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正确执行运算符重载和多态性

friend-function virtual-functions operator-overloading polymorphism c++

-1

Jayendra Parmar · 技术社区 · 7 年前

我有两个用于string和int的包装器类,一个用于表示二进制操作和重载运算符 << 以字符串格式写入它们。不幸的是,我不能超载 << 没有 friend virtual 不能使用具有 事实上的 !!

#include <iostream>
#include <string>

class AST {};
class expr: public AST {};

class operator_: public AST {};

class BinOp: public expr {
private:
  expr *_left;
  expr *_right;
  operator_ *_op;
public:
  BinOp(expr *p_left, expr *p_right, operator_ *p_op):_left(p_left),_right(p_right),_op(p_op) {}
  friend std::ostream &operator<< (std::ostream &out, const BinOp &binop) {
    out << (binop._left) << " " << (binop._op) << " " << (binop._right);
  }
};

class LShift: public operator_ {
public:
  LShift() {}
  friend std::ostream &operator<< (std::ostream &out, const LShift &lshift) {
    out << "<<";
  }
};

class Name: public expr {
private:
  std::string _id;
public:
  Name(std::string p_id) { _id = p_id; }
  friend std::ostream &operator<< (std::ostream &out, const Name &name) {
    out << name._id;
  }
};

class Num: public expr {
private:
  int n;
public:
  Num(int p_n) { n = p_n; }
  friend std::ostream &operator<< (std::ostream &out, const Num &num) {
    out << num.n;
  }
};

int main() {

  auto name = Name("x");
  auto num  = Num(5);
  auto lshift = LShift();
  auto binop = BinOp(&name, &num, &lshift);

  std::cout << binop << '\n';

  return 0;
}

所以上面提到的程序输出指针,

0x7ffd05935db0 0x7ffd05935d8b 0x7ffd05935d8c

x << 5

1 回复 | 直到 7 年前

1

4

Sebastian Redl 7 年前

但我要说的是,流插入操作符是一个例外。

解决方案是重载操作符一次,并将其转发到更易于重写的函数。

class AST {
public:
  virtual void print(std::ostream& s) const = 0;
  virtual ~AST() {} // you probably want this one too
};

// no need for friendliness
std::ostream& operator <<(std::ostream& s, const AST& node) {
  node.print(s);
  return s;
}

print 每个类的重写。

但我也觉得有必要指出 AST 在AST中,树中的单个节点。运算符是表达式的一部分。整棵树都不一样。

您应该有单独的层次结构根 operator_ 和 expr