    template<typename T> void setsockopt(int option_, T const &optval)
+    {
+        setsockopt(option_, &optval, sizeof(T));
+    }
+
+    void setsockopt(int option_, const void *optval_, size_t optvallen_)
+    {
+        int rc = zmq_setsockopt(_handle, option_, optval_, optvallen_);
+        if (rc != 0)
+            throw error_t();
+    }
+
+    void getsockopt(int option_, void *optval_, size_t *optvallen_) const
+    {
+        int rc = zmq_getsockopt(_handle, option_, optval_, optvallen_);
+        if (rc != 0)
+            throw error_t();
+    }
+
+    template<typename T> T getsockopt(int option_) const
+    {
+        T optval;
+        size_t optlen = sizeof(T);
+        getsockopt(option_, &optval, &optlen);
+        return optval;
+    }
+
+    void bind(std::string const &addr) { bind(addr.c_str()); }
+
+    void bind(const char *addr_)
+    {
+        int rc = zmq_bind(_handle, addr_);
+        if (rc != 0)
+            throw error_t();
+    }
+
+    void unbind(std::string const &addr) { unbind(addr.c_str()); }
+
+    void unbind(const char *addr_)
+    {
+        int rc = zmq_unbind(_handle, addr_);
+        if (rc != 0)
+            throw error_t();
+    }
+
+    void connect(std::string const &addr) { connect(addr.c_str()); }
+
+    void connect(const char *addr_)
+    {
+        int rc = zmq_connect(_handle, addr_);
+        if (rc != 0)
+            throw error_t();
+    }
+
+    void disconnect(std::string const &addr) { disconnect(addr.c_str()); }
+
+    void disconnect(const char *addr_)
+    {
+        int rc = zmq_disconnect(_handle, addr_);
+        if (rc != 0)
+            throw error_t();
+    }
+
+    bool connected() const ZMQ_NOTHROW { return (_handle != ZMQ_NULLPTR); }
+
+    size_t send(const void *buf_, size_t len_, int flags_ = 0)
+    {
+        int nbytes = zmq_send(_handle, buf_, len_, flags_);
+        if (nbytes >= 0)
+            return (size_t) nbytes;
+        if (zmq_errno() == EAGAIN)
+            return 0;
+        throw error_t();
+    }
+
+    bool send(message_t &msg_, int flags_ = 0)
+    {
+        int nbytes = zmq_msg_send(msg_.handle(), _handle, flags_);
+        if (nbytes >= 0)
+            return true;
+        if (zmq_errno() == EAGAIN)
+            return false;
+        throw error_t();
+    }
+
+    template<typename T> bool send(T first, T last, int flags_ = 0)
+    {
+        zmq::message_t msg(first, last);
+        return send(msg, flags_);
+    }
+
+#ifdef ZMQ_HAS_RVALUE_REFS
+    bool send(message_t &&msg_, int flags_ = 0) { return send(msg_, flags_); }
+#endif
+
+    size_t recv(void *buf_, size_t len_, int flags_ = 0)
+    {
+        int nbytes = zmq_recv(_handle, buf_, len_, flags_);
+        if (nbytes >= 0)
+            return (size_t) nbytes;
+        if (zmq_errno() == EAGAIN)
+            return 0;
+        throw error_t();
+    }
+
+    bool recv(message_t *msg_, int flags_ = 0)
+    {
+        int nbytes = zmq_msg_recv(msg_->handle(), _handle, flags_);
+        if (nbytes >= 0)
+            return true;
+        if (zmq_errno() == EAGAIN)
+            return false;
+        throw error_t();
+    }
+
+#if defined(ZMQ_BUILD_DRAFT_API) && ZMQ_VERSION >= ZMQ_MAKE_VERSION(4, 2, 0)
+    void join(const char* group)
+    {
+        int rc = zmq_join(_handle, group);
+        if (rc != 0)
+            throw error_t();
+    }
+
+    void leave(const char* group)
+    {
+        int rc = zmq_leave(_handle, group);
+        if (rc != 0)
+            throw error_t();
+    }
+#endif
+
+    ZMQ_NODISCARD void *handle() ZMQ_NOTHROW { return _handle; }
+    ZMQ_NODISCARD const void *handle() const ZMQ_NOTHROW { return _handle; }
+
+    ZMQ_EXPLICIT operator bool() const ZMQ_NOTHROW { return _handle != ZMQ_NULLPTR; }
+    // note: non-const operator bool can be removed once
+    // operator void* is removed from socket_t
+    ZMQ_EXPLICIT operator bool() ZMQ_NOTHROW { return _handle != ZMQ_NULLPTR; }
+
+protected:
+    void *_handle;
+};
+} // namespace detail
+
 #ifdef ZMQ_CPP11
 enum class socket_type : int
 {
@@ -636,22 +793,106 @@ enum class socket_type : int
 };
 #endif
 
-class socket_t
+struct from_handle_t
+{
+    struct _private {}; // disabling use other than with from_handle
+    ZMQ_CONSTEXPR_FN ZMQ_EXPLICIT from_handle_t(_private /*p*/) ZMQ_NOTHROW {}
+};
+
+ZMQ_CONSTEXPR_VAR from_handle_t from_handle = from_handle_t(from_handle_t::_private());
+
+// A non-owning nullable reference to a socket.
+// The reference is invalidated on socket close or destruction.
+class socket_ref : public detail::socket_base
+{
+  public:
+    socket_ref() ZMQ_NOTHROW : detail::socket_base() {}
+#ifdef ZMQ_CPP11
+    socket_ref(std::nullptr_t) ZMQ_NOTHROW : detail::socket_base() {}
+#endif
+    socket_ref(from_handle_t /*fh*/, void *handle) ZMQ_NOTHROW
+        : detail::socket_base(handle) {}
+};
+
+#ifdef ZMQ_CPP11
+inline bool operator==(socket_ref sr, std::nullptr_t /*p*/) ZMQ_NOTHROW
+{
+    return sr.handle() == nullptr;
+}
+inline bool operator==(std::nullptr_t /*p*/, socket_ref sr) ZMQ_NOTHROW
+{
+    return sr.handle() == nullptr;
+}
+inline bool operator!=(socket_ref sr, std::nullptr_t /*p*/) ZMQ_NOTHROW
+{
+    return !(sr == nullptr);
+}
+inline bool operator!=(std::nullptr_t /*p*/, socket_ref sr) ZMQ_NOTHROW
+{
+    return !(sr == nullptr);
+}
+#endif
+
+inline bool operator==(socket_ref a, socket_ref b) ZMQ_NOTHROW
+{
+    return std::equal_to<void*>()(a.handle(), b.handle());
+}
+inline bool operator!=(socket_ref a, socket_ref b) ZMQ_NOTHROW
+{
+    return !(a == b);
+}
+inline bool operator<(socket_ref a, socket_ref b) ZMQ_NOTHROW
+{
+    return std::less<void*>()(a.handle(), b.handle());
+}
+inline bool operator>(socket_ref a, socket_ref b) ZMQ_NOTHROW
+{
+    return b < a;
+}
+inline bool operator<=(socket_ref a, socket_ref b) ZMQ_NOTHROW
+{
+    return !(a > b);
+}
+inline bool operator>=(socket_ref a, socket_ref b) ZMQ_NOTHROW
+{
+    return !(a < b);
+}
+
+} // namespace zmq
+
+#ifdef ZMQ_CPP11
+namespace std
+{
+template<>
+struct hash<zmq::socket_ref>
+{
+    size_t operator()(zmq::socket_ref sr) const ZMQ_NOTHROW
+    {
+        return hash<void*>()(sr.handle());
+    }
+};
+} // namespace std
+#endif
+
+namespace zmq
+{
+
+class socket_t : public detail::socket_base
 {
     friend class monitor_t;
 
   public:
     socket_t() ZMQ_NOTHROW
-      : ptr(ZMQ_NULLPTR)
+      : detail::socket_base(ZMQ_NULLPTR)
       , ctxptr(ZMQ_NULLPTR)
     {
     }
 
     socket_t(context_t &context_, int type_)
-        : ptr(zmq_socket(static_cast<void*>(context_), type_))
+        : detail::socket_base(zmq_socket(static_cast<void*>(context_), type_))
         , ctxptr(static_cast<void*>(context_))
     {
-        if (ptr == ZMQ_NULLPTR)
+        if (_handle == ZMQ_NULLPTR)
             throw error_t();
     }
 
@@ -663,177 +904,59 @@ class socket_t
 #endif
 
 #ifdef ZMQ_HAS_RVALUE_REFS
-    socket_t(socket_t &&rhs) ZMQ_NOTHROW : ptr(rhs.ptr), ctxptr(rhs.ctxptr)
+    socket_t(socket_t &&rhs) ZMQ_NOTHROW : detail::socket_base(rhs._handle), ctxptr(rhs.ctxptr)
     {
-        rhs.ptr = ZMQ_NULLPTR;
+        rhs._handle = ZMQ_NULLPTR;
         rhs.ctxptr = ZMQ_NULLPTR;
     }
     socket_t &operator=(socket_t &&rhs) ZMQ_NOTHROW
     {
-        std::swap(ptr, rhs.ptr);
+        std::swap(_handle, rhs._handle);
         return *this;
     }
 #endif
 
     ~socket_t() ZMQ_NOTHROW { close(); }
 
-    operator void *() ZMQ_NOTHROW { return ptr; }
+    operator void *() ZMQ_NOTHROW { return _handle; }
 
-    operator void const *() const ZMQ_NOTHROW { return ptr; }
+    operator void const *() const ZMQ_NOTHROW { return _handle; }
 
     void close() ZMQ_NOTHROW
     {
-        if (ptr == ZMQ_NULLPTR)
+        if (_handle == ZMQ_NULLPTR)
             // already closed
             return;
-        int rc = zmq_close(ptr);
+        int rc = zmq_close(_handle);
         ZMQ_ASSERT(rc == 0);
-        ptr = ZMQ_NULLPTR;
+        _handle = ZMQ_NULLPTR;
     }
 
-    template<typename T> void setsockopt(int option_, T const &optval)
-    {
-        setsockopt(option_, &optval, sizeof(T));
-    }
-
-    void setsockopt(int option_, const void *optval_, size_t optvallen_)
-    {
-        int rc = zmq_setsockopt(ptr, option_, optval_, optvallen_);
-        if (rc != 0)
-            throw error_t();
-    }
-
-    void getsockopt(int option_, void *optval_, size_t *optvallen_) const
-    {
-        int rc = zmq_getsockopt(ptr, option_, optval_, optvallen_);
-        if (rc != 0)
-            throw error_t();
-    }
-
-    template<typename T> T getsockopt(int option_) const
-    {
-        T optval;
-        size_t optlen = sizeof(T);
-        getsockopt(option_, &optval, &optlen);
-        return optval;
-    }
-
-    void bind(std::string const &addr) { bind(addr.c_str()); }
-
-    void bind(const char *addr_)
-    {
-        int rc = zmq_bind(ptr, addr_);
-        if (rc != 0)
-            throw error_t();
-    }
-
-    void unbind(std::string const &addr) { unbind(addr.c_str()); }
-
-    void unbind(const char *addr_)
-    {
-        int rc = zmq_unbind(ptr, addr_);
-        if (rc != 0)
-            throw error_t();
-    }
-
-    void connect(std::string const &addr) { connect(addr.c_str()); }
-
-    void connect(const char *addr_)
-    {
-        int rc = zmq_connect(ptr, addr_);
-        if (rc != 0)
-            throw error_t();
-    }
-
-    void disconnect(std::string const &addr) { disconnect(addr.c_str()); }
-
-    void disconnect(const char *addr_)
-    {
-        int rc = zmq_disconnect(ptr, addr_);
-        if (rc != 0)
-            throw error_t();
-    }
-
-    bool connected() const ZMQ_NOTHROW { return (ptr != ZMQ_NULLPTR); }
-
-    size_t send(const void *buf_, size_t len_, int flags_ = 0)
-    {
-        int nbytes = zmq_send(ptr, buf_, len_, flags_);
-        if (nbytes >= 0)
-            return (size_t) nbytes;
-        if (zmq_errno() == EAGAIN)
-            return 0;
-        throw error_t();
-    }
-
-    bool send(message_t &msg_, int flags_ = 0)
-    {
-        int nbytes = zmq_msg_send(&(msg_.msg), ptr, flags_);
-        if (nbytes >= 0)
-            return true;
-        if (zmq_errno() == EAGAIN)
-            return false;
-        throw error_t();
-    }
-
-    template<typename T> bool send(T first, T last, int flags_ = 0)
-    {
-        zmq::message_t msg(first, last);
-        return send(msg, flags_);
-    }
-
-#ifdef ZMQ_HAS_RVALUE_REFS
-    bool send(message_t &&msg_, int flags_ = 0) { return send(msg_, flags_); }
-#endif
-
-    size_t recv(void *buf_, size_t len_, int flags_ = 0)
-    {
-        int nbytes = zmq_recv(ptr, buf_, len_, flags_);
-        if (nbytes >= 0)
-            return (size_t) nbytes;
-        if (zmq_errno() == EAGAIN)
-            return 0;
-        throw error_t();
-    }
-
-    bool recv(message_t *msg_, int flags_ = 0)
-    {
-        int nbytes = zmq_msg_recv(&(msg_->msg), ptr, flags_);
-        if (nbytes >= 0)
-            return true;
-        if (zmq_errno() == EAGAIN)
-            return false;
-        throw error_t();
-    }
-
-#if defined(ZMQ_BUILD_DRAFT_API) && ZMQ_VERSION >= ZMQ_MAKE_VERSION(4, 2, 0)
-    void join(const char* group)
-    {
-        int rc = zmq_join(ptr, group);
-        if (rc != 0)
-            throw error_t();
-    }
-
-    void leave(const char* group)
-    {
-        int rc = zmq_leave(ptr, group);
-        if (rc != 0)
-            throw error_t();
-    }
-#endif
-
     void swap(socket_t &other) ZMQ_NOTHROW
     {
-        std::swap(ptr, other.ptr);
+        std::swap(_handle, other._handle);
         std::swap(ctxptr, other.ctxptr);
     }
 
+    operator socket_ref() ZMQ_NOTHROW
+    {
+        return socket_ref(from_handle, _handle);
+    }
+
   private:
-    void *ptr;
     void *ctxptr;
 
     socket_t(const socket_t &) ZMQ_DELETED_FUNCTION;
     void operator=(const socket_t &) ZMQ_DELETED_FUNCTION;
+
+    // used by monitor_t
+    socket_t(void *context_, int type_)
+        : detail::socket_base(zmq_socket(context_, type_))
+        , ctxptr(context_)
+    {
+        if (_handle == ZMQ_NULLPTR)
+            throw error_t();
+    }
 };
 
 inline void swap(socket_t &a, socket_t &b) ZMQ_NOTHROW {
@@ -848,13 +971,12 @@ inline void proxy(void *frontend, void *backend, void *capture)
         throw error_t();
 }
 
-inline void proxy(socket_t &frontend, socket_t &backend, socket_t *capture = ZMQ_NULLPTR)
+inline void
+proxy(socket_ref frontend, socket_ref backend, socket_ref capture = socket_ref())
 {
-    int rc = zmq_proxy(static_cast<void *>(frontend),
-                       static_cast<void *>(backend),
-                       capture ? static_cast<void *>(*capture) : ZMQ_NULLPTR);
+    int rc = zmq_proxy(frontend.handle(), backend.handle(), capture.handle());
     if (rc != 0)
-       throw error_t();
+        throw error_t();
 }
 
 #ifdef ZMQ_HAS_PROXY_STEERABLE
@@ -867,13 +989,13 @@ proxy_steerable(void *frontend, void *backend, void *capture, void *control)
         throw error_t();
 }
 
-inline void
-proxy_steerable(socket_t &frontend, socket_t &backend, socket_t *capture, socket_t *control)
+inline void proxy_steerable(socket_ref frontend,
+                            socket_ref backend,
+                            socket_ref capture,
+                            socket_ref control)
 {
-    int rc = zmq_proxy_steerable(static_cast<void *>(frontend),
-                                 static_cast<void *>(backend),
-                                 capture ? static_cast<void *>(*capture) : ZMQ_NULLPTR,
-                                 control ? static_cast<void *>(*control) : ZMQ_NULLPTR);
+    int rc = zmq_proxy_steerable(frontend.handle(), backend.handle(),
+                                 capture.handle(), control.handle());
     if (rc != 0)
         throw error_t();
 }
@@ -882,7 +1004,7 @@ proxy_steerable(socket_t &frontend, socket_t &backend, socket_t *capture, socket
 class monitor_t
 {
   public:
-    monitor_t() : socketPtr(ZMQ_NULLPTR), monitor_socket(ZMQ_NULLPTR) {}
+    monitor_t() : _socket(), _monitor_socket() {}
 
     virtual ~monitor_t()
     {
@@ -890,20 +1012,18 @@ class monitor_t
     }
 
 #ifdef ZMQ_HAS_RVALUE_REFS
-    monitor_t(monitor_t &&rhs) ZMQ_NOTHROW : socketPtr(rhs.socketPtr),
-                                             monitor_socket(rhs.monitor_socket)
+    monitor_t(monitor_t &&rhs) ZMQ_NOTHROW : _socket(), _monitor_socket()
     {
-        rhs.socketPtr = ZMQ_NULLPTR;
-        rhs.monitor_socket = ZMQ_NULLPTR;
+        std::swap(_socket, rhs._socket);
+        std::swap(_monitor_socket, rhs._monitor_socket);
     }
 
     monitor_t &operator=(monitor_t &&rhs) ZMQ_NOTHROW
     {
         close();
-        socketPtr = ZMQ_NULLPTR;
-        monitor_socket = ZMQ_NULLPTR;
-        std::swap(socketPtr, rhs.socketPtr);
-        std::swap(monitor_socket, rhs.monitor_socket);
+        _socket = socket_ref();
+        std::swap(_socket, rhs._socket);
+        std::swap(_monitor_socket, rhs._monitor_socket);
         return *this;
     }
 #endif
@@ -930,35 +1050,32 @@ class monitor_t
 
     void init(socket_t &socket, const char *addr_, int events = ZMQ_EVENT_ALL)
     {
-        int rc = zmq_socket_monitor(socket.ptr, addr_, events);
+        int rc = zmq_socket_monitor(socket.handle(), addr_, events);
         if (rc != 0)
             throw error_t();
 
-        socketPtr = socket.ptr;
-        monitor_socket = zmq_socket(socket.ctxptr, ZMQ_PAIR);
-        assert(monitor_socket);
-
-        rc = zmq_connect(monitor_socket, addr_);
-        assert(rc == 0);
+        _socket = socket;
+        _monitor_socket = socket_t(socket.ctxptr, ZMQ_PAIR);
+        _monitor_socket.connect(addr_);
 
         on_monitor_started();
     }
 
     bool check_event(int timeout = 0)
     {
-        assert(monitor_socket);
+        assert(_monitor_socket);
 
         zmq_msg_t eventMsg;
         zmq_msg_init(&eventMsg);
 
         zmq::pollitem_t items[] = {
-          {monitor_socket, 0, ZMQ_POLLIN, 0},
+          {_monitor_socket.handle(), 0, ZMQ_POLLIN, 0},
         };
 
         zmq::poll(&items[0], 1, timeout);
 
         if (items[0].revents & ZMQ_POLLIN) {
-            int rc = zmq_msg_recv(&eventMsg, monitor_socket, 0);
+            int rc = zmq_msg_recv(&eventMsg, _monitor_socket.handle(), 0);
             if (rc == -1 && zmq_errno() == ETERM)
                 return false;
             assert(rc != -1);
@@ -982,7 +1099,7 @@ class monitor_t
 #ifdef ZMQ_NEW_MONITOR_EVENT_LAYOUT
         zmq_msg_t addrMsg;
         zmq_msg_init(&addrMsg);
-        int rc = zmq_msg_recv(&addrMsg, monitor_socket, 0);
+        int rc = zmq_msg_recv(&addrMsg, _monitor_socket.handle(), 0);
         if (rc == -1 && zmq_errno() == ETERM) {
             zmq_msg_close(&eventMsg);
             return false;
@@ -1071,10 +1188,10 @@ class monitor_t
 #ifdef ZMQ_EVENT_MONITOR_STOPPED
     void abort()
     {
-        if (socketPtr)
-            zmq_socket_monitor(socketPtr, ZMQ_NULLPTR, 0);
+        if (_socket)
+            zmq_socket_monitor(_socket.handle(), ZMQ_NULLPTR, 0);
 
-        socketPtr = ZMQ_NULLPTR;
+        _socket = socket_ref();
     }
 #endif
     virtual void on_monitor_started() {}
@@ -1179,16 +1296,14 @@ class monitor_t
     monitor_t(const monitor_t &) ZMQ_DELETED_FUNCTION;
     void operator=(const monitor_t &) ZMQ_DELETED_FUNCTION;
 
-    void *socketPtr;
-    void *monitor_socket;
+    socket_ref _socket;
+    socket_t _monitor_socket;
 
     void close() ZMQ_NOTHROW
     {
-        if (socketPtr)
-            zmq_socket_monitor(socketPtr, ZMQ_NULLPTR, 0);
-
-        if (monitor_socket)
-            zmq_close(monitor_socket);
+        if (_socket)
+            zmq_socket_monitor(_socket.handle(), ZMQ_NULLPTR, 0);
+        _monitor_socket.close();
     }
 };
 
@@ -1198,26 +1313,26 @@ template<typename T = void> class poller_t
   public:
     poller_t() = default;
 
-    void add(zmq::socket_t &socket, short events, T *user_data)
+    void add(zmq::socket_ref socket, short events, T *user_data)
     {
         if (0
-            != zmq_poller_add(poller_ptr.get(), static_cast<void *>(socket),
+            != zmq_poller_add(poller_ptr.get(), socket.handle(),
                               user_data, events)) {
             throw error_t();
         }
     }
 
-    void remove(zmq::socket_t &socket)
+    void remove(zmq::socket_ref socket)
     {
-        if (0 != zmq_poller_remove(poller_ptr.get(), static_cast<void *>(socket))) {
+        if (0 != zmq_poller_remove(poller_ptr.get(), socket.handle())) {
             throw error_t();
         }
     }
 
-    void modify(zmq::socket_t &socket, short events)
+    void modify(zmq::socket_ref socket, short events)
     {
         if (0
-            != zmq_poller_modify(poller_ptr.get(), static_cast<void *>(socket),
+            != zmq_poller_modify(poller_ptr.get(), socket.handle(),
                                  events)) {
             throw error_t();
         }

zmq_addon.hpp

@@ -373,12 +373,12 @@ class active_poller_t
 
     using handler_t = std::function<void(short)>;
 
-    void add(zmq::socket_t &socket, short events, handler_t handler)
+    void add(zmq::socket_ref socket, short events, handler_t handler)
     {
         auto it = decltype(handlers)::iterator{};
         auto inserted = bool{};
         std::tie(it, inserted) =
-          handlers.emplace(static_cast<void *>(socket),
+          handlers.emplace(socket,
                            std::make_shared<handler_t>(std::move(handler)));
         try {
             base_poller.add(socket, events,
@@ -388,20 +388,20 @@ class active_poller_t
         catch (const zmq::error_t &) {
             // rollback
             if (inserted) {
-                handlers.erase(static_cast<void *>(socket));
+                handlers.erase(socket);
             }
             throw;
         }
     }
 
-    void remove(zmq::socket_t &socket)
+    void remove(zmq::socket_ref socket)
     {
         base_poller.remove(socket);
-        handlers.erase(static_cast<void *>(socket));
+        handlers.erase(socket);
         need_rebuild = true;
     }
 
-    void modify(zmq::socket_t &socket, short events)
+    void modify(zmq::socket_ref socket, short events)
     {
         base_poller.modify(socket, events);
     }
@@ -435,7 +435,7 @@ class active_poller_t
     bool need_rebuild{false};
 
     poller_t<handler_t> base_poller{};
-    std::unordered_map<void *, std::shared_ptr<handler_t>> handlers{};
+    std::unordered_map<socket_ref, std::shared_ptr<handler_t>> handlers{};
     std::vector<zmq_poller_event_t> poller_events{};
     std::vector<std::shared_ptr<handler_t>> poller_handlers{};
 };     // class active_poller_t