1=============================
2Introduction to the Clang AST
3=============================
4
5This document gives a gentle introduction to the mysteries of the Clang
6AST. It is targeted at developers who either want to contribute to
7Clang, or use tools that work based on Clang's AST, like the AST
8matchers.
9
10Introduction
11============
12
13Clang's AST is different from ASTs produced by some other compilers in
14that it closely resembles both the written C++ code and the C++
15standard. For example, parenthesis expressions and compile time
16constants are available in an unreduced form in the AST. This makes
17Clang's AST a good fit for refactoring tools.
18
19Documentation for all Clang AST nodes is available via the generated
20`Doxygen <http://clang.llvm.org/doxygen>`_. The doxygen online
21documentation is also indexed by your favorite search engine, which will
22make a search for clang and the AST node's class name usually turn up
23the doxygen of the class you're looking for (for example, search for:
24clang ParenExpr).
25
26Examining the AST
27=================
28
29A good way to familarize yourself with the Clang AST is to actually look
30at it on some simple example code. Clang has a builtin AST-dump modes,
31which can be enabled with the flags ``-ast-dump`` and ``-ast-dump-xml``. Note
32that ``-ast-dump-xml`` currently only works with debug builds of clang.
33
34Let's look at a simple example AST:
35
36::
37
38    $ cat test.cc
39    int f(int x) {
40      int result = (x / 42);
41      return result;
42    }
43
44    # Clang by default is a frontend for many tools; -cc1 tells it to directly
45    # use the C++ compiler mode. -undef leaves out some internal declarations.
46    $ clang -cc1 -undef -ast-dump-xml test.cc
47    ... cutting out internal declarations of clang ...
48    <TranslationUnit ptr="0x4871160">
49     <Function ptr="0x48a5800" name="f" prototype="true">
50      <FunctionProtoType ptr="0x4871de0" canonical="0x4871de0">
51       <BuiltinType ptr="0x4871250" canonical="0x4871250"/>
52       <parameters>
53        <BuiltinType ptr="0x4871250" canonical="0x4871250"/>
54       </parameters>
55      </FunctionProtoType>
56      <ParmVar ptr="0x4871d80" name="x" initstyle="c">
57       <BuiltinType ptr="0x4871250" canonical="0x4871250"/>
58      </ParmVar>
59      <Stmt>
60    (CompoundStmt 0x48a5a38 <t2.cc:1:14, line:4:1>
61      (DeclStmt 0x48a59c0 <line:2:3, col:24>
62        0x48a58c0 "int result =
63          (ParenExpr 0x48a59a0 <col:16, col:23> 'int'
64            (BinaryOperator 0x48a5978 <col:17, col:21> 'int' '/'
65              (ImplicitCastExpr 0x48a5960 <col:17> 'int' <LValueToRValue>
66                (DeclRefExpr 0x48a5918 <col:17> 'int' lvalue ParmVar 0x4871d80 'x' 'int'))
67              (IntegerLiteral 0x48a5940 <col:21> 'int' 42)))")
68      (ReturnStmt 0x48a5a18 <line:3:3, col:10>
69        (ImplicitCastExpr 0x48a5a00 <col:10> 'int' <LValueToRValue>
70          (DeclRefExpr 0x48a59d8 <col:10> 'int' lvalue Var 0x48a58c0 'result' 'int'))))
71
72      </Stmt>
73     </Function>
74    </TranslationUnit>
75
76In general, ``-ast-dump-xml`` dumps declarations in an XML-style format and
77statements in an S-expression-style format. The toplevel declaration in
78a translation unit is always the `translation unit
79declaration <http://clang.llvm.org/doxygen/classclang_1_1TranslationUnitDecl.html>`_.
80In this example, our first user written declaration is the `function
81declaration <http://clang.llvm.org/doxygen/classclang_1_1FunctionDecl.html>`_
82of "``f``". The body of "``f``" is a `compound
83statement <http://clang.llvm.org/doxygen/classclang_1_1CompoundStmt.html>`_,
84whose child nodes are a `declaration
85statement <http://clang.llvm.org/doxygen/classclang_1_1DeclStmt.html>`_
86that declares our result variable, and the `return
87statement <http://clang.llvm.org/doxygen/classclang_1_1ReturnStmt.html>`_.
88
89AST Context
90===========
91
92All information about the AST for a translation unit is bundled up in
93the class
94`ASTContext <http://clang.llvm.org/doxygen/classclang_1_1ASTContext.html>`_.
95It allows traversal of the whole translation unit starting from
96`getTranslationUnitDecl <http://clang.llvm.org/doxygen/classclang_1_1ASTContext.html#abd909fb01ef10cfd0244832a67b1dd64>`_,
97or to access Clang's `table of
98identifiers <http://clang.llvm.org/doxygen/classclang_1_1ASTContext.html#a4f95adb9958e22fbe55212ae6482feb4>`_
99for the parsed translation unit.
100
101AST Nodes
102=========
103
104Clang's AST nodes are modeled on a class hierarchy that does not have a
105common ancestor. Instead, there are multiple larger hierarchies for
106basic node types like
107`Decl <http://clang.llvm.org/doxygen/classclang_1_1Decl.html>`_ and
108`Stmt <http://clang.llvm.org/doxygen/classclang_1_1Stmt.html>`_. Many
109important AST nodes derive from
110`Type <http://clang.llvm.org/doxygen/classclang_1_1Type.html>`_,
111`Decl <http://clang.llvm.org/doxygen/classclang_1_1Decl.html>`_,
112`DeclContext <http://clang.llvm.org/doxygen/classclang_1_1DeclContext.html>`_
113or `Stmt <http://clang.llvm.org/doxygen/classclang_1_1Stmt.html>`_, with
114some classes deriving from both Decl and DeclContext.
115
116There are also a multitude of nodes in the AST that are not part of a
117larger hierarchy, and are only reachable from specific other nodes, like
118`CXXBaseSpecifier <http://clang.llvm.org/doxygen/classclang_1_1CXXBaseSpecifier.html>`_.
119
120Thus, to traverse the full AST, one starts from the
121`TranslationUnitDecl <http://clang.llvm.org/doxygen/classclang_1_1TranslationUnitDecl.html>`_
122and then recursively traverses everything that can be reached from that
123node - this information has to be encoded for each specific node type.
124This algorithm is encoded in the
125`RecursiveASTVisitor <http://clang.llvm.org/doxygen/classclang_1_1RecursiveASTVisitor.html>`_.
126See the `RecursiveASTVisitor
127tutorial <http://clang.llvm.org/docs/RAVFrontendAction.html>`_.
128
129The two most basic nodes in the Clang AST are statements
130(`Stmt <http://clang.llvm.org/doxygen/classclang_1_1Stmt.html>`_) and
131declarations
132(`Decl <http://clang.llvm.org/doxygen/classclang_1_1Decl.html>`_). Note
133that expressions
134(`Expr <http://clang.llvm.org/doxygen/classclang_1_1Expr.html>`_) are
135also statements in Clang's AST.
136