fixed build issues on ubuntu 18.04

examples/melody.cpp

@@ -9,6 +9,7 @@
 
 #include <cmath>
 #include <array>
+#include <atomic>
 #include <thread>
 #include <audio>
 
@@ -99,4 +100,4 @@ int main() {
   while (!stop.load()) {
     std::this_thread::sleep_for(std::chrono::milliseconds(50));
   }
-}
+}

include/audio_backend/__null_backend.h

@@ -8,6 +8,8 @@
 #include <string_view>
 #include <chrono>
 #include <cassert>
+#include <forward_list>
+#include <functional>
 
 _LIBSTDAUDIO_NAMESPACE_BEGIN
 
@@ -74,7 +76,14 @@ class audio_device {
     return false;
   }
 
-  bool start() {
+  using no_op_t = std::function<void(audio_device&)>;
+
+  template <typename _StartCallbackType = no_op_t,
+            typename _StopCallbackType = no_op_t,
+            // TODO: is_nothrow_invocable_t does not compile, temporarily replaced with is_invocable_t
+            typename = enable_if_t<is_invocable_v<_StartCallbackType, audio_device&> && is_invocable_v<_StopCallbackType, audio_device&>>>
+  bool start(_StartCallbackType&& start_callback = [](audio_device&) noexcept {},
+             _StopCallbackType&& stop_callback = [](audio_device&) noexcept {}) {
     return false;
   }
 
@@ -98,28 +107,13 @@ class audio_device {
   constexpr bool has_unprocessed_io() const noexcept {
     return false;
   }
-};
 
-class audio_device_list
-{
-private:
-  class iterator {
-  public:
-    auto operator==(const iterator&) const noexcept { return true; }
-    auto operator!=(const iterator&) const noexcept { return false; }
-    auto operator++() -> const iterator& { assert(false); return *this; }
-    auto operator*() -> audio_device& {
-      assert(false); static audio_device device{};
-      return device;
-    }
-  };
+  template <typename _CallbackType>
+  void connect(_CallbackType callback) {
+  }
+};
 
-public:
-  auto begin() -> iterator { return {}; }
-  auto end() -> iterator { return {}; }
-  auto begin() const -> iterator { return {}; }
-  auto end() const -> iterator { return {}; }
-  auto empty() const -> bool { return true; }
+class audio_device_list : public forward_list<audio_device> {
 };
 
 optional<audio_device> get_default_audio_input_device() {
@@ -138,4 +132,8 @@ audio_device_list get_audio_output_device_list() {
   return {};
 }
 
-_LIBSTDAUDIO_NAMESPACE_END
+template <typename F, typename /* = enable_if_t<is_nothrow_invocable_v<F>> */ >
+void set_audio_device_list_callback(audio_device_list_event event, F&& cb) {
+}
+
+_LIBSTDAUDIO_NAMESPACE_END