How to get the information of DeclRefExpr from the basic block in Clang:CFG?

Question

How to get the information of DeclRefExpr from the basic block in Clang:CFG? This is code i have coded

class CFGPrinter : public MatchFinder::MatchCallback {
public:
  virtual void run(const MatchFinder::MatchResult &Result) {
    if (const FunctionDecl *funcDecl =
        Result.Nodes.getNodeAs<clang::FunctionDecl>("mainFunction")) {
      ASTContext *context = Result.Context;
      Stmt *funcBody = funcDecl->getBody();
      static std::unique_ptr<CFG> sourceCFG = CFG::buildCFG(funcDecl, funcBody, context, clang::CFG::BuildOptions());
      for(CFG::const_iterator it = sourceCFG->begin(), ei = sourceCFG->end(); it != ei; ++it){
        const CFGBlock* block = *it;
        for(CFGBlock::const_iterator bi = block->begin(), be = block->end(); bi != be; ++bi){
          const CFGElement &elem = *bi;
          const Stmt *stmt_tmp = elem.castAs<CFGStmt>().getStmt();
          /*I want to get DeclRefExpr from next*/            
          }
        }
      }
    }
  }
};