Decided to use signal slot mechanism

The automatic cross thread activation  is probably worth it.

pglab/QueryTab.cpp

@@ -32,10 +32,8 @@ QueryTab::QueryTab(MainWindow *win, QWidget *parent) :
 {
 	ui->setupUi(this);
 
-	m_dbConnection.setStateChangeReceiver(
-				[this, win](auto s) { win->QueueTask([this, s]() { connectionStateChanged(s); }); });
-	m_dbConnection.setNoticeReceiver(
-				[this, win](auto n) { win->QueueTask([this, n]() { receiveNotice(n); }); });
+    connect(&m_dbConnection, &ASyncDBConnection::onStateChanged, this, &QueryTab::connectionStateChanged);
+    connect(&m_dbConnection, &ASyncDBConnection::onNotice, this, &QueryTab::receiveNotice);
   
 	QFont font;
 	font.setFamily("Source Code Pro");

pglablib/ASyncDBConnection.cpp

@@ -4,19 +4,19 @@
 
 using namespace boost::asio;
 
-//namespace {
+namespace {
 
-//    class registerMetaTypes {
-//    public:
-//        registerMetaTypes()
-//        {
-//            qRegisterMetaType<ASyncDBConnection::State>();
-//            qRegisterMetaType<Pgsql::ErrorDetails>();
-//        }
-//    } registerMetaTypes_instance;
+    class registerMetaTypes {
+    public:
+        registerMetaTypes()
+        {
+            qRegisterMetaType<ASyncDBConnection::State>();
+            qRegisterMetaType<Pgsql::ErrorDetails>();
+        }
+    } registerMetaTypes_instance;
     
     
-//}
+}
 
 ASyncDBConnection::ASyncDBConnection(boost::asio::io_service &ios)
     : m_asioSock(ios)
@@ -87,8 +87,7 @@ void ASyncDBConnection::doStateCallback(State state)
         m_connection.setNoticeReceiver(
             [this](const PGresult *result) { processNotice(result); });
     }
-	if (m_stateChangeReceiver)
-		m_stateChangeReceiver(state);
+	emit onStateChanged(state);	
 }
 
 
@@ -137,7 +136,7 @@ void ASyncDBConnection::async_query_handler(boost::system::error_code ec, std::s
             while ( ! finished && ! m_connection.isBusy()) {
                 auto res = m_connection.getResult();
                 qint64 ms = m_timer.restart();
-				on_result(res, ms); // do we really want to send null result to???
+                on_result(res, ms);
                 if (res == nullptr) {
                     m_timer.invalidate();
                     doStateCallback(State::Connected);
@@ -170,16 +169,5 @@ bool ASyncDBConnection::cancel()
 void ASyncDBConnection::processNotice(const PGresult *result)
 {
     Pgsql::ErrorDetails details = Pgsql::ErrorDetails::createErrorDetailsFromPGresult(result);
-	if (m_noticeReceiver)
-		m_noticeReceiver(details);
-}
-
-void ASyncDBConnection::setStateChangeReceiver(StateChangeReceiver state_change_receiver)
-{
-	m_stateChangeReceiver = state_change_receiver;
-}
-
-void ASyncDBConnection::setNoticeReceiver(NoticeReceiver notice_receiver)
-{
-	m_noticeReceiver = notice_receiver;
+	emit onNotice(details);
 }

pglablib/ASyncDBConnection.h

@@ -1,13 +1,14 @@
 #ifndef ASYNCDBCONNECTION_H
 #define ASYNCDBCONNECTION_H
 
+#include <QObject>
+
 #include "Pgsql_Connection.h"
 #include "Pgsql_Params.h"
 #include "Pgsql_Result.h"
 #include "Expected.h"
 #include "ConnectionConfig.h"
 #include <QElapsedTimer>
-#include <functional>
 #include <mutex>
 #include <boost/asio/ip/tcp.hpp>
 #include <boost/asio/io_service.hpp>
@@ -15,12 +16,10 @@
 /** \brief Class that handles asynchronous execution of queries.
  *
  * Queries are passed to this class with a routine to call on completion
- * when the result is received that routine is called. The asynchronous
- * processing is done using boost::asio.
- *
- * You can cancel a running query but even then you have to wait for the result.
+ * when the result is on that routine is called.
  */
-class ASyncDBConnection {
+class ASyncDBConnection: public QObject {
+    Q_OBJECT
 public:
 	enum class State {
 		NotConnected,
@@ -40,36 +39,20 @@ public:
 	void setupConnection(const ConnectionConfig &config);
 	void closeConnection();
 
-	/** \defgroup send These function send queries to the server.
-	 *
-	 * The query string can actually contain multiple queries.
-	 * These functions report their results through a callback function.
-	 * If the command gives multiple results on_result will be called for each result.
-	 *
-	 * \note when the io_service that was passed to the constructor is running
-	 * on a different thread the callback also will be done on that thread. Forward
-	 * the call to your own thread if needed.
-	 *
-	 * \note while you can pass multiple queries to a single send call it is not allowed to
-	 *   send new queries while not all results have been received.
-	 */
+	/** Sends command to the server.
 
-	/** \ingroup send
-	 * \brief Function to send a command without parameters to the server.
+	When the result is in on_result will be	called directly within the thread.
+
+	If the command gives multiple results on_result will be called for each result.
 	*/
 	bool send(const std::string &command, on_result_callback on_result);
-	/** \ingroup send
-	 * \brief Function to send a a command with parameters to the server.
-	 */
 	bool send(const std::string &command, Pgsql::Params params, on_result_callback on_result);
 
 	bool cancel();
     
-	using StateChangeReceiver = std::function<void(ASyncDBConnection::State)>;
-	void setStateChangeReceiver(StateChangeReceiver state_change_receiver);
-
-	using NoticeReceiver = std::function<void(Pgsql::ErrorDetails)>;
-	void setNoticeReceiver(NoticeReceiver notice_receiver);
+signals:
+    void onStateChanged(ASyncDBConnection::State state);
+    void onNotice(Pgsql::ErrorDetails notice);
 
 private:
 	Pgsql::Connection m_connection;
@@ -77,8 +60,6 @@ private:
 	ConnectionConfig m_config;
 	State m_state = State::NotConnected;
     Pgsql::Canceller m_canceller;
-	StateChangeReceiver m_stateChangeReceiver;
-	NoticeReceiver m_noticeReceiver;
     
     QElapsedTimer m_timer;
     
@@ -88,4 +69,8 @@ private:
     void processNotice(const PGresult *result);
 };
 
+Q_DECLARE_METATYPE(ASyncDBConnection::State);
+Q_DECLARE_METATYPE(Pgsql::ErrorDetails);
+
+
 #endif // ASYNCDBCONNECTION_H