.. ot-topic:: cxx11.new_language_features.move_pf.livehack_string_move
   :dependencies: cxx11.new_language_features.move_pf.overview

.. include:: <mmlalias.txt>


Implementing Move Semantics (``class String``, Live Hacked)
===========================================================

.. contents::
   :local:

Copy Semantics, Revisited
-------------------------

.. sidebar::
   
   **See also**

   * :doc:`/trainings/material/soup/cxx03/020-data-encapsulation/object-copy`

* Copy semantics is complicated when resources are maintained
  (e.g. dynamic memory is allocated)
* Compiler generated copy semantics is exactly *counter-bugfree* in
  such situations
* |longrightarrow| *pointers* are copied, not the resources that they
  point to

``class String``, Without Copy
------------------------------
  
* ``class String`` again
* *Copy* explicitly deleted (added again later)
* |longrightarrow| neither *copy* nor *move* (remember: move is
  generated by compiler only when no dtor or copy is there

* Implementation

  .. literalinclude:: code/string-10.h
     :caption: :download:`code/string-10.h`
     :language: c++

* Basic functionality/usage (other than copy/move)

  .. literalinclude:: code/string-10-suite.cpp
     :caption: :download:`code/string-10-suite.cpp`
     :language: c++

Return From Function: *Copy (Or Move) Wanted*
---------------------------------------------

* Copy explicitly deleted
* Rules: no *move* semantics either (we don't know what that is
  anyway)
* *Return from function?*

.. literalinclude:: code/string-10-suite-bogus.cpp
   :caption: :download:`code/string-10-suite-bogus.cpp`
   :language: c++

* No move, no copy
* |longrightarrow| error

.. code-block:: console

   code/string-10-suite-bogus.cpp:8:12: error: use of deleted function ‘String::String(const String&)’

Move Constructor? Move?
-----------------------

.. code-block:: c++

   String make_a_string_from(const char* s)
   {
       String ret_s(s);
       return ret_s;      // <--- proven: ret_s not accessible past this point
   }

* Local variable ``ret_s`` not accessible anymore after ``return``,
  obviously
* No need for a *copy* altogether which would be

  * Allocate destination (caller's function value)
  * Copy source (``ret_s`` to destination)
  * Free source (``ret_s``)

* |longrightarrow| enter *move*: transfer source to destination
  without copying
* Compiler knows all that
* |longrightarrow| can insert move constructor to transfer returned
  value to final location

Implementing Move Constructor
-----------------------------

* Prototype: ``String(String&& from) noexcept``
* Transfers guts of ``from`` into ``this``
* ``noexcept`` is good practice but not required
* Invalidates ``from`` |longrightarrow| resources not deleted twice
* *Attention*: invalidated object should still be usable!

.. literalinclude:: code/string-20.h
   :caption: :download:`code/string-20.h`
   :language: c++

.. literalinclude:: code/string-20-suite.cpp
   :caption: :download:`code/string-20-suite.cpp`
   :language: c++

.. note::

   The manual operations (move ``_c_str`` from there to here, and
   invalidated there) can also be written as

   .. code-block:: c++

      #include <utility>

      String(String&& from) noexcept
      : _c_str(std::exchange(from._c_str, nullptr) {}

And Move Assignment?
--------------------

.. literalinclude:: code/string-20-suite-bogus.cpp
   :caption: :download:`code/string-20-suite-bogus.cpp`
   :language: c++

* (Copy) assignment was deleted
* No move assignment
* |longrightarrow| no assigment

.. code-block:: console

   code/string-20-suite-bogus.cpp:14:35: error: use of deleted function ‘String& String::operator=(const String&)’

Implementing Move Assignment
----------------------------
  
* Prototype: ``String& operator=(String&&) noexcept``
* Transfers guts, and invalidates source (just like move constructor)
* Much like copy assignment |longrightarrow| *self assignment check*

.. literalinclude:: code/string-30.h
   :caption: :download:`code/string-30.h`
   :language: c++