在派生类中重新定义静态常量值C++

Question

如果我在层次结构的基类中创建静态常量，我可以在派生类中重新定义它的值吗？

编辑：

#include <iostream> 

class Base 
{ 
public: 
    static const int i = 1; 
}; 

class Derived : public Base 
{ 
public: 
    static const int i = 2; 
}; 

int main() 
{ 
    std::cout << "Base::i == " << Base::i << std::endl; 
    std::cout << "Derived::i == " << Derived::i << std::endl; 

    Base * ptr; 

    ptr= new Derived; 

    std::cout<< "ptr=" << ptr->i << std::endl; 

    return 0; 
} 

... ptr指Base::i，这是不可取的。

Answer 1

访问静态成员是通过其声明类型Base *而不是它的运行时类型（有时Base *，有时Derived *）。你可以用你的程序如下平凡扩张看到这一点：

#include <iostream> 

class Base 
{ 
public: 
    static const int i = 1; 
}; 

class Derived : public Base 
{ 
public: 
    static const int i = 2; 
}; 

int main() 
{ 
    std::cout << "Base::i == " << Base::i << std::endl; 
    std::cout << "Derived::i == " << Derived::i << std::endl; 

    Base *b_ptr = new Derived; 
    std::cout<< "b_ptr=" << b_ptr->i << std::endl; 

    Derived *d_ptr = new Derived; 
    std::cout<< "d_ptr=" << d_ptr->i << std::endl; 

    return 0; 
} 

输出：

Base::i == 1 
Derived::i == 2 
b_ptr=1 
d_ptr=2 

Answer 2

您必须在构造函数成员初始化列表中启动const memember变量。

你不能修改const变量。

Answer 3

不是，这是const，所以你不能修改它的值。

但是你可以为派生类声明一个新名称为static const的相同名称，并在那里定义它的值。经由ptr

#include <iostream> 

class Base 
{ 
public: 
    static const int i = 1; 
}; 

class Derived : public Base 
{ 
public: 
    static const int i = 2; 
}; 

int main() 
{ 
    std::cout << "Base::i == " << Base::i << std::endl; 
    std::cout << "Derived::i == " << Derived::i << std::endl; 
    return 0; 
}