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Sean Silva3872b462012-12-12 23:44:55 +00001======================
2Matching the Clang AST
3======================
4
5This document explains how to use Clang's LibASTMatchers to match interesting
6nodes of the AST and execute code that uses the matched nodes. Combined with
7:doc:`LibTooling`, LibASTMatchers helps to write code-to-code transformation
8tools or query tools.
9
Sean Silva159cc9e2013-01-02 13:07:47 +000010We assume basic knowledge about the Clang AST. See the :doc:`Introduction
11to the Clang AST <IntroductionToTheClangAST>` if you want to learn more
12about how the AST is structured.
Sean Silva3872b462012-12-12 23:44:55 +000013
14.. FIXME: create tutorial and link to the tutorial
15
16Introduction
17------------
18
19LibASTMatchers provides a domain specific language to create predicates on
20Clang's AST. This DSL is written in and can be used from C++, allowing users
21to write a single program to both match AST nodes and access the node's C++
22interface to extract attributes, source locations, or any other information
23provided on the AST level.
24
25AST matchers are predicates on nodes in the AST. Matchers are created by
26calling creator functions that allow building up a tree of matchers, where
27inner matchers are used to make the match more specific.
28
29For example, to create a matcher that matches all class or union declarations
30in the AST of a translation unit, you can call `recordDecl()
31<LibASTMatchersReference.html#recordDecl0Anchor>`_. To narrow the match down,
32for example to find all class or union declarations with the name "``Foo``",
33insert a `hasName <LibASTMatchersReference.html#hasName0Anchor>`_ matcher: the
34call ``recordDecl(hasName("Foo"))`` returns a matcher that matches classes or
35unions that are named "``Foo``", in any namespace. By default, matchers that
36accept multiple inner matchers use an implicit `allOf()
37<LibASTMatchersReference.html#allOf0Anchor>`_. This allows further narrowing
38down the match, for example to match all classes that are derived from
39"``Bar``": ``recordDecl(hasName("Foo"), isDerivedFrom("Bar"))``.
40
41How to create a matcher
42-----------------------
43
44With more than a thousand classes in the Clang AST, one can quickly get lost
45when trying to figure out how to create a matcher for a specific pattern. This
46section will teach you how to use a rigorous step-by-step pattern to build the
47matcher you are interested in. Note that there will always be matchers missing
48for some part of the AST. See the section about :ref:`how to write your own
49AST matchers <astmatchers-writing>` later in this document.
50
51.. FIXME: why is it linking back to the same section?!
52
53The precondition to using the matchers is to understand how the AST for what you
54want to match looks like. The
Sean Silva159cc9e2013-01-02 13:07:47 +000055:doc:`Introduction to the Clang AST <IntroductionToTheClangAST>` teaches you
Sean Silva3872b462012-12-12 23:44:55 +000056how to dump a translation unit's AST into a human readable format.
57
58.. FIXME: Introduce link to ASTMatchersTutorial.html
59.. FIXME: Introduce link to ASTMatchersCookbook.html
60
61In general, the strategy to create the right matchers is:
62
63#. Find the outermost class in Clang's AST you want to match.
64#. Look at the `AST Matcher Reference <LibASTMatchersReference.html>`_ for
65 matchers that either match the node you're interested in or narrow down
66 attributes on the node.
67#. Create your outer match expression. Verify that it works as expected.
68#. Examine the matchers for what the next inner node you want to match is.
69#. Repeat until the matcher is finished.
70
71.. _astmatchers-bind:
72
73Binding nodes in match expressions
74----------------------------------
75
76Matcher expressions allow you to specify which parts of the AST are interesting
77for a certain task. Often you will want to then do something with the nodes
78that were matched, like building source code transformations.
79
80To that end, matchers that match specific AST nodes (so called node matchers)
81are bindable; for example, ``recordDecl(hasName("MyClass")).bind("id")`` will
82bind the matched ``recordDecl`` node to the string "``id``", to be later
83retrieved in the `match callback
84<http://clang.llvm.org/doxygen/classclang_1_1ast__matchers_1_1MatchFinder_1_1MatchCallback.html>`_.
85
86.. FIXME: Introduce link to ASTMatchersTutorial.html
87.. FIXME: Introduce link to ASTMatchersCookbook.html
88
89Writing your own matchers
90-------------------------
91
92There are multiple different ways to define a matcher, depending on its type
93and flexibility.
94
95``VariadicDynCastAllOfMatcher<Base, Derived>``
96^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
97
98Those match all nodes of type *Base* if they can be dynamically casted to
99*Derived*. The names of those matchers are nouns, which closely resemble
100*Derived*. ``VariadicDynCastAllOfMatchers`` are the backbone of the matcher
101hierarchy. Most often, your match expression will start with one of them, and
102you can :ref:`bind <astmatchers-bind>` the node they represent to ids for later
103processing.
104
105``VariadicDynCastAllOfMatchers`` are callable classes that model variadic
106template functions in C++03. They take an aribtrary number of
107``Matcher<Derived>`` and return a ``Matcher<Base>``.
108
109``AST_MATCHER_P(Type, Name, ParamType, Param)``
110^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
111
112Most matcher definitions use the matcher creation macros. Those define both
113the matcher of type ``Matcher<Type>`` itself, and a matcher-creation function
114named *Name* that takes a parameter of type *ParamType* and returns the
115corresponding matcher.
116
117There are multiple matcher definition macros that deal with polymorphic return
118values and different parameter counts. See `ASTMatchersMacros.h
119<http://clang.llvm.org/doxygen/ASTMatchersMacros_8h.html>`_.
120
121.. _astmatchers-writing:
122
123Matcher creation functions
124^^^^^^^^^^^^^^^^^^^^^^^^^^
125
126Matchers are generated by nesting calls to matcher creation functions. Most of
127the time those functions are either created by using
128``VariadicDynCastAllOfMatcher`` or the matcher creation macros (see below).
129The free-standing functions are an indication that this matcher is just a
130combination of other matchers, as is for example the case with `callee
131<LibASTMatchersReference.html#callee1Anchor>`_.
132
133.. FIXME: "... macros (see below)" --- there isn't anything below
134