.. include:: <mmlalias.txt>


``std::shared_ptr`` (Alternative Take: Transition From Raw Pointers)
====================================================================

.. topic:: See also

   * :doc:`topic`

This is a task that I gave to students, expanding on :doc:`topic`:
refactoring memory management in an existing program by replacing raw
pointers with something better.

The (stripped-down) program can be called like so,

* Using the test/mock implementation of some interface

  .. code-block:: console
     
     $ ./program test

* Using the real implementation of some interface (usually using some
  real hardware on a Linux system, like GPIO or I2C)

  .. code-block:: console
     
     $ ./program real

Original Version: Raw Pointers
------------------------------

A heap allocation of either ``TestImplemention`` or
``RealImplemention`` is made, depending on the value of the
commandline parameter, and assigned to an ``Interface`` (the abstract
base class of both) pointer ``my_obj``.

**Problem**: avoiding memory leaks is hard - the allocation must be
freed, so we must take care to ``delete`` the object that we
allocated.

.. literalinclude:: code/shared-ptr-raw.cpp
   :caption: :download:`code/shared-ptr-raw.cpp`
   :language: c++

Replacing *Raw* With *Shared* Pointers (Clumsy Version)
-------------------------------------------------------

.. topic:: See also

   * :doc:`topic`

**Solution**: using shared pointers. I the program below we replaced
all raw pointer types with their correspondingly typed
``std::shared_ptr<>`` template instances. As ``my_obj`` goes out of
scope (as a consequence of the return statement), the allocation is
freed *automatically*.

**Problem**: much redundant writing

.. literalinclude:: code/shared-ptr-explicit-shared-ptr.cpp
   :caption: :download:`code/shared-ptr-explicit-shared-ptr.cpp`
   :language: c++

Using ``std::make_shared<>()`` (Less Clumsy)
--------------------------------------------

Reducing redundancy by using the ``std::make_shared<>()`` template
function. 

Note:

* ``new`` is not called directly
* The constructor is not called directly
* The parameters of the ``RealImplemention`` constructor are
  *forwarded* through by ``std::make_shared<>()`` (see :doc:`here </trainings/material/soup/cxx/cxx11/perfect-forwarding/topic>` for more)

.. literalinclude:: code/shared-ptr-make_shared.cpp
   :caption: :download:`code/shared-ptr-make_shared.cpp`
   :language: c++