C++: pointer to class members

整理自：Thinking in C++

---

#include <iostream>
using namespace std;

class Data {
private:
	int i;
public:
    int a, b, c;
    void print() const {
        cout << "a = " << a << ", b = " << b
             << ", c = " << c << endl;
    }
};

int main() {
    Data d;
	Data *pd = &d;
	
    int Data::*pmInt = &Data::a;
    pd->*pmInt = 47;
    
	pmInt = &Data::b;
    d.*pmInt = 48;
    
	pmInt = &Data::c;
    pd->*pmInt = 49;
    
    pd->print();
	
	pmInt = &Data::i; // ERROR. 'int Data::i' is private
}

// output: a = 47, b = 48, c = 49

• int Data::*pmInt = &Data::a; 定义一个 pointer 指向 Data class 内的一个 int member，并初始化为 member a;
  • 这句其实很像在 Data 外部凭空给 Data 加了一个 pointer member
  • &Data::a 这个奇葩的语法只能在这个场合用
  • Pointers to class members are quite limited: they can be assigned only to a specific location inside a class. You could not, for example, increment or compare them as you can with ordinary pointers.
• int *pda = &(d.a); 这样直接从外部取 member 的地址其实并不违法
  • 从这个角度来说，pointer to class member 的一个好处就是它被限定在了 class member 的范围内，不像普通指针那么野
• 注意 d.*pmInt = 48; 和 pd->*pmInt = 49; 这两种写法
• pointer to class member 无法指向 private member

pointer to class member 的语法还能扩展到 member function：

class Simple2 {
public:
    int f(float) const {
        return 1;
    }
};


int main() {
    int (Simple2::*fp)(float) const;
    fp = &Simple2::f;

    int (Simple2::*fp2)(float) const = &Simple2::f;
    
    Simple2 s;
    Simple2 *ps = &s;
    
    (s.*fp)(1.0F);
    (ps->*fp2)(1.0F);
}

• 说老实话，这段我看着十分难受……
• 普通的 function，其名称就是它的地址。但是在这里不行，&Simple2::f 里的 & 必须要写
• You can give the function identifier without an argument list, because overload resolution can be determined by the type of the pointer to member.
• 考虑到 member 多是 private，可能 pointer to class member function 的市场还稍微大些

书上还有个更复杂的例子，需要时可以看看。