以下示例是我的演示。首先,我创建一个名为Expr_node
的抽象类。之后,我创建另一个类Int_node
。为了正常访问私有成员和受保护成员函数,我将Expr
类设置为Expr_node
的朋友类。但是,我仍然无法通过重载print
来访问operator<<
功能。
#include <iostream>
#include <string>
#include <utility>
using namespace std;
class Expr_node;
class Int_node;
class Expr {
friend ostream& operator<<(ostream&, const Expr&);
Expr_node* p;
public:
Expr(int);
Expr(const string&, const Expr&);
Expr(const string&, const Expr&, const Expr&);
Expr(const Expr& t);
Expr& operator=(const Expr&);
};
class Expr_node {
friend ostream& operator<< (ostream&, const Expr_node&);
friend class Expr;
int use;
protected:
Expr_node(): use(1) {}
virtual ~Expr_node() = default;
virtual void print(ostream&) const = 0;
};
ostream& operator<< (ostream& o, const Expr_node& e) {
e.print(o);
return o;
}
ostream& operator<< (ostream& o, const Expr& e) {
e.p->print(o);
return o;
}
class Int_node: public Expr_node {
friend class Expr;
int n;
explicit Int_node(int k) : n(k) {}
void print(ostream& o) const override { o << n;}
};
是operator<<
的问题。它不是类成员,而是朋友功能。在Expr类内实现函数,该类在print
上调用p
。然后在Expr参数上调用它。像这样的东西:
#include <iostream>
#include <string>
#include <utility>
using namespace std;
class Expr_node;
class Int_node;
class Expr {
friend ostream& operator<<(ostream&, const Expr&);
Expr_node* p;
public:
Expr(int);
Expr(const string&, const Expr&);
Expr(const string&, const Expr&, const Expr&);
Expr(const Expr& t);
Expr& operator=(const Expr&);
void print(ostream& o) const
{
this->p->print(o);
}
};
class Expr_node {
friend ostream& operator<< (ostream&, const Expr_node&);
friend class Expr;
int use;
protected:
Expr_node(): use(1) {}
virtual ~Expr_node() = default;
virtual void print(ostream&) const = 0;
};
ostream& operator<< (ostream& o, const Expr_node& e) {
e.print(o);
return o;
}
ostream& operator<< (ostream& o, const Expr& e) {
e.print(o);
return o;
}