error: use of undeclared identifier 'proc' ... why?? -> solved. Required to specify the rvalue of '->*'

gistfile1.cpp

//  ref: http://okwave.jp/qa/q3884248.html

#include <iostream>
#include <stdio.h>
#include <functional>

class SuperClass {
public:
    virtual ~SuperClass() {}

    typedef void (SuperClass::*method_type)();
    method_type proc;
    std::function<void(void)> func;

protected:
    SuperClass() { }
    void SetSubClassFunction(method_type theProc) {
        proc = theProc;
        func = std::bind(theProc, this);
    }
public:
    void CallSubClassFunction() {
        (this->*proc)();      // OK
    }
    virtual void Set() = 0;
};

class SubClass : public SuperClass {
public:
    void Set() {
        SetSubClassFunction(reinterpret_cast<method_type>(&SubClass::TestFunction));
    }
    void TestFunction() {
        printf("SubClass::TestFunction()\n");
    }
};

int main(int, char **) {
    SuperClass *obj = new SubClass();
    obj->Set();
    obj->CallSubClassFunction(); // ←OK
    // (obj->*proc)();           // ←NG（compilation error）
    (obj->*(obj->proc))();       // ←OK!

    SuperClass::method_type p = reinterpret_cast<SuperClass::method_type>(&SubClass::TestFunction);
    (obj->*p)();                 // ←OK!!

    (obj->func)();               // ←OK!!

    delete obj;
    return 0;
}