From 3807e31cc5c0de2c1bb752afd3f45ca724518df1 Mon Sep 17 00:00:00 2001
From: Luigi Ballabio <luigi.ballabio@gmail.com>
Date: Mon, 5 Apr 2021 16:16:10 +0200
Subject: [PATCH] Use equality-comparable function object.

Comparison between `boost::bind` objects doesn't work
in gcc when using C++20 mode.
---
 ql/patterns/observable.cpp | 50 +++++++++++++++++++-------------------
 1 file changed, 25 insertions(+), 25 deletions(-)

diff --git a/ql/patterns/observable.cpp b/ql/patterns/observable.cpp
index 87da71d0b8d..078a66eee28 100644
--- a/ql/patterns/observable.cpp
+++ b/ql/patterns/observable.cpp
@@ -87,21 +87,6 @@ namespace QuantLib {
 
 #else
 
-#include <ql/functional.hpp>
-
-#if defined(QL_USE_STD_FUNCTION)
-#if defined(__GNUC__) && (((__GNUC__ == 4) && (__GNUC_MINOR__ >= 8)) || (__GNUC__ > 4))
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wunused-local-typedefs"
-#endif
-
-#include <boost/bind/bind.hpp>
-
-#if defined(__GNUC__) && (((__GNUC__ == 4) && (__GNUC_MINOR__ >= 8)) || (__GNUC__ > 4))
-#pragma GCC diagnostic pop
-#endif
-#endif
-
 #include <boost/signals2/signal_type.hpp>
 
 namespace QuantLib {
@@ -131,17 +116,35 @@ namespace QuantLib {
             signal_type sig_;
         };
 
+        template <class T>
+        class ProxyUpdater {
+            T* proxy_;
+          public:
+            explicit ProxyUpdater(const ext::shared_ptr<T>& observerProxy)
+            : proxy_(observerProxy.get()) {}
+
+            void operator()() const {
+                proxy_->update();
+            }
+
+            bool operator==(const ProxyUpdater<T>& other) const {
+                return proxy_ == other.proxy_;
+            }
+
+            bool operator!=(const ProxyUpdater<T>& other) const {
+                return proxy_ != other.proxy_;
+            }
+        };
+
     }
 
-    void Observable::registerObserver(
-        const ext::shared_ptr<Observer::Proxy>& observerProxy) {
+    void Observable::registerObserver(const ext::shared_ptr<Observer::Proxy>& observerProxy) {
         {
             boost::lock_guard<boost::recursive_mutex> lock(mutex_);
             observers_.insert(observerProxy);
         }
 
-        detail::Signal::signal_type::slot_type slot(&Observer::Proxy::update,
-                                    observerProxy.get());
+        detail::Signal::signal_type::slot_type slot {detail::ProxyUpdater<Observer::Proxy>(observerProxy)};
         #if defined(QL_USE_STD_SHARED_PTR)
         sig_->connect(slot.track_foreign(observerProxy));
         #else
@@ -149,9 +152,8 @@ namespace QuantLib {
         #endif
     }
 
-    void Observable::unregisterObserver(
-        const ext::shared_ptr<Observer::Proxy>& observerProxy,
-        bool disconnect) {
+    void Observable::unregisterObserver(const ext::shared_ptr<Observer::Proxy>& observerProxy,
+                                        bool disconnect) {
         {
             boost::lock_guard<boost::recursive_mutex> lock(mutex_);
             observers_.erase(observerProxy);
@@ -165,9 +167,7 @@ namespace QuantLib {
         }
 
         if (disconnect) {
-            // signals2 needs boost::bind, std::bind does not work
-            sig_->disconnect(boost::bind(&Observer::Proxy::update,
-                                 observerProxy.get()));
+            sig_->disconnect(detail::ProxyUpdater<Observer::Proxy>(observerProxy));
         }
     }