# Clang 11 documentation

## How to write RecursiveASTVisitor based ASTFrontendActions.

## Introduction

In this tutorial you will learn how to create a FrontendAction that uses a RecursiveASTVisitor to find CXXRecordDecl AST nodes with a specified name.

## Creating a FrontendAction

When writing a clang based tool like a Clang Plugin or a standalone tool based on LibTooling, the common entry point is the FrontendAction. FrontendAction is an interface that allows execution of user specific actions as part of the compilation. To run tools over the AST clang provides the convenience interface ASTFrontendAction, which takes care of executing the action. The only part left is to implement the CreateASTConsumer method that returns an ASTConsumer per translation unit.

    class FindNamedClassAction : public clang::ASTFrontendAction {
    public:
      virtual std::unique_ptr<clang::ASTConsumer> CreateASTConsumer(
        clang::CompilerInstance &Compiler, llvm::StringRef InFile) {
        return std::unique_ptr<clang::ASTConsumer>(
            new FindNamedClassConsumer);
      }
    };

## Creating an ASTConsumer

ASTConsumer is an interface used to write generic actions on an AST, regardless of how the AST was produced. ASTConsumer provides many different entry points, but for our use case the only one needed is HandleTranslationUnit, which is called with the ASTContext for the translation unit.

    class FindNamedClassConsumer : public clang::ASTConsumer {
    public:
      virtual void HandleTranslationUnit(clang::ASTContext &Context) {
        // Traversing the translation unit decl via a RecursiveASTVisitor
        // will visit all nodes in the AST.
        Visitor.TraverseDecl(Context.getTranslationUnitDecl());
      }
    private:
      // A RecursiveASTVisitor implementation.
      FindNamedClassVisitor Visitor;
    };

## Using the RecursiveASTVisitor

Now that everything is hooked up, the next step is to implement a RecursiveASTVisitor to extract the relevant information from the AST.

The RecursiveASTVisitor provides hooks of the form bool VisitNodeType(NodeType *) for most AST nodes; the exception are TypeLoc nodes, which are passed by-value. We only need to implement the methods for the relevant node types.

Let’s start by writing a RecursiveASTVisitor that visits all CXXRecordDecl’s.

    class FindNamedClassVisitor
      : public RecursiveASTVisitor<FindNamedClassVisitor> {
    public:
      bool VisitCXXRecordDecl(CXXRecordDecl *Declaration) {
        // For debugging, dumping the AST nodes will show which nodes are already
        // being visited.
        Declaration->dump();

        // The return value indicates whether we want the visitation to proceed.
        // Return false to stop the traversal of the AST.
        return true;
      }
    };

In the methods of our RecursiveASTVisitor we can now use the full power of the Clang AST to drill through to the parts that are interesting for us. For example, to find all class declaration with a certain name, we can check for a specific qualified name:

    bool VisitCXXRecordDecl(CXXRecordDecl *Declaration) {
      if (Declaration->getQualifiedNameAsString() == "n::m::C")
        Declaration->dump();
      return true;
    }

## Accessing the SourceManager and ASTContext

Some of the information about the AST, like source locations and global identifier information, are not stored in the AST nodes themselves, but in the ASTContext and its associated source manager. To retrieve them we need to hand the ASTContext into our RecursiveASTVisitor implementation.

The ASTContext is available from the CompilerInstance during the call to CreateASTConsumer. We can thus extract it there and hand it into our freshly created FindNamedClassConsumer:

    virtual std::unique_ptr<clang::ASTConsumer> CreateASTConsumer(
      clang::CompilerInstance &Compiler, llvm::StringRef InFile) {
      return std::unique_ptr<clang::ASTConsumer>(
          new FindNamedClassConsumer(&Compiler.getASTContext()));
    }

Now that the ASTContext is available in the RecursiveASTVisitor, we can do more interesting things with AST nodes, like looking up their source locations:

    bool VisitCXXRecordDecl(CXXRecordDecl *Declaration) {
      if (Declaration->getQualifiedNameAsString() == "n::m::C") {
        // getFullLoc uses the ASTContext's SourceManager to resolve the source
        // location and break it up into its line and column parts.
        FullSourceLoc FullLocation = Context->getFullLoc(Declaration->getBeginLoc());
        if (FullLocation.isValid())
          llvm::outs() << "Found declaration at "
                       << FullLocation.getSpellingLineNumber() << ":"
                       << FullLocation.getSpellingColumnNumber() << "\n";
      }
      return true;
    }

## Putting it all together

Now we can combine all of the above into a small example program:

    #include "clang/AST/ASTConsumer.h"
    #include "clang/AST/RecursiveASTVisitor.h"
    #include "clang/Frontend/CompilerInstance.h"
    #include "clang/Frontend/FrontendAction.h"
    #include "clang/Tooling/Tooling.h"

    using namespace clang;

    class FindNamedClassVisitor
      : public RecursiveASTVisitor<FindNamedClassVisitor> {
    public:
      explicit FindNamedClassVisitor(ASTContext *Context)
        : Context(Context) {}

      bool VisitCXXRecordDecl(CXXRecordDecl *Declaration) {
        if (Declaration->getQualifiedNameAsString() == "n::m::C") {
          FullSourceLoc FullLocation = Context->getFullLoc(Declaration->getBeginLoc());
          if (FullLocation.isValid())
            llvm::outs() << "Found declaration at "
                         << FullLocation.getSpellingLineNumber() << ":"
                         << FullLocation.getSpellingColumnNumber() << "\n";
        }
        return true;
      }

    private:
      ASTContext *Context;
    };

    class FindNamedClassConsumer : public clang::ASTConsumer {
    public:
      explicit FindNamedClassConsumer(ASTContext *Context)
        : Visitor(Context) {}

      virtual void HandleTranslationUnit(clang::ASTContext &Context) {
        Visitor.TraverseDecl(Context.getTranslationUnitDecl());
      }
    private:
      FindNamedClassVisitor Visitor;
    };

    class FindNamedClassAction : public clang::ASTFrontendAction {
    public:
      virtual std::unique_ptr<clang::ASTConsumer> CreateASTConsumer(
        clang::CompilerInstance &Compiler, llvm::StringRef InFile) {
        return std::unique_ptr<clang::ASTConsumer>(
            new FindNamedClassConsumer(&Compiler.getASTContext()));
      }
    };

    int main(int argc, char **argv) {
      if (argc > 1) {
        clang::tooling::runToolOnCode(std::make_unique<FindNamedClassAction>(), argv[1]);
      }
    }

We store this into a file called FindClassDecls.cpp and create the following CMakeLists.txt to link it:

    add_clang_executable(find-class-decls FindClassDecls.cpp)

    target_link_libraries(find-class-decls clangTooling)

When running this tool over a small code snippet it will output all declarations of a class n::m::C it found:

    $ ./bin/find-class-decls "namespace n { namespace m { class C {}; } }"
    Found declaration at 1:29