<<C++ 沉思录>> 中文人民邮电出版 勘误
这本中文版里面有各种坑爹的小错误. 比方说变量名的大小写, 同一个变量, 出现了大小写不一致,等等问题都有.
然后今天感觉遇到了个语法问题. 关于继承权限的问题.
书中第八章的demo里面, 关于class Expr_node.
使用了protected关键字. 但是这里Expr_node是基类, 继承就会出现问题.
具体的代码如下:
class Expr_node{friend ostream& operator << (ostream&, const Expr_node&);friend class Expr;int use;// @use is a counter to avoid copying objects.//protected:public:Expr_node(): use(1) { }virtual void print(ostream&) const = 0;virtual ~Expr_node() { }virtual int eval() const = 0;};这里用protected就会error.
protected本来就是为了明确不发生继承的区域. 这里这堆虚函数是要发生继承的.就是为了占坑了让子类去实现.
这里应该使用public, 而不是 protected.
由于可能在"挑战权威" , 所以把问题抛出来, 希望有心人能一起讨论.
下面是完整的代码. 可供测试
/*Programmer : EOFDate: 2015.05.19File: 8.4.cppE-mail: jasonleaster@gmail.com */#include <iostream>#include <string>using namespace std;/* This @Expr_node is the base-class. */class Expr_node{friend ostream& operator << (ostream&, const Expr_node&);friend class Expr;int use;// @use is a counter to avoid copying objects.protected://public:Expr_node(): use(1) { }virtual void print(ostream&) const = 0;virtual ~Expr_node() { }virtual int eval() const = 0;};class Expr{friend ostream& operator<<(ostream&, const Expr&);Expr_node* p;public:Expr():p(NULL){}Expr(int);Expr(const string&, Expr);Expr(const string&, Expr, Expr);Expr(const Expr& t) { p = t.p; ++p->use; };Expr& operator=(const Expr&);~Expr() { if(–p->use == 0) delete p;}int eval() const {return p->eval();}};ostream&operator<<(ostream& o, const Expr_node& e){e.print(o);return o;}Expr&Expr::operator=(const Expr& rhs){rhs.p->use++;if(–p->use == 0){delete p;}p = rhs.p;return *this;}ostream&operator<<(ostream& o, const Expr& t){t.p->print(o);return o;}class Int_node: public Expr_node{friend class Expr;int n;Int_node(int k): n(k) { }void print(ostream& o) const { o << n;}int eval() const { return n;}};class Unary_node: public Expr_node{friend class Expr;string op;Expr opnd;Unary_node(const string& a, Expr b):op(a), opnd(b) { }void print(ostream& o) const{o << "(" << op << opnd << ")";}int eval() const{if(op == "-"){return -opnd.eval();}throw "error, bad op" + op + "int UnaryNode";}};class Binary_node: public Expr_node{friend class Expr;string op;Expr left;Expr right;Binary_node(const string& a, Expr b, Expr c):op(a), left(b), right(c) { }void print(ostream& o) const{o << "(" << left << op << right << ")";}int eval() const{int op1 = left.eval();int op2 = right.eval();if(op == "-") return op1 – op2;if(op == "+") return op1 + op2;if(op == "*") return op1 * op2;if(op == "/") return op1 / op2;if(op == "/" && op2 != 0) return op1/ op2;throw "error, bad op" + op + "int BinaryNode";}};Expr::Expr(int n){p = new Int_node(n);}Expr::Expr(const string& op, Expr t){p = new Unary_node(op, t);}Expr::Expr(const string& op, Expr left, Expr right){p = new Binary_node(op, left, right);}int main(){Expr t = Expr("*", Expr("-", 5), Expr("+", 3, 4));cout << t << " = " << t.eval() << endl;t = Expr("*", t, t);cout << t << " = " << t.eval() << endl;return 0;}
，去追寻那飞翔的翅膀。落叶随风淡定了它漂泊的方向。
