.. title:: clang-tidy - readability-function-cognitive-complexity

readability-function-cognitive-complexity
=========================================

Checks function Cognitive Complexity metric.

The metric is implemented as per the `COGNITIVE COMPLEXITY by SonarSource
<https://www.sonarsource.com/docs/CognitiveComplexity.pdf>`_ specification
version 1.2 (19 April 2017).

Options
-------

.. option:: Threshold

   Flag functions with Cognitive Complexity exceeding this number.
   The default is `25`.

.. option:: DescribeBasicIncrements

   If set to `true`, then for each function exceeding the complexity threshold
   the check will issue additional diagnostics on every piece of code (loop,
   `if` statement, etc.) which contributes to that complexity. See also the
   examples below. Default is `true`.

.. option:: IgnoreMacros

   If set to `true`, the check will ignore code inside macros. Note, that also
   any macro arguments are ignored, even if they should count to the complexity.
   As this might change in the future, this option isn't guaranteed to be
   forward-compatible. Default is `false`.

Building blocks
---------------

There are three basic building blocks of a Cognitive Complexity metric:

Increment
^^^^^^^^^

The following structures increase the function's Cognitive Complexity metric
(by `1`):

* Conditional operators:

   - ``if()``
   - ``else if()``
   - ``else``
   - ``cond ? true : false``

* ``switch()``
* Loops:

   - ``for()``
   - C++11 range-based ``for()``
   - ``while()``
   - ``do while()``

* ``catch ()``
* ``goto LABEL``, ``goto *(&&LABEL))``,
* sequences of binary logical operators:

   - ``boolean1 || boolean2``
   - ``boolean1 && boolean2``

Nesting level
^^^^^^^^^^^^^

While by itself the nesting level does not change the function's Cognitive
Complexity metric, it is tracked, and is used by the next, third building block.
The following structures increase the nesting level (by `1`):

* Conditional operators:

   - ``if()``
   - ``else if()``
   - ``else``
   - ``cond ? true : false``

* ``switch()``
* Loops:

   - ``for()``
   - C++11 range-based ``for()``
   - ``while()``
   - ``do while()``

* ``catch ()``
* Nested functions:

   - C++11 Lambda
   - Nested ``class``
   - Nested ``struct``
* GNU statement expression
* Apple Block Declaration

Nesting increment
^^^^^^^^^^^^^^^^^

This is where the previous basic building block, `Nesting level`_, matters.
The following structures increase the function's Cognitive Complexity metric by
the current `Nesting level`_:

* Conditional operators:

   - ``if()``
   - ``cond ? true : false``

* ``switch()``
* Loops:

   - ``for()``
   - C++11 range-based ``for()``
   - ``while()``
   - ``do while()``

* ``catch ()``

Examples
--------

The simplest case. This function has Cognitive Complexity of `0`.

.. code-block:: c++

  void function0() {}

Slightly better example. This function has Cognitive Complexity of `1`.

.. code-block:: c++

  int function1(bool var) {
    if(var) // +1, nesting level +1
      return 42;
    return 0;
  }

Full example. This function has Cognitive Complexity of `3`.

.. code-block:: c++

  int function3(bool var1, bool var2) {
    if(var1) { // +1, nesting level +1
      if(var2)  // +2 (1 + current nesting level of 1), nesting level +1
        return 42;
    }

    return 0;
  }

In the last example, the check will flag `function3` if the option Threshold is
set to `2` or smaller. If the option DescribeBasicIncrements is set to `true`,
it will additionally flag the two `if` statements with the amounts by which they
increase to the complexity of the function and the current nesting level.

Limitations
-----------

The metric is implemented with two notable exceptions:
   * `preprocessor conditionals` (``#ifdef``, ``#if``, ``#elif``, ``#else``,
     ``#endif``) are not accounted for.
   * `each method in a recursion cycle` is not accounted for. It can't be fully
     implemented, because cross-translational-unit analysis would be needed,
     which is currently not possible in clang-tidy.