# Introduction to the Clang AST

This document gives a gentle introduction to the mysteries of the Clang AST. It is targeted at developers who either want to contribute to Clang, or use tools that work based on Clang's AST, like the AST matchers.

## Introduction

Clang's AST is different from ASTs produced by some other compilers in that it closely resembles both the written C++ code and the C++ standard. For example, parenthesis expressions and compile time constants are available in an unreduced form in the AST. This makes Clang's AST a good fit for refactoring tools.

Documentation for all Clang AST nodes is available via the generated Doxygen. The doxygen online documentation is also indexed by your favorite search engine, which will make a search for clang and the AST node's class name usually turn up the doxygen of the class you're looking for (for example, search for: clang ParenExpr).

## Examining the AST

A good way to familarize yourself with the Clang AST is to actually look at it on some simple example code. Clang has a builtin AST-dump modes, which can be enabled with the flags -ast-dump and -ast-dump-xml. Note that -ast-dump-xml currently only works with debug-builds of clang.

Let's look at a simple example AST:

    # cat test.cc
    int f(int x) {
      int result = (x / 42);
      return result;
    }

    # Clang by default is a frontend for many tools; -cc1 tells it to directly
    # use the C++ compiler mode. -undef leaves out some internal declarations.
    $ clang -cc1 -undef -ast-dump-xml test.cc
    ... cutting out internal declarations of clang ...
    <TranslationUnit ptr="0x4871160">
     <Function ptr="0x48a5800" name="f" prototype="true">
      <FunctionProtoType ptr="0x4871de0" canonical="0x4871de0">
       <BuiltinType ptr="0x4871250" canonical="0x4871250"/>
       <parameters>
        <BuiltinType ptr="0x4871250" canonical="0x4871250"/>
       </parameters>
      </FunctionProtoType>
      <ParmVar ptr="0x4871d80" name="x" initstyle="c">
       <BuiltinType ptr="0x4871250" canonical="0x4871250"/>
      </ParmVar>
      <Stmt>
    (CompoundStmt 0x48a5a38 <t2.cc:1:14, line:4:1>
      (DeclStmt 0x48a59c0 <line:2:3, col:24>
        0x48a58c0 "int result =
          (ParenExpr 0x48a59a0 <col:16, col:23> 'int'
            (BinaryOperator 0x48a5978 <col:17, col:21> 'int' '/'
              (ImplicitCastExpr 0x48a5960 <col:17> 'int' <LValueToRValue>
                (DeclRefExpr 0x48a5918 <col:17> 'int' lvalue ParmVar 0x4871d80 'x' 'int'))
              (IntegerLiteral 0x48a5940 <col:21> 'int' 42)))")
      (ReturnStmt 0x48a5a18 <line:3:3, col:10>
        (ImplicitCastExpr 0x48a5a00 <col:10> 'int' <LValueToRValue>
          (DeclRefExpr 0x48a59d8 <col:10> 'int' lvalue Var 0x48a58c0 'result' 'int'))))

      </Stmt>
     </Function>
    </TranslationUnit>

In general, -ast-dump-xml dumps declarations in an XML-style format and statements in an S-expression-style format. The toplevel declaration in a translation unit is always the translation unit declaration. In this example, our first user written declaration is the function declaration of 'f'. The body of 'f' is a compound statement, whose child nodes are a declaration statement that declares our result variable, and the return statement.

## AST Context

All information about the AST for a translation unit is bundled up in the class ASTContext. It allows traversal of the whole translation unit starting from getTranslationUnitDecl, or to access Clang's table of identifiers for the parsed translation unit.

## AST Nodes

Clang's AST nodes are modeled on a class hierarchy that does not have a common ancestor. Instead, there are multiple larger hierarchies for basic node types like Decl and Stmt. Many important AST nodes derive from Type, Decl, DeclContext or Stmt, with some classes deriving from both Decl and DeclContext.

There are also a multitude of nodes in the AST that are not part of a larger hierarchy, and are only reachable from specific other nodes, like CXXBaseSpecifier.

Thus, to traverse the full AST, one starts from the TranslationUnitDecl and then recursively traverses everything that can be reached from that node - this information has to be encoded for each specific node type. This algorithm is encoded in the RecursiveASTVisitor. See the RecursiveASTVisitor tutorial.

The two most basic nodes in the Clang AST are statements (Stmt) and declarations (Decl). Note that expressions (Expr) are also statements in Clang's AST.