CMake/Help/manual/cmake-developer.7.rst

.. cmake-manual-description: CMake Developer Reference

cmake-developer(7)
******************

.. only:: html or latex

   .. contents::

Introduction
============

This manual is intended for reference by developers modifying the CMake
source tree itself.


Permitted C++ Subset
====================

CMake is required to build with ancient C++ compilers and standard library
implementations.  Some common C++ constructs may not be used in CMake in order
to build with such toolchains.

std::vector::at
---------------

The ``at()`` member function of ``std::vector`` may not be used. Use
``operator[]`` instead:

.. code-block:: c++

  std::vector<int> someVec = getVec();
  int i1 = someVec.at(5); // Wrong
  int i2 = someVec[5];    // Ok

std::string::append and std::string::clear
------------------------------------------

The ``append()`` and ``clear()`` member functions of ``std::string`` may not
be used. Use ``operator+=`` and ``operator=`` instead:

.. code-block:: c++

  std::string stringBuilder;
  stringBuilder.append("chunk"); // Wrong
  stringBuilder.clear(); // Wrong
  stringBuilder += "chunk";      // Ok
  stringBuilder = "";      // Ok

std::set const iterators
------------------------

The ``find()`` member function of a ``const`` ``std::set`` instance may not be
used in a comparison with the iterator returned by ``end()``:

.. code-block:: c++

  const std::set<std::string>& someSet = getSet();
  if (someSet.find("needle") == someSet.end()) // Wrong
    {
    // ...
    }

The return value of ``find()`` must be assigned to an intermediate
``const_iterator`` for comparison:

.. code-block:: c++

  const std::set<std::string>& someSet;
  const std::set<std::string>::const_iterator i = someSet.find("needle");
  if (i != propSet.end()) // Ok
    {
    // ...
    }

Char Array to ``string`` Conversions with Algorithms
----------------------------------------------------

In some implementations, algorithms operating on iterators to a container of
``std::string`` can not accept a ``const char*`` value:

.. code-block:: c++

  const char* dir = /*...*/;
  std::vector<std::string> vec;
  // ...
  std::binary_search(vec.begin(), vec.end(), dir); // Wrong

The ``std::string`` may need to be explicitly constructed:

.. code-block:: c++

  const char* dir = /*...*/;
  std::vector<std::string> vec;
  // ...
  std::binary_search(vec.begin(), vec.end(), std::string(dir)); // Ok

std::auto_ptr
-------------

Some implementations have a ``std::auto_ptr`` which can not be used as a
return value from a function. ``std::auto_ptr`` may not be used. Use
``cmsys::auto_ptr`` instead.

std::vector::insert and std::set
--------------------------------

Use of ``std::vector::insert`` with an iterator whose ``element_type`` requires
conversion is not allowed:

.. code-block:: c++

  std::set<const char*> theSet;
  std::vector<std::string> theVector;
  theVector.insert(theVector.end(), theSet.begin(), theSet.end()); // Wrong

A loop must be used instead:

.. code-block:: c++

  std::set<const char*> theSet;
  std::vector<std::string> theVector;
  for(std::set<const char*>::iterator li = theSet.begin();
      li != theSet.end(); ++li)
    {
    theVector.push_back(*li);
    }

std::set::insert
----------------

Use of ``std::set::insert`` is not allowed with any source container:

.. code-block:: c++

  std::set<cmTarget*> theSet;
  theSet.insert(targets.begin(), targets.end()); // Wrong

A loop must be used instead:

.. code-block:: c++

  ConstIterator it = targets.begin();
  const ConstIterator end = targets.end();
  for ( ; it != end; ++it)
    {
    theSet.insert(*it);
    }

.. MSVC6, SunCC 5.9

Template Parameter Defaults
---------------------------

On ancient compilers, C++ template must use template parameters in function
arguments.  If no parameter of that type is needed, the common workaround is
to add a defaulted pointer to the type to the templated function. However,
this does not work with other ancient compilers:

.. code-block:: c++

  template<typename PropertyType>
  PropertyType getTypedProperty(cmTarget* tgt, const char* prop,
                                PropertyType* = 0) // Wrong
    {

    }

.. code-block:: c++

  template<typename PropertyType>
  PropertyType getTypedProperty(cmTarget* tgt, const char* prop,
                                PropertyType*) // Ok
    {

    }

and invoke it with the value ``0`` explicitly in all cases.

std::min and std::max
---------------------

``min`` and ``max`` are defined as macros on some systems. ``std::min`` and
``std::max`` may not be used.  Use ``cmMinimum`` and ``cmMaximum`` instead.

size_t
------

Various implementations have differing implementation of ``size_t``.  When
assigning the result of ``.size()`` on a container for example, the result
should not be assigned to an ``unsigned int`` or similar. ``std::size_t`` must
not be used.

Templates
---------

Some template code is permitted, but with some limitations. Member templates
may not be used, and template friends may not be used.

Help
====

The ``Help`` directory contains CMake help manual source files.
They are written using the `reStructuredText`_ markup syntax and
processed by `Sphinx`_ to generate the CMake help manuals.

.. _`reStructuredText`: http://docutils.sourceforge.net/docs/ref/rst/introduction.html
.. _`Sphinx`: http://sphinx-doc.org

Markup Constructs
-----------------

In addition to using Sphinx to generate the CMake help manuals, we
also use a C++-implemented document processor to print documents for
the ``--help-*`` command-line help options.  It supports a subset of
reStructuredText markup.  When authoring or modifying documents,
please verify that the command-line help looks good in addition to the
Sphinx-generated html and man pages.

The command-line help processor supports the following constructs
defined by reStructuredText, Sphinx, and a CMake extension to Sphinx.

..
 Note: This list must be kept consistent with the cmRST implementation.

CMake Domain directives
 Directives defined in the `CMake Domain`_ for defining CMake
 documentation objects are printed in command-line help output as
 if the lines were normal paragraph text with interpretation.

CMake Domain interpreted text roles
 Interpreted text roles defined in the `CMake Domain`_ for
 cross-referencing CMake documentation objects are replaced by their
 link text in command-line help output.  Other roles are printed
 literally and not processed.

``code-block`` directive
 Add a literal code block without interpretation.  The command-line
 help processor prints the block content without the leading directive
 line and with common indentation replaced by one space.

``include`` directive
 Include another document source file.  The command-line help
 processor prints the included document inline with the referencing
 document.

literal block after ``::``
 A paragraph ending in ``::`` followed by a blank line treats
 the following indented block as literal text without interpretation.
 The command-line help processor prints the ``::`` literally and
 prints the block content with common indentation replaced by one
 space.

``note`` directive
 Call out a side note.  The command-line help processor prints the
 block content as if the lines were normal paragraph text with
 interpretation.

``parsed-literal`` directive
 Add a literal block with markup interpretation.  The command-line
 help processor prints the block content without the leading
 directive line and with common indentation replaced by one space.

``productionlist`` directive
 Render context-free grammar productions.  The command-line help
 processor prints the block content as if the lines were normal
 paragraph text with interpretation.

``replace`` directive
 Define a ``|substitution|`` replacement.
 The command-line help processor requires a substitution replacement
 to be defined before it is referenced.

``|substitution|`` reference
 Reference a substitution replacement previously defined by
 the ``replace`` directive.  The command-line help processor
 performs the substitution and replaces all newlines in the
 replacement text with spaces.

``toctree`` directive
 Include other document sources in the Table-of-Contents
 document tree.  The command-line help processor prints
 the referenced documents inline as part of the referencing
 document.

Inline markup constructs not listed above are printed literally in the
command-line help output.  We prefer to use inline markup constructs that
look correct in source form, so avoid use of \\-escapes in favor of inline
literals when possible.

Explicit markup blocks not matching directives listed above are removed from
command-line help output.  Do not use them, except for plain ``..`` comments
that are removed by Sphinx too.

Note that nested indentation of blocks is not recognized by the
command-line help processor.  Therefore:

* Explicit markup blocks are recognized only when not indented
  inside other blocks.

* Literal blocks after paragraphs ending in ``::`` but not
  at the top indentation level may consume all indented lines
  following them.

Try to avoid these cases in practice.

CMake Domain
------------

CMake adds a `Sphinx Domain`_ called ``cmake``, also called the
"CMake Domain".  It defines several "object" types for CMake
documentation:

``command``
 A CMake language command.

``generator``
 A CMake native build system generator.
 See the :manual:`cmake(1)` command-line tool's ``-G`` option.

``manual``
 A CMake manual page, like this :manual:`cmake-developer(7)` manual.

``module``
 A CMake module.
 See the :manual:`cmake-modules(7)` manual
 and the :command:`include` command.

``policy``
 A CMake policy.
 See the :manual:`cmake-policies(7)` manual
 and the :command:`cmake_policy` command.

``prop_cache, prop_dir, prop_gbl, prop_sf, prop_test, prop_tgt``
 A CMake cache, directory, global, source file, test, or target
 property, respectively.  See the :manual:`cmake-properties(7)` manual
 and the :command:`set_property` command.

``variable``
 A CMake language variable.
 See the :manual:`cmake-variables(7)` manual
 and the :command:`set` command.

Documentation objects in the CMake Domain come from two sources.
First, the CMake extension to Sphinx transforms every document named
with the form ``Help/<type>/<file-name>.rst`` to a domain object with
type ``<type>``.  The object name is extracted from the document title,
which is expected to be of the form::

 <object-name>
 -------------

and to appear at or near the top of the ``.rst`` file before any other
lines starting in a letter, digit, or ``<``.  If no such title appears
literally in the ``.rst`` file, the object name is the ``<file-name>``.
If a title does appear, it is expected that ``<file-name>`` is equal
to ``<object-name>`` with any ``<`` and ``>`` characters removed.

Second, the CMake Domain provides directives to define objects inside
other documents:

.. code-block:: rst

 .. command:: <command-name>

  This indented block documents <command-name>.

 .. variable:: <variable-name>

  This indented block documents <variable-name>.

Object types for which no directive is available must be defined using
the first approach above.

.. _`Sphinx Domain`: http://sphinx-doc.org/domains.html

Cross-References
----------------

Sphinx uses reStructuredText interpreted text roles to provide
cross-reference syntax.  The `CMake Domain`_ provides for each
domain object type a role of the same name to cross-reference it.
CMake Domain roles are inline markup of the forms::

 :type:`name`
 :type:`text <name>`

where ``type`` is the domain object type and ``name`` is the
domain object name.  In the first form the link text will be
``name`` (or ``name()`` if the type is ``command``) and in
the second form the link text will be the explicit ``text``.
For example, the code:

.. code-block:: rst

 * The :command:`list` command.
 * The :command:`list(APPEND)` sub-command.
 * The :command:`list() command <list>`.
 * The :command:`list(APPEND) sub-command <list>`.
 * The :variable:`CMAKE_VERSION` variable.
 * The :prop_tgt:`OUTPUT_NAME_<CONFIG>` target property.

produces:

* The :command:`list` command.
* The :command:`list(APPEND)` sub-command.
* The :command:`list() command <list>`.
* The :command:`list(APPEND) sub-command <list>`.
* The :variable:`CMAKE_VERSION` variable.
* The :prop_tgt:`OUTPUT_NAME_<CONFIG>` target property.

Note that CMake Domain roles differ from Sphinx and reStructuredText
convention in that the form ``a<b>``, without a space preceding ``<``,
is interpreted as a name instead of link text with an explicit target.
This is necessary because we use ``<placeholders>`` frequently in
object names like ``OUTPUT_NAME_<CONFIG>``.  The form ``a <b>``,
with a space preceding ``<``, is still interpreted as a link text
with an explicit target.

Style
-----

1)
  Command signatures should be marked up as plain literal blocks, not as
  cmake ``code-blocks``.

2)
  Signatures are separated from preceding content by a horizontal
  line. That is, use:

  .. code-block:: rst

    ... preceding paragraph.

    ---------------------------------------------------------------------

    ::

      add_library(<lib> ...)

    This signature is used for ...

3)
  Use "``OFF``" and "``ON``" for boolean values which can be modified by
  the user, such as :prop_tgt:`POSITION_INDEPENDENT_CODE`. Such properties
  may be "enabled" and "disabled". Use "``True``" and "``False``" for
  inherent values which can't be modified after being set, such as the
  :prop_tgt:`IMPORTED` property of a build target.

4)
  Use two spaces for indentation.  Use two spaces between sentences in
  prose.

5)
  Prefer to mark the start of literal blocks with ``::`` at the end of
  the preceding paragraph. In cases where the following block gets
  a ``code-block`` marker, put a single ``:`` at the end of the preceding
  paragraph.

6)
  Prefer to restrict the width of lines to 75-80 columns.  This is not a
  hard restriction, but writing new paragraphs wrapped at 75 columns
  allows space for adding minor content without significant re-wrapping of
  content.

7)
  Mark up self-references with  ``inline-literal`` syntax. For example,
  within the add_executable command documentation, use

  .. code-block:: rst

    ``add_executable``

  not

  .. code-block:: rst

    :command:`add_executable`

  which is used elsewhere.

8)
  Mark up all other linkable references as links, including repeats. An
  alternative, which is used by wikipedia (`<http://en.wikipedia.org/wiki/WP:REPEATLINK>`_),
  is to link to a reference only once per article. That style is not used
  in CMake documentation.

9)
  Mark up references to keywords in signatures, file names, and other
  technical terms with ``inline-literl`` syntax, for example:

  .. code-block:: rst

    If ``WIN32`` is used with :command:`add_executable`, the
    :prop_tgt:`WIN32_EXECUTABLE` target property is enabled. That command
    creates the file ``<name>.exe`` on Windows.


10)
  If referring to a concept which corresponds to a property, and that
  concept is described in a high-level manual, prefer to link to the
  manual section instead of the property. For example:

  .. code-block:: rst

    This command creates an :ref:`Imported Target <Imported Targets>`.

  instead of:

  .. code-block:: rst

    This command creates an :prop_tgt:`IMPORTED` target.

  The latter should be used only when referring specifically to the
  property.

  References to manual sections are not automatically created by creating
  a section, but code such as:

  .. code-block:: rst

    .. _`Imported Targets`:

  creates a suitable anchor.  Use an anchor name which matches the name
  of the corresponding section.  Refer to the anchor using a
  cross-reference with specified text.

  Imported Targets need the ``IMPORTED`` term marked up with care in
  particular because the term may refer to a command keyword
  (``IMPORTED``), a target property (:prop_tgt:`IMPORTED`), or a
  concept (:ref:`Imported Targets`).

11)
  Where a property, command or variable is related conceptually to others,
  by for example, being related to the buildsystem description, generator
  expressions or Qt, each relevant property, command or variable should
  link to the primary manual, which provides high-level information.  Only
  particular information relating to the command should be in the
  documentation of the command.

12)
  When marking section titles, make the section decoration line as long as
  the title text.  Use only a line below the title, not above. For
  example:

  .. code-block:: rst

    Title Text
    ----------

  Capitalize the first letter of each non-minor word in the title.

13)
  When referring to properties, variables, commands etc, prefer to link
  to the target object and follow that with the type of object it is.
  For example:

  .. code-block:: rst

    Set the :prop_tgt:`AUTOMOC` target property to ``ON``.

  Instead of

  .. code-block:: rst

    Set the target property :prop_tgt:`AUTOMOC` to ``ON``.

  The ``policy`` directive is an exception, and the type us usually
  referred to before the link:

  .. code-block:: rst

    If policy :prop_tgt:`CMP0022` is set to ``NEW`` the behavior is ...

14)
  Signatures of commands should wrap optional parts with square brackets,
  and should mark list of optional arguments with an ellipsis (``...``).
  Elements of the signature which are specified by the user should be
  specified with angle brackets, and may be referred to in prose using
  ``inline-literal`` syntax.

15)
  Use American English spellings in prose.


Modules
=======

The ``Modules`` directory contains CMake-language ``.cmake`` module files.

Module Documentation
--------------------

To document CMake module ``Modules/<module-name>.cmake``, modify
``Help/manual/cmake-modules.7.rst`` to reference the module in the
``toctree`` directive, in sorted order, as::

 /module/<module-name>

Then add the module document file ``Help/module/<module-name>.rst``
containing just the line::

 .. cmake-module:: ../../Modules/<module-name>.cmake

The ``cmake-module`` directive will scan the module file to extract
reStructuredText markup from comment blocks that start in ``.rst:``.
Add to the top of ``Modules/<module-name>.cmake`` a
:ref:`Line Comment` block of the form:

.. code-block:: cmake

 #.rst:
 # <module-name>
 # -------------
 #
 # <reStructuredText documentation of module>

or a :ref:`Bracket Comment` of the form:

.. code-block:: cmake

 #[[.rst:
 <module-name>
 -------------

 <reStructuredText documentation of module>
 #]]

Any number of ``=`` may be used in the opening and closing brackets
as long as they match.  Content on the line containing the closing
bracket is excluded if and only if the line starts in ``#``.

Additional such ``.rst:`` comments may appear anywhere in the module file.
All such comments must start with ``#`` in the first column.

For example, a ``Modules/Findxxx.cmake`` module may contain:

.. code-block:: cmake

 #.rst:
 # FindXxx
 # -------
 #
 # This is a cool module.
 # This module does really cool stuff.
 # It can do even more than you think.
 #
 # It even needs two paragraphs to tell you about it.
 # And it defines the following variables:
 #
 # * VAR_COOL: this is great isn't it?
 # * VAR_REALLY_COOL: cool right?

 <code>

 #[========================================[.rst:
 .. command:: xxx_do_something

  This command does something for Xxx::

   xxx_do_something(some arguments)
 #]========================================]
 macro(xxx_do_something)
   <code>
 endmacro()

Find Modules
------------

A "find module" is a ``Modules/Find<package>.cmake`` file to be loaded
by the :command:`find_package` command when invoked for ``<package>``.

We would like all ``FindXxx.cmake`` files to produce consistent variable
names.  Please use the following consistent variable names for general use.

Xxx_INCLUDE_DIRS
 The final set of include directories listed in one variable for use by client
 code.  This should not be a cache entry.

Xxx_LIBRARIES
 The libraries to link against to use Xxx. These should include full paths.
 This should not be a cache entry.

Xxx_DEFINITIONS
 Definitions to use when compiling code that uses Xxx. This really shouldn't
 include options such as (-DHAS_JPEG)that a client source-code file uses to
 decide whether to #include <jpeg.h>

Xxx_EXECUTABLE
 Where to find the Xxx tool.

Xxx_Yyy_EXECUTABLE
 Where to find the Yyy tool that comes with Xxx.

Xxx_LIBRARY_DIRS
 Optionally, the final set of library directories listed in one variable for
 use by client code.  This should not be a cache entry.

Xxx_ROOT_DIR
 Where to find the base directory of Xxx.

Xxx_VERSION_Yy
 Expect Version Yy if true. Make sure at most one of these is ever true.

Xxx_WRAP_Yy
 If False, do not try to use the relevant CMake wrapping command.

Xxx_Yy_FOUND
 If False, optional Yy part of Xxx sytem is not available.

Xxx_FOUND
 Set to false, or undefined, if we haven't found, or don't want to use Xxx.

Xxx_NOT_FOUND_MESSAGE
 Should be set by config-files in the case that it has set Xxx_FOUND to FALSE.
 The contained message will be printed by the find_package() command and by
 find_package_handle_standard_args() to inform the user about the problem.

Xxx_RUNTIME_LIBRARY_DIRS
 Optionally, the runtime library search path for use when running an
 executable linked to shared libraries.  The list should be used by user code
 to create the PATH on windows or LD_LIBRARY_PATH on unix.  This should not be
 a cache entry.

Xxx_VERSION_STRING
 A human-readable string containing the version of the package found, if any.

Xxx_VERSION_MAJOR
 The major version of the package found, if any.

Xxx_VERSION_MINOR
 The minor version of the package found, if any.

Xxx_VERSION_PATCH
 The patch version of the package found, if any.

You do not have to provide all of the above variables. You should provide
Xxx_FOUND under most circumstances.  If Xxx is a library, then Xxx_LIBRARIES,
should also be defined, and Xxx_INCLUDE_DIRS should usually be defined (I
guess libm.a might be an exception)

The following names should not usually be used in CMakeLists.txt files, but
they may be usefully modified in users' CMake Caches to control stuff.

Xxx_LIBRARY
 Name of Xxx Library. A User may set this and Xxx_INCLUDE_DIR to ignore to
 force non-use of Xxx.

Xxx_Yy_LIBRARY
 Name of Yy library that is part of the Xxx system. It may or may not be
 required to use Xxx.

Xxx_INCLUDE_DIR
 Where to find xxx.h, etc.  (Xxx_INCLUDE_PATH was considered bad because a path
 includes an actual filename.)

Xxx_Yy_INCLUDE_DIR
 Where to find xxx_yy.h, etc.

For tidiness's sake, try to keep as many options as possible out of the cache,
leaving at least one option which can be used to disable use of the module, or
locate a not-found library (e.g. Xxx_ROOT_DIR).  For the same reason, mark
most cache options as advanced.

If you need other commands to do special things then it should still begin
with ``Xxx_``. This gives a sort of namespace effect and keeps things tidy for the
user. You should put comments describing all the exported settings, plus
descriptions of any the users can use to control stuff.

You really should also provide backwards compatibility any old settings that
were actually in use.  Make sure you comment them as deprecated, so that
no-one starts using them.

To add a module to the CMake documentation, follow the steps in the
`Module Documentation`_ section above.  Test the documentation formatting
by running ``cmake --help-module FindXxx``, and also by enabling the
``SPHINX_HTML`` and ``SPHINX_MAN`` options to build the documentation.
Edit the comments until generated documentation looks satisfactory.
To have a .cmake file in this directory NOT show up in the modules
documentation, simply leave out the ``Help/module/<module-name>.rst`` file
and the ``Help/manual/cmake-modules.7.rst`` toctree entry.

After the documentation, leave a *BLANK* line, and then add a
copyright and licence notice block like this one::

 #=============================================================================
 # Copyright 2009-2011 Your Name
 #
 # Distributed under the OSI-approved BSD License (the "License");
 # see accompanying file Copyright.txt for details.
 #
 # This software is distributed WITHOUT ANY WARRANTY; without even the
 # implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 # See the License for more information.
 #=============================================================================
 # (To distribute this file outside of CMake, substitute the full
 #  License text for the above reference.)

The layout of the notice block is strictly enforced by the ``ModuleNotices``
test.  Only the year range and name may be changed freely.

A FindXxx.cmake module will typically be loaded by the command::

 FIND_PACKAGE(Xxx [major[.minor[.patch[.tweak]]]] [EXACT]
              [QUIET] [[REQUIRED|COMPONENTS] [components...]])

If any version numbers are given to the command it will set the following
variables before loading the module:

Xxx_FIND_VERSION
 full requested version string

Xxx_FIND_VERSION_MAJOR
 major version if requested, else 0

Xxx_FIND_VERSION_MINOR
 minor version if requested, else 0

Xxx_FIND_VERSION_PATCH
 patch version if requested, else 0

Xxx_FIND_VERSION_TWEAK
 tweak version if requested, else 0

Xxx_FIND_VERSION_COUNT
 number of version components, 0 to 4

Xxx_FIND_VERSION_EXACT
 true if EXACT option was given

If the find module supports versioning it should locate a version of
the package that is compatible with the version requested.  If a
compatible version of the package cannot be found the module should
not report success.  The version of the package found should be stored
in "Xxx_VERSION..." version variables documented by the module.

If the QUIET option is given to the command it will set the variable
Xxx_FIND_QUIETLY to true before loading the FindXxx.cmake module.  If
this variable is set the module should not complain about not being
able to find the package.  If the
REQUIRED option is given to the command it will set the variable
Xxx_FIND_REQUIRED to true before loading the FindXxx.cmake module.  If
this variable is set the module should issue a FATAL_ERROR if the
package cannot be found.
If neither the QUIET nor REQUIRED options are given then the
FindXxx.cmake module should look for the package and complain without
error if the module is not found.

FIND_PACKAGE() will set the variable CMAKE_FIND_PACKAGE_NAME to
contain the actual name of the package.

A package can provide sub-components.
Those components can be listed after the COMPONENTS (or REQUIRED) or
OPTIONAL_COMPONENTS keywords.  The set of all listed components will be
specified in a Xxx_FIND_COMPONENTS variable.
For each package-specific component, say Yyy, a variable Xxx_FIND_REQUIRED_Yyy
will be set to true if it listed after COMPONENTS and it will be set to false
if it was listed after OPTIONAL_COMPONENTS.
Using those variables a FindXxx.cmake module and also a XxxConfig.cmake
package configuration file can determine whether and which components have
been requested, and whether they were requested as required or as optional.
For each of the requested components a Xxx_Yyy_FOUND variable should be set
accordingly.
The per-package Xxx_FOUND variable should be only set to true if all requested
required components have been found. A missing optional component should not
keep the Xxx_FOUND variable from being set to true.
If the package provides Xxx_INCLUDE_DIRS and Xxx_LIBRARIES variables, the
include dirs and libraries for all components which were requested and which
have been found should be added to those two variables.

To get this behavior you can use the FIND_PACKAGE_HANDLE_STANDARD_ARGS()
macro, as an example see FindJPEG.cmake.

For internal implementation, it's a generally accepted convention that
variables starting with underscore are for temporary use only. (variable
starting with an underscore are not intended as a reserved prefix).