Introduction to the Clang AST — Clang 21.0.0git documentation (original) (raw)

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 generatedDoxygen. 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 familiarize 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 <> ... 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 declarationof “f”. The body of “f” is a compound statement, whose child nodes are a declaration statementthat declares our result variable, and the return statement.

AST Context¶

All information about the AST for a translation unit is bundled up in the classASTContext. It allows traversal of the whole translation unit starting fromgetTranslationUnitDecl, or to access Clang’s table of identifiersfor 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 likeDecl andStmt. Many important AST nodes derive fromType,Decl,DeclContextor 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, likeCXXBaseSpecifier.

Thus, to traverse the full AST, one starts from theTranslationUnitDecland 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 theRecursiveASTVisitor. 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.