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.

Slides

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 mode, which can be enabled with the flag -ast-dump.

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; -Xclang is used to pass
# options directly to the C++ frontend.
$ clang -Xclang -ast-dump -fsyntax-only test.cc
TranslationUnitDecl 0x5aea0d0 <<invalid sloc>>
... cutting out internal declarations of clang ...
`-FunctionDecl 0x5aeab50 <test.cc:1:1, line:4:1> f 'int (int)'
  |-ParmVarDecl 0x5aeaa90 <line:1:7, col:11> x 'int'
  `-CompoundStmt 0x5aead88 <col:14, line:4:1>
    |-DeclStmt 0x5aead10 <line:2:3, col:24>
    | `-VarDecl 0x5aeac10 <col:3, col:23> result 'int'
    |   `-ParenExpr 0x5aeacf0 <col:16, col:23> 'int'
    |     `-BinaryOperator 0x5aeacc8 <col:17, col:21> 'int' '/'
    |       |-ImplicitCastExpr 0x5aeacb0 <col:17> 'int' <LValueToRValue>
    |       | `-DeclRefExpr 0x5aeac68 <col:17> 'int' lvalue ParmVar 0x5aeaa90 'x' 'int'
    |       `-IntegerLiteral 0x5aeac90 <col:21> 'int' 42
    `-ReturnStmt 0x5aead68 <line:3:3, col:10>
      `-ImplicitCastExpr 0x5aead50 <col:10> 'int' <LValueToRValue>
        `-DeclRefExpr 0x5aead28 <col:10> 'int' lvalue Var 0x5aeac10 'result' 'int'

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.