Alignment typed storage (#1023)

* Add etl::typed_storage based on etl::aligned_storage_as

* Have create() instead of emplace()

Author: rolandreichweinbmw

include/etl/alignment.h

@@ -36,6 +36,7 @@ SOFTWARE.
 #include "static_assert.h"
 #include "error_handler.h"
 #include "exception.h"
+#include "utility.h"
 
 #include <stdint.h>
 
@@ -71,6 +72,19 @@ namespace etl
     }
   };
 
+  //***************************************************************************
+  /// Typed storage exception.
+  //***************************************************************************
+  class typed_storage_error : public alignment_exception
+  {
+  public:
+
+    typed_storage_error(string_type file_name_, numeric_type line_number_)
+      : alignment_exception(ETL_ERROR_TEXT("typed_storage:error", ETL_ALIGNMENT_FILE_ID"B"), file_name_, line_number_)
+    {
+    }
+  };
+
   //*****************************************************************************
   /// Check that 'p' has 'required_alignment'.
   //*****************************************************************************
@@ -333,6 +347,109 @@ namespace etl
 #if ETL_USING_CPP11
   template <size_t Length, typename T>
   using aligned_storage_as_t = typename aligned_storage_as<Length, T>::type;
+
+  //***************************************************************************
+  /// Wrapper class that provides a memory area and lets the user emplace and
+  /// instance of T in this memory at runtime. This class also erases the
+  /// destructor call of T, i.e. if typed_storage goes out of scope, the
+  /// destructor if the wrapped type will not be called. This can be done
+  /// explicitly by calling destroy().
+  /// \tparam T    Type of element stored in this instance of typed_storage.
+  //***************************************************************************
+  template <typename T>
+  class typed_storage
+  {
+  public:
+
+    using value_type      = T;
+    using reference       = T&;
+    using const_reference = T const&;
+    using pointer         = T*;
+    using const_pointer   = T const*;
+
+    // Constructor
+    typed_storage()
+      : valid(false)
+    {
+    }
+
+    //***************************************************************************
+    /// Default destructor which will NOT call the destructor of the object which
+    /// was created by calling emplace().
+    //***************************************************************************
+    ~typed_storage() = default;
+
+    //***************************************************************************
+    /// Calls the destructor of the wrapped object and asserts if has_value() is false.
+    //***************************************************************************
+    void destroy()
+    {
+      ETL_ASSERT(has_value(), ETL_ERROR(etl::typed_storage_error));
+      data.template get_reference<T>().~T();
+      valid = false;
+    }
+
+    //***************************************************************************
+    /// \returns true if object has been constructed using emplace().
+    /// \returns false otherwise.
+    //***************************************************************************
+    bool has_value() const
+    {
+      return valid;
+    }
+
+    //***************************************************************************
+    /// Constructs the instance of T forwarding the given \p args to its constructor and
+    /// asserts if has_value() is true before calling emplace().
+    ///
+    /// \returns the instance of T which has been constructed in the internal byte array.
+    //***************************************************************************
+    template<typename... Args>
+    reference create(Args&&... args)
+    {
+      ETL_ASSERT(!has_value(), ETL_ERROR(etl::typed_storage_error));
+      valid = true;
+      return *::new (data.template get_address<char>()) value_type(etl::forward<Args>(args)...);
+    }
+
+    //***************************************************************************
+    /// \returns a pointer of type T and asserts if has_value() is false.
+    //***************************************************************************
+    pointer operator->()
+    {
+      ETL_ASSERT(has_value(), ETL_ERROR(etl::typed_storage_error));
+      return data.template get_address<value_type>();
+    }
+
+    //***************************************************************************
+    /// \returns a const pointer of type T and asserts if has_value() is false.
+    //***************************************************************************
+    const_pointer operator->() const
+    {
+      return operator->();
+    }
+
+    //***************************************************************************
+    /// \returns reference of type T and asserts if has_value() is false.
+    //***************************************************************************
+    reference operator*()
+    {
+      return *operator->();
+    }
+
+    //***************************************************************************
+    /// \returns const reference of type T and asserts if has_value() is false.
+    //***************************************************************************
+    const_reference operator*() const
+    {
+      return *operator->();
+    }
+
+  private:
+
+    typename aligned_storage_as<sizeof(value_type), value_type>::type data;
+    bool valid;
+  };
 #endif
 }
 

test/test_alignment.cpp

@@ -48,6 +48,23 @@ void f(int)
 {
 }
 
+struct A_t
+{
+  A_t(uint32_t v_x, uint8_t v_y)
+   : x(v_x)
+   , y(v_y)
+  {
+  }
+
+  bool operator==(A_t& other)
+  {
+    return other.x == x && other.y == y;
+  }
+
+  uint32_t x;
+  uint8_t y;
+};
+
 namespace
 {
   SUITE(test_alignment)
@@ -155,5 +172,31 @@ namespace
       CHECK_EQUAL(32, alignof(etl::type_with_alignment_t<32>));
       CHECK_EQUAL(64, alignof(etl::type_with_alignment_t<64>));
     }
+
+    //*************************************************************************
+#if ETL_USING_CPP11
+    TEST(test_typed_storage)
+    {
+      etl::typed_storage<A_t> a;
+
+      CHECK_EQUAL(false, a.has_value());
+
+      auto& b = a.create(123, 4);
+
+      CHECK_EQUAL(true, a.has_value());
+
+      CHECK_EQUAL(a->x, 123);
+      CHECK_EQUAL(a->y, 4);
+
+      CHECK_EQUAL(b.x, 123);
+      CHECK_EQUAL(b.y, 4);
+
+      CHECK_TRUE(*a == b);
+
+      CHECK_EQUAL(true, a.has_value());
+      a.destroy();
+      CHECK_EQUAL(false, a.has_value());
+    }
+#endif
   };
 }