Add execution_context::service_maker abstract base class.

A service_maker is an object that is passed to an execution context's
constructor, and allows services to be added at context construction
time. Additional constructor overloads have been added to io_context and
thread_pool that accept a service_maker. For example:

  class my_service_maker : public execution_context::service_maker
  {
  public:
    void make(execution_context& ctx) override
    {
      make_service<my_service>(ctx);
    }
  };

  io_context ctx{my_service_maker{}};

asio/include/asio/execution_context.hpp

@@ -107,11 +107,23 @@ class execution_context
 public:
   class id;
   class service;
+  class service_maker;
 
 public:
   /// Constructor.
   ASIO_DECL execution_context();
 
+  /// Constructor.
+  /**
+   * Construct with a service maker, to create an initial set of services that
+   * will be installed into the execution context at construction time.
+   *
+   * @param initial_services Used to create the initial services. The @c make
+   * function will be called once at the end of execution_context construction.
+   */
+  ASIO_DECL explicit execution_context(
+      const service_maker& initial_services);
+
   /// Destructor.
   ASIO_DECL ~execution_context();
 
@@ -287,7 +299,7 @@ class execution_context::id
   id() {}
 };
 
-/// Base class for all io_context services.
+/// Base class for all execution context services.
 class execution_context::service
   : private noncopyable
 {
@@ -330,6 +342,23 @@ class execution_context::service
   service* next_;
 };
 
+/// Base class for all execution context service makers.
+/**
+ * A service maker is called by the execution context to create services that
+ * need to be installed into the execution context at construction time.
+ */
+class execution_context::service_maker
+  : private noncopyable
+{
+public:
+  /// Make services to be added to the execution context.
+  virtual void make(execution_context& context) const = 0;
+
+protected:
+  /// Destructor.
+  ASIO_DECL virtual ~service_maker();
+};
+
 /// Exception thrown when trying to add a duplicate service to an
 /// execution_context.
 class service_already_exists

asio/include/asio/impl/execution_context.ipp

@@ -28,6 +28,13 @@ execution_context::execution_context()
 {
 }
 
+execution_context::execution_context(
+    const execution_context::service_maker& initial_services)
+  : service_registry_(new asio::detail::service_registry(*this))
+{
+  initial_services.make(*this);
+}
+
 execution_context::~execution_context()
 {
   shutdown();
@@ -65,6 +72,10 @@ void execution_context::service::notify_fork(execution_context::fork_event)
 {
 }
 
+execution_context::service_maker::~service_maker()
+{
+}
+
 service_already_exists::service_already_exists()
   : std::logic_error("Service already exists.")
 {

asio/include/asio/impl/io_context.ipp

@@ -45,6 +45,13 @@ io_context::io_context(int concurrency_hint)
 {
 }
 
+io_context::io_context(const execution_context::service_maker& initial_services)
+  : execution_context(initial_services),
+    impl_(add_impl(new impl_type(*this,
+          ASIO_CONCURRENCY_HINT_DEFAULT, false)))
+{
+}
+
 io_context::impl_type& io_context::add_impl(io_context::impl_type* impl)
 {
   asio::detail::scoped_ptr<impl_type> scoped_impl(impl);

asio/include/asio/impl/thread_pool.ipp

@@ -94,6 +94,19 @@ thread_pool::thread_pool(std::size_t num_threads)
   threads_.create_threads(f, static_cast<std::size_t>(num_threads_));
 }
 
+thread_pool::thread_pool(std::size_t num_threads,
+    const execution_context::service_maker& initial_services)
+  : execution_context(initial_services),
+    scheduler_(add_scheduler(new detail::scheduler(
+          *this, num_threads == 1 ? 1 : 0, false))),
+    num_threads_(detail::clamp_thread_pool_size(num_threads))
+{
+  scheduler_.work_started();
+
+  thread_function f = { &scheduler_ };
+  threads_.create_threads(f, static_cast<std::size_t>(num_threads_));
+}
+
 thread_pool::~thread_pool()
 {
   stop();

asio/include/asio/io_context.hpp

@@ -230,6 +230,17 @@ class io_context
    */
   ASIO_DECL explicit io_context(int concurrency_hint);
 
+  /// Constructor.
+  /**
+   * Construct with a service maker, to create an initial set of services that
+   * will be installed into the execution context at construction time.
+   *
+   * @param initial_services Used to create the initial services. The @c make
+   * function will be called once at the end of execution_context construction.
+   */
+  ASIO_DECL explicit io_context(
+      const execution_context::service_maker& initial_services);
+
   /// Destructor.
   /**
    * On destruction, the io_context performs the following sequence of

asio/include/asio/thread_pool.hpp

@@ -92,6 +92,19 @@ class thread_pool
   /// Constructs a pool with a specified number of threads.
   ASIO_DECL thread_pool(std::size_t num_threads);
 
+  /// Constructs a pool with a specified number of threads.
+  /**
+   * Construct with a service maker, to create an initial set of services that
+   * will be installed into the execution context at construction time.
+   *
+   * @param num_threads The number of threads required.
+   *
+   * @param initial_services Used to create the initial services. The @c make
+   * function will be called once at the end of execution_context construction.
+   */
+  ASIO_DECL thread_pool(std::size_t num_threads,
+      const execution_context::service_maker& initial_services);
+
   /// Destructor.
   /**
    * Automatically stops and joins the pool, if not explicitly done beforehand.

asio/src/doc/quickref.xml

@@ -155,6 +155,7 @@
             <member><link linkend="asio.reference.execution_context">execution_context</link></member>
             <member><link linkend="asio.reference.execution_context__id">execution_context::id</link></member>
             <member><link linkend="asio.reference.execution_context__service">execution_context::service</link></member>
+            <member><link linkend="asio.reference.execution_context__service_maker">execution_context::service_maker</link></member>
             <member><link linkend="asio.reference.executor">executor</link></member>
             <member><link linkend="asio.reference.executor_arg_t">executor_arg_t</link></member>
             <member><link linkend="asio.reference.invalid_service_owner">invalid_service_owner</link></member>
@@ -163,6 +164,7 @@
             <member><link linkend="asio.reference.io_context__service">io_context::service</link></member>
             <member><link linkend="asio.reference.io_context__strand">io_context::strand</link></member>
             <member><link linkend="asio.reference.multiple_exceptions">multiple_exceptions</link></member>
+            <member><link linkend="asio.reference.no_error_t">no_error_t</link></member>
             <member><link linkend="asio.reference.partial_as_tuple">partial_as_tuple</link></member>
             <member><link linkend="asio.reference.partial_redirect_error">partial_redirect_error</link></member>
             <member><link linkend="asio.reference.service_already_exists">service_already_exists</link></member>
@@ -196,6 +198,7 @@
             <member><link linkend="asio.reference.cancellation_slot_binder">cancellation_slot_binder</link></member>
             <member><link linkend="asio.reference.consign_t">consign_t</link></member>
             <member><link linkend="asio.reference.deferred_t">deferred_t</link></member>
+            <member><link linkend="asio.reference.disposition_traits">disposition_traits</link></member>
             <member><link linkend="asio.reference.executor_binder">executor_binder</link></member>
             <member><link linkend="asio.reference.executor_work_guard">executor_work_guard</link></member>
             <member><link linkend="asio.reference.experimental__as_single_t">experimental::as_single_t</link></member>
@@ -267,6 +270,8 @@
             <member><link linkend="asio.reference.spawn">spawn</link></member>
             <member><link linkend="asio.reference.this_coro__reset_cancellation_state">this_coro::reset_cancellation_state</link></member>
             <member><link linkend="asio.reference.this_coro__throw_if_cancelled">this_coro::throw_if_cancelled</link></member>
+            <member><link linkend="asio.reference.throw_exception">throw_exception</link></member>
+            <member><link linkend="asio.reference.to_exception_ptr">to_exception_ptr</link></member>
             <member><link linkend="asio.reference.execution_context.use_service">use_service</link></member>
           </simplelist>
           <bridgehead renderas="sect3">Special Values</bridgehead>
@@ -277,6 +282,7 @@
             <member><link linkend="asio.reference.executor_arg">executor_arg</link></member>
             <member><link linkend="asio.reference.experimental__use_coro">experimental::use_coro</link></member>
             <member><link linkend="asio.reference.experimental__use_promise">experimental::use_promise</link></member>
+            <member><link linkend="asio.reference.no_error">no_error</link></member>
             <member><link linkend="asio.reference.this_coro__cancellation_state">this_coro::cancellation_state</link></member>
             <member><link linkend="asio.reference.this_coro__executor">this_coro::executor</link></member>
             <member><link linkend="asio.reference.use_awaitable">use_awaitable</link></member>

asio/src/tests/unit/io_context.cpp

@@ -268,14 +268,56 @@ class test_service : public asio::io_context::service
 {
 public:
   static asio::io_context::id id;
+
   test_service(asio::io_context& s)
-    : asio::io_context::service(s) {}
+    : asio::io_context::service(s)
+  {
+  }
+
 private:
-  virtual void shutdown() {}
+  void shutdown() override
+  {
+  }
 };
 
 asio::io_context::id test_service::id;
 
+class test_context_service : public asio::execution_context::service
+{
+public:
+  static asio::execution_context::id id;
+
+  test_context_service(asio::execution_context& c, int value = 0)
+    : asio::execution_context::service(c),
+      value_(value)
+  {
+  }
+
+  int get_value() const
+  {
+    return value_;
+  }
+
+private:
+  void shutdown() override
+  {
+  }
+
+  int value_;
+};
+
+asio::execution_context::id test_context_service::id;
+
+class test_context_service_maker :
+  public asio::execution_context::service_maker
+{
+public:
+  void make(asio::execution_context& ctx) const override
+  {
+    (void)asio::make_service<test_context_service>(ctx, 42);
+  }
+};
+
 void io_context_service_test()
 {
   asio::io_context ioc1;
@@ -332,6 +374,14 @@ void io_context_service_test()
   delete svc4;
 
   ASIO_CHECK(!asio::has_service<test_service>(ioc3));
+
+  // Initial service registration.
+
+  asio::io_context ioc4{test_context_service_maker{}};
+
+  ASIO_CHECK(asio::has_service<test_context_service>(ioc4));
+  ASIO_CHECK(asio::use_service<test_context_service>(ioc4).get_value()
+      == 42);
 }
 
 void io_context_executor_query_test()

asio/src/tests/unit/thread_pool.cpp

@@ -99,6 +99,40 @@ class test_service : public asio::execution_context::service
 asio::execution_context::id test_service::id;
 #endif // defined(ASIO_NO_TYPEID)
 
+class test_context_service : public asio::execution_context::service
+{
+public:
+  static asio::execution_context::id id;
+
+  test_context_service(asio::execution_context& c, int value = 0)
+    : asio::execution_context::service(c),
+      value_(value)
+  {
+  }
+
+  int get_value() const
+  {
+    return value_;
+  }
+
+private:
+  virtual void shutdown() {}
+
+  int value_;
+};
+
+asio::execution_context::id test_context_service::id;
+
+class test_context_service_maker :
+  public asio::execution_context::service_maker
+{
+public:
+  void make(asio::execution_context& ctx) const override
+  {
+    (void)asio::make_service<test_context_service>(ctx, 42);
+  }
+};
+
 void thread_pool_service_test()
 {
   asio::thread_pool pool1(1);
@@ -154,6 +188,14 @@ void thread_pool_service_test()
   delete svc4;
 
   ASIO_CHECK(!asio::has_service<test_service>(pool3));
+
+  // Initial service registration.
+
+  asio::thread_pool pool4{1, test_context_service_maker{}};
+
+  ASIO_CHECK(asio::has_service<test_context_service>(pool4));
+  ASIO_CHECK(asio::use_service<test_context_service>(pool4).get_value()
+      == 42);
 }
 
 void thread_pool_executor_query_test()