.. ot-topic:: cxx03.data_encapsulation.ctor_default
   :dependencies: cxx03.data_encapsulation.ctor_custom

.. include:: <mmlalias.txt>


Default Constructor
===================

.. contents::
   :local:

Compiler Generated Default Constructor
--------------------------------------

.. admonition:: **Rule**

   A default constructor is automatically generated by the compiler
   *when no user defined constructors are present in the class*.

   That constructor *default-initializes* all members.

.. literalinclude:: code/c++03-default-ctor-compiler-generated.cpp
   :caption: :download:`code/c++03-default-ctor-compiler-generated.cpp`
   :language: c++

.. code-block:: console

   $ ./code/c++03-default-ctor-compiler-generated 
   sth.s: ""
   sth.ints.size(): 0

Compiler Generated Default Constructor: Pitfall: Built-In Types
---------------------------------------------------------------

.. caution::

   **Built-in datatypes (e.g. integers) are not initialized**

This one reads uninitialized memory from the stack ...

.. literalinclude:: code/c++03-default-ctor-compiler-generated-pitfall.cpp
   :caption: :download:`code/c++03-default-ctor-compiler-generated-pitfall.cpp`
   :language: c++

.. code-block:: console

   $ ./code/c++03-default-ctor-compiler-generated-pitfall 
   sth.i: 666

**Solution**: write default constructor *manually*!!

Manually Written Default Constructor
------------------------------------

**C++ pre 11**

.. code-block:: c++

   class Something
   {
   public:
       Something() : i(0) {}     // <--- explicit initialization
   
       int i;
   };

**C++11 onwards**

.. code-block:: c++

   class Something
   {
   public:
       int i{};                  // <--- default value specified in member definition
   };

What If User-Defined Constructors Are In Place?
-----------------------------------------------

* No default constructor is generated
* |longrightarrow| user has to supply one (if desired)

**C++ pre 11**

.. code-block:: c++

   class Something
   {
   public:
       Something() : i(0) {}               // <--- user supplied default ctor
       Something(int value) : i(value) {}  // <--- user defined ctor
   
       int i;
   };

**C++11 onwards**

.. code-block:: c++

   class Something
   {
   public:
       Something() = default;              // <--- force default ctor generation
       Something(int value) : i(value) {}  // <--- user defined ctor
       int i{};                            // <--- default value specified in member definition
   };