clang-tools-extra/docs/cpp11-migrate.rst

===========================
cpp11-migrate User's Manual
===========================

.. toctree::
   :hidden:

   UseAutoTransform

:program:`cpp11-migrate` is a standalone tool used to automatically convert
C++98 and C++03 code to use features of the new C++11 standard where
appropriate.

Basic Usage
===========

``cpp11-migrate [options] <source0> [... <sourceN>]``

``<source0>...`` specify the paths of files in the CMake source tree,
with the same requirements as other tools built on LibTooling.

Command Line Options
--------------------

.. option:: -fatal-assembler-warnings

  Treat all compiler warnings as errors.

.. option:: -help

  Displays tool usage instructions and command line options.

.. option:: -loop-convert

  Makes use of C++11 range-based for loops where possible.
  See :ref:`loop-convert-transformation`.

.. option:: -use-auto

  Replace the type specifier of variable declarations with the ``auto`` type
  specifier. See :doc:`UseAutoTransform`.

.. option:: -p=<build-path>

  ``<build-path>`` is a CMake build directory containing a file named
  ``compile_commands.json`` which provides compiler options for building
  each source file and can be generated by specifying
  ``-DCMAKE_EXPORT_COMPILE_COMMANDS`` when running CMake. If ``<build-path>``
  is not provided the ``compile_commands.json`` file is searched for through
  all parent directories.

  Alternatively, one can provide compile options to be applied to every
  source file after the optional ``--``.

.. option:: -risk=<risk-level>

  Some transformations may cause a change in semantics. In such cases the
  maximum acceptable risk level specified through the ``-risk`` command
  line option decides whether or not a transformation is applied.

  Three different risk level options are available:

    ``-risk=safe``
      Perform only safe transformations.
    ``-risk=reasonable`` (default)
      Enable transformations that may change semantics.
    ``-risk=risky``
      Enable transformations that are likely to change semantics.

  See :ref:`transformations` for further details for
  risk in individual transformations.

.. option:: -use-nullptr

  Makes use of the new C++11 keyword ``nullptr`` where possible.
  See :ref:`nullptr-convert-transformation`.

.. option:: -version

  Displays the version information of this tool.

.. _transformations:

Transformations
===============

.. _loop-convert-transformation:

Loop Convert
------------

Loop convert is a transformation to convert ``for(...; ...; ...)`` loops to use
the new range-based loops in C++11.

Three kinds of loops can be converted:

-  Loops over statically allocated arrays
-  Loops over containers, using iterators
-  Loops over array-like containers, using ``operator[]`` and ``at()``

Risk
^^^^

TODO: Add code examples for which incorrect transformations are performed
when the risk level is set to "Risky" or "Reasonable".

Risky
"""""

In loops where the container expression is more complex than just a
reference to a declared expression (a variable, function, enum, etc.),
and some part of it appears elsewhere in the loop, we lower our confidence
in the transformation due to the increased risk of changing semantics.
Transformations for these loops are marked as `risky`, and thus will only
be converted if the acceptable risk level is set to ``-risk=risky``.

.. code-block:: c++

  int arr[10][20];
  int l = 5;

  for (int j = 0; j < 20; ++j)
    int k = arr[l][j] + l; // using l outside arr[l] is considered risky

  for (int i = 0; i < obj.getVector().size(); ++i)
    obj.foo(10); // using 'obj' is considered risky

Reasonable (Default)
""""""""""""""""""""

If a loop calls ``.end()`` or ``.size()`` after each iteration, the
transformation for that loop is marked as `reasonable`, and thus will
be converted if the acceptable risk level is set to ``-risk=reasonable``
(default) or higher.

.. code-block:: c++

  // using size() is considered reasonable
  for (int i = 0; i < container.size(); ++i)
    cout << container[i];

Safe
""""

Any other loops that do not match the above criteria to be marked as
`risky` or `reasonable` are marked `safe`, and thus will be converted
if the acceptable risk level is set to ``-risk=safe`` or higher.

.. code-block:: c++

  int arr[] = {1,2,3};

  for (int i = 0; i < 3; ++i)
    cout << arr[i];

Example
^^^^^^^

Original
""""""""

.. code-block:: c++

  const int N = 5;
  int arr[] = {1,2,3,4,5};
  vector<int> v;
  v.push_back(1);
  v.push_back(2);
  v.push_back(3);

  // safe transform
  for (int i = 0; i < N; ++i)
    cout << arr[i];

  // reasonable transform
  for (vector<int>::iterator it = v.begin(); it != v.end(); ++it)
    cout << *it;

  // reasonable transform
  for (int i = 0; i < v.size(); ++i)
    cout << v[i];


After transformation
""""""""""""""""""""
With risk level set to ``-risk=reasonable`` (default).

.. code-block:: c++

  const int N = 5;
  int arr[] = {1,2,3,4,5};
  vector<int> v;
  v.push_back(1);
  v.push_back(2);
  v.push_back(3);

  // safe transform
  for (auto & elem : arr)
    cout << elem;

  // reasonable transform
  for (auto & elem : v)
    cout << elem;

  // reasonable transform
  for (auto & elem : v)
    cout << elem;

.. _nullptr-convert-transformation:

Nullptr Convert
---------------

Nullptr convert is a transformation to convert the usage of null pointer
constants (eg. ``NULL``, ``0``) to use the new C++11 ``nullptr`` keyword.

Example
^^^^^^^

Original
""""""""

.. code-block:: c++

  void assignment() {
    char *a = NULL;
    char *b = 0;
    char c = 0;
  }

  int *ret_ptr() {
    return 0;
  }


After transformation
""""""""""""""""""""

.. code-block:: c++

  void assignment() {
    char *a = nullptr;
    char *b = nullptr;
    char c = 0;
  }

  int *ret_ptr() {
    return nullptr;
  }