Using Clang to get AST - clang

Question

I would like to use Clang to get to its AST to get some information about the variables and methods in a particular source file. However, I do not want to use the LibTooling facilities. I would like to, by hand, to write to code that would call the methods to parse the .cpp and then get the tree. I can not find any resources that tell me how to do this. Can anybody help?

Answer 1

If your goal is to learn how to drive Clang components to work with the compilation database, configure the compiler instance, and so on, then the Clang source code is a resource. Perhaps the source for the ClangTool::buildASTs() method would be a good place to start: see Tooling.cpp in the lib/Tooling/ directory of the source tree.
If your goal is to do an analysis that LibTooling doesn't support, and you just want to get the AST with minimal fuss, then either ClangTool::buildASTs or clang::tooling::buildASTFromCode might be of service. The ClangTool approach would be better if you need the compilation database to express compiler options, include paths, and so on. buildASTFromCode is fine if you have a standalone bit of code for lightweight tests. Here's an example of the ClangTool approach:
#include "clang/Tooling/CommonOptionsParser.h"
#include "clang/Tooling/Tooling.h"
#include "llvm/Support/CommandLine.h"
#include <memory>
#include <vector>
static llvm::cl::OptionCategory MyOpts("Ignored");
int main(int argc, const char ** argv)
{
using namespace clang;
using namespace clang::tooling;
CommonOptionsParser opt_prs(argc, argv, MyOpts);
ClangTool tool(opt_prs.getCompilations(), opt_prs.getSourcePathList());
using ast_vec_t = std::vector<std::unique_ptr<ASTUnit>>;
ast_vec_t asts;
tool.buildASTs(asts);
// now you the AST for each translation unit
...
Here's an example of buildASTFromCode:
#include "clang/Frontend/ASTUnit.h"
#include "clang/Tooling/Tooling.h"
...
std::string code = "struct A{public: int i;}; void f(A & a}{}";
std::unique_ptr<clang::ASTUnit> ast(clang::tooling::buildASTFromCode(code));
// now you have the AST for the code snippet
clang::ASTContext * pctx = &(ast->getASTContext());
clang::TranslationUnitDecl * decl = pctx->getTranslationUnitDecl();
...