.. include:: <mmlalias.txt>

.. ot-topic:: cxx11.new_language_features.default


``= default``
=============

.. contents::
   :local:

Problem: Default Constructor Not Automatically Generated
--------------------------------------------------------

**Rule**: C++ does not generate a default constructor if a constructor
has been defined manually

.. literalinclude:: code/c++11-default-missing.cpp
   :caption: :download:`code/c++11-default-missing.cpp`
   :language: c++

.. code-block:: console

   code/c++11-default-explicit.cpp:15:31: error: no matching function for call to ‘LackingDefaultConstructor::LackingDefaultConstructor()’
      15 |     LackingDefaultConstructor ldc;
         |                               ^~~
   code/c++11-default-explicit.cpp:7:5: note: candidate: ‘LackingDefaultConstructor::LackingDefaultConstructor(const string&)’
       7 |     LackingDefaultConstructor(const std::string& s) : _s{s} {}
         |     ^~~~~~~~~~~~~~~~~~~~~~~~~
   code/c++11-default-explicit.cpp:7:5: note:   candidate expects 1 argument, 0 provided
   code/c++11-default-explicit.cpp:4:7: note: candidate: ‘LackingDefaultConstructor::LackingDefaultConstructor(const LackingDefaultConstructor&)’
       4 | class LackingDefaultConstructor
         |       ^~~~~~~~~~~~~~~~~~~~~~~~~

C++ < 11 Solution: Write Default Constructor |:angry:| |:thumbsdown:|
---------------------------------------------------------------------

* Write possibly large amounts of code that could have generated by
  the compiler
* Well, a real-life default-ctor implementation might be a bit more
  complicated than in this example |:rolling_eyes:|

.. literalinclude:: code/c++11-default-explicit.cpp
   :caption: :download:`code/c++11-default-explicit.cpp`
   :language: c++

C++ >= 11 Solution: ``= default``
---------------------------------

.. literalinclude:: code/c++11-default-default.cpp
   :caption: :download:`code/c++11-default-default.cpp`
   :language: c++