forked from OSchip/llvm-project
135 lines
5.9 KiB
ReStructuredText
135 lines
5.9 KiB
ReStructuredText
======================
|
|
Matching the Clang AST
|
|
======================
|
|
|
|
This document explains how to use Clang's LibASTMatchers to match interesting
|
|
nodes of the AST and execute code that uses the matched nodes. Combined with
|
|
:doc:`LibTooling`, LibASTMatchers helps to write code-to-code transformation
|
|
tools or query tools.
|
|
|
|
We assume basic knowledge about the Clang AST. See the :doc:`Introduction
|
|
to the Clang AST <IntroductionToTheClangAST>` if you want to learn more
|
|
about how the AST is structured.
|
|
|
|
.. FIXME: create tutorial and link to the tutorial
|
|
|
|
Introduction
|
|
------------
|
|
|
|
LibASTMatchers provides a domain specific language to create predicates on
|
|
Clang's AST. This DSL is written in and can be used from C++, allowing users
|
|
to write a single program to both match AST nodes and access the node's C++
|
|
interface to extract attributes, source locations, or any other information
|
|
provided on the AST level.
|
|
|
|
AST matchers are predicates on nodes in the AST. Matchers are created by
|
|
calling creator functions that allow building up a tree of matchers, where
|
|
inner matchers are used to make the match more specific.
|
|
|
|
For example, to create a matcher that matches all class or union declarations
|
|
in the AST of a translation unit, you can call `recordDecl()
|
|
<LibASTMatchersReference.html#recordDecl0Anchor>`_. To narrow the match down,
|
|
for example to find all class or union declarations with the name "``Foo``",
|
|
insert a `hasName <LibASTMatchersReference.html#hasName0Anchor>`_ matcher: the
|
|
call ``recordDecl(hasName("Foo"))`` returns a matcher that matches classes or
|
|
unions that are named "``Foo``", in any namespace. By default, matchers that
|
|
accept multiple inner matchers use an implicit `allOf()
|
|
<LibASTMatchersReference.html#allOf0Anchor>`_. This allows further narrowing
|
|
down the match, for example to match all classes that are derived from
|
|
"``Bar``": ``recordDecl(hasName("Foo"), isDerivedFrom("Bar"))``.
|
|
|
|
How to create a matcher
|
|
-----------------------
|
|
|
|
With more than a thousand classes in the Clang AST, one can quickly get lost
|
|
when trying to figure out how to create a matcher for a specific pattern. This
|
|
section will teach you how to use a rigorous step-by-step pattern to build the
|
|
matcher you are interested in. Note that there will always be matchers missing
|
|
for some part of the AST. See the section about :ref:`how to write your own
|
|
AST matchers <astmatchers-writing>` later in this document.
|
|
|
|
.. FIXME: why is it linking back to the same section?!
|
|
|
|
The precondition to using the matchers is to understand how the AST for what you
|
|
want to match looks like. The
|
|
:doc:`Introduction to the Clang AST <IntroductionToTheClangAST>` teaches you
|
|
how to dump a translation unit's AST into a human readable format.
|
|
|
|
.. FIXME: Introduce link to ASTMatchersTutorial.html
|
|
.. FIXME: Introduce link to ASTMatchersCookbook.html
|
|
|
|
In general, the strategy to create the right matchers is:
|
|
|
|
#. Find the outermost class in Clang's AST you want to match.
|
|
#. Look at the `AST Matcher Reference <LibASTMatchersReference.html>`_ for
|
|
matchers that either match the node you're interested in or narrow down
|
|
attributes on the node.
|
|
#. Create your outer match expression. Verify that it works as expected.
|
|
#. Examine the matchers for what the next inner node you want to match is.
|
|
#. Repeat until the matcher is finished.
|
|
|
|
.. _astmatchers-bind:
|
|
|
|
Binding nodes in match expressions
|
|
----------------------------------
|
|
|
|
Matcher expressions allow you to specify which parts of the AST are interesting
|
|
for a certain task. Often you will want to then do something with the nodes
|
|
that were matched, like building source code transformations.
|
|
|
|
To that end, matchers that match specific AST nodes (so called node matchers)
|
|
are bindable; for example, ``recordDecl(hasName("MyClass")).bind("id")`` will
|
|
bind the matched ``recordDecl`` node to the string "``id``", to be later
|
|
retrieved in the `match callback
|
|
<http://clang.llvm.org/doxygen/classclang_1_1ast__matchers_1_1MatchFinder_1_1MatchCallback.html>`_.
|
|
|
|
.. FIXME: Introduce link to ASTMatchersTutorial.html
|
|
.. FIXME: Introduce link to ASTMatchersCookbook.html
|
|
|
|
Writing your own matchers
|
|
-------------------------
|
|
|
|
There are multiple different ways to define a matcher, depending on its type
|
|
and flexibility.
|
|
|
|
``VariadicDynCastAllOfMatcher<Base, Derived>``
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Those match all nodes of type *Base* if they can be dynamically casted to
|
|
*Derived*. The names of those matchers are nouns, which closely resemble
|
|
*Derived*. ``VariadicDynCastAllOfMatchers`` are the backbone of the matcher
|
|
hierarchy. Most often, your match expression will start with one of them, and
|
|
you can :ref:`bind <astmatchers-bind>` the node they represent to ids for later
|
|
processing.
|
|
|
|
``VariadicDynCastAllOfMatchers`` are callable classes that model variadic
|
|
template functions in C++03. They take an arbitrary number of
|
|
``Matcher<Derived>`` and return a ``Matcher<Base>``.
|
|
|
|
``AST_MATCHER_P(Type, Name, ParamType, Param)``
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Most matcher definitions use the matcher creation macros. Those define both
|
|
the matcher of type ``Matcher<Type>`` itself, and a matcher-creation function
|
|
named *Name* that takes a parameter of type *ParamType* and returns the
|
|
corresponding matcher.
|
|
|
|
There are multiple matcher definition macros that deal with polymorphic return
|
|
values and different parameter counts. See `ASTMatchersMacros.h
|
|
<http://clang.llvm.org/doxygen/ASTMatchersMacros_8h.html>`_.
|
|
|
|
.. _astmatchers-writing:
|
|
|
|
Matcher creation functions
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Matchers are generated by nesting calls to matcher creation functions. Most of
|
|
the time those functions are either created by using
|
|
``VariadicDynCastAllOfMatcher`` or the matcher creation macros (see below).
|
|
The free-standing functions are an indication that this matcher is just a
|
|
combination of other matchers, as is for example the case with `callee
|
|
<LibASTMatchersReference.html#callee1Anchor>`_.
|
|
|
|
.. FIXME: "... macros (see below)" --- there isn't anything below
|
|
|