定義抽象語法樹，也就是各種表達(dá)式的數(shù)據(jù)結(jié)構(gòu)。

class BaseAST {
 public:
  virtual ~BaseAST();
};

class VariableAST : public BaseAST {
  std::string var_name_;
 public:
  VariableAST(std::string &name) : var_name_(name) {}
};

class NumericAST : public BaseAST {
  int numeric_val_;
 public:
  NumericAST(int val) : numeric_val_(val) {}
};

class BinaryAST : public BaseAST {
  std::string operator_;
  BaseAST *lhs_, *rhs_;
 public:
  BinaryAST(std::string op, BaseAST* lhs, BaseAST* rhs) :
      operator_(op), lhs_(lhs), rhs_(rhs) {}
};

class FunctionDeclAST {
  std::string func_name_;
  std::vector<std::string> args_;
 public:
  FunctionDeclAST(const std::string &name, const std::vector<std::string> &args) :
      func_name_(name), args_(args) {}
};

class FunctionDefnAST {
  FunctionDeclAST *func_decl_;
  BaseAST* body_;
 public:
  FunctionDefnAST(FunctionDeclAST *proto, BaseAST *body) :
      func_decl_(proto), body_(body) {}
};

class FunctionCallAST : public BaseAST {
  std::string callee_;
  std::vector<BaseAST*> args_;
 public:
  FunctionCallAST(const std::string &callee, const std::vector<BaseAST*> args) :
      callee_(callee), args_(args) {}
};