add std::optional example

Chapter07/error_handling/app/src/main_exceptions.cpp

@@ -21,7 +21,6 @@ template <class T, std::size_t N> struct ring_buffer {
   }
 };
 
-
 int main()
 {
     hal::init();

Chapter07/error_handling/app/src/main_expected.cpp

@@ -0,0 +1,28 @@
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+
+
+#include <stm32f072xb.h>
+
+#include <hal.hpp>
+#include <uart_stm32.hpp>
+
+#include <retarget.hpp>
+
+int main()
+{
+    hal::init();
+
+    hal::uart_stm32 uart(USART2);
+    uart.init();
+
+    retarget::set_stdio_uart(&uart);
+
+    printf("std::expected example\r\n");
+
+    while(true)
+    {
+    }
+}
+ 

Chapter07/error_handling/app/src/main_optional.cpp

@@ -0,0 +1,58 @@
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <optional>
+
+#include <stm32f072xb.h>
+
+#include <hal.hpp>
+#include <uart_stm32.hpp>
+
+#include <retarget.hpp>
+
+struct sensor {
+    struct data {
+        int x;
+        int y;
+    };
+    static inline bool ret_val = true;
+    static std::optional<data> get_data() {
+        ret_val = !ret_val;
+        if(ret_val) {
+            return data{4, 5};
+        } 
+        else {
+            return std::nullopt;
+        }
+    }
+};
+
+int main()
+{
+    hal::init();
+
+    hal::uart_stm32 uart(USART2);
+    uart.init();
+
+    retarget::set_stdio_uart(&uart);
+
+    printf("std::optional example\r\n");
+
+    auto get_data_from_main = [] () {
+      auto result = sensor::get_data();
+      if(result) {
+          printf("x = %d, y = %d\r\n", (*result).x, (*result).y);
+      } 
+      else {
+          printf("No data!\r\n");
+      }
+    };
+
+    get_data_from_main();
+    get_data_from_main();
+
+    while(true)
+    {
+    }
+}
+