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gerrit-public.fairphone.software / platform / external / clang / 278057fd9f44684af832695cb01676c02a257b14 / . / docs / LibASTMatchers.html
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15<h1>Matching the Clang AST</h1>
16<p>This document explains how to use Clang's LibASTMatchers to match interesting
17nodes of the AST and execute code that uses the matched nodes. Combined with
18<a href="LibTooling.html">LibTooling</a>, LibASTMatchers helps to write
19code-to-code transformation tools or query tools.</p>
20
21<p>We assume basic knowledge about the Clang AST. See the
22<a href="IntroductionToTheClangAST.html">Introduction to the Clang AST</a> if
23you want to learn more about how the AST is structured.</p>
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28<h2 id="intro">Introduction</h2>
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31<p>LibASTMatchers provides a domain specific language to create predicates on Clang's
32AST. This DSL is written in and can be used from C++, allowing users to write
33a single program to both match AST nodes and access the node's C++ interface
34to extract attributes, source locations, or any other information provided on
35the AST level.</p>
36
37<p>AST matchers are predicates on nodes in the AST. Matchers are created
38by calling creator functions that allow building up a tree of matchers, where
39inner matchers are used to make the match more specific.</p>
40
41</p>For example, to create a matcher that matches all class or union declarations
42in the AST of a translation unit, you can call
43<a href="LibASTMatchersReference.html#recordDecl0Anchor">recordDecl()</a>.
44To narrow the match down, for example to find all class or union declarations with the name "Foo",
45insert a <a href="LibASTMatchersReference.html#hasName0Anchor">hasName</a>
46matcher: the call recordDecl(hasName("Foo")) returns a matcher that matches classes
47or unions that are named "Foo", in any namespace. By default, matchers that accept
48multiple inner matchers use an implicit <a href="LibASTMatchersReference.html#allOf0Anchor">allOf()</a>.
49This allows further narrowing down the match, for example to match all classes
50that are derived from "Bar": recordDecl(hasName("Foo"), isDerivedFrom("Bar")).</p>
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53<h2 id="writing">How to create a matcher</h2>
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56<p>With more than a thousand classes in the Clang AST, one can quickly get lost
57when trying to figure out how to create a matcher for a specific pattern. This
58section will teach you how to use a rigorous step-by-step pattern to build the
59matcher you are interested in. Note that there will always be matchers missing
60for some part of the AST. See the section about <a href="#writing">how to write
61your own AST matchers</a> later in this document.</p>
62
63<p>The precondition to using the matchers is to understand how the AST
64for what you want to match looks like. The <a href="IntroductionToTheClangAST.html">Introduction to the Clang AST</a>
65teaches you how to dump a translation unit's AST into a human readable format.</p>
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69
70<p>In general, the strategy to create the right matchers is:</p>
71<ol>
72<li>Find the outermost class in Clang's AST you want to match.</li>
73<li>Look at the <a href="LibASTMatchersReference.html">AST Matcher Reference</a> for matchers that either match the
74node you're interested in or narrow down attributes on the node.</li>
75<li>Create your outer match expression. Verify that it works as expected.</li>
76<li>Examine the matchers for what the next inner node you want to match is.</li>
77<li>Repeat until the matcher is finished.</li>
78</ol>
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81<h2 id="binding">Binding nodes in match expressions</h2>
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83
84<p>Matcher expressions allow you to specify which parts of the AST are interesting
85for a certain task. Often you will want to then do something with the nodes
86that were matched, like building source code transformations.</p>
87
88<p>To that end, matchers that match specific AST nodes (so called node matchers)
89are bindable; for example, recordDecl(hasName("MyClass")).bind("id") will bind
90the matched recordDecl node to the string "id", to be later retrieved in the
91<a href="http://clang.llvm.org/doxygen/classclang_1_1ast__matchers_1_1MatchFinder_1_1MatchCallback.html">match callback</a>.</p>
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97<h2 id="writing">Writing your own matchers</h2>
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99
100<p>There are multiple different ways to define a matcher, depending on its
101type and flexibility.</p>
102<ul>
103<li><b>VariadicDynCastAllOfMatcher&ltBase, Derived></b><p>Those match all nodes
104of type <i>Base</i> if they can be dynamically casted to <i>Derived</i>. The
105names of those matchers are nouns, which closely resemble <i>Derived</i>.
106VariadicDynCastAllOfMatchers are the backbone of the matcher hierarchy. Most
107often, your match expression will start with one of them, and you can
108<a href="#binding">bind</a> the node they represent to ids for later processing.</p>
109<p>VariadicDynCastAllOfMatchers are callable classes that model variadic
110template functions in C++03. They take an aribtrary number of Matcher&lt;Derived>
111and return a Matcher&lt;Base>.</p></li>
112<li><b>AST_MATCHER_P(Type, Name, ParamType, Param)</b><p> Most matcher definitions
113use the matcher creation macros. Those define both the matcher of type Matcher&lt;Type>
114itself, and a matcher-creation function named <i>Name</i> that takes a parameter
115of type <i>ParamType</i> and returns the corresponding matcher.</p>
116<p>There are multiple matcher definition macros that deal with polymorphic return
117values and different parameter counts. See <a href="http://clang.llvm.org/doxygen/ASTMatchersMacros_8h.html">ASTMatchersMacros.h</a>.
118</p></li>
119<li><b>Matcher creation functions</b><p>Matchers are generated by nesting
120calls to matcher creation functions. Most of the time those functions are either
121created by using VariadicDynCastAllOfMatcher or the matcher creation macros
122(see below). The free-standing functions are an indication that this matcher
123is just a combination of other matchers, as is for example the case with
124<a href="LibASTMatchersReference.html#callee1Anchor">callee</a>.</p></li>
125</ul>
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