C++:为什么我的DerivedClass的构造不能访问BaseClass的的保护领域？

class BaseClass {
public:
    // ...
protected:
    int protected_member;
};

class DerivedClass : public BaseClass {
public:
    DerivedClass() {
        protected_member = 42; // 现在可以访问了
    }
};

class BaseClass {
public:
    // ...
protected:
    int protected_member;

    friend class DerivedClass; // 将DerivedClass声明为友元类
};

class DerivedClass {
public:
    DerivedClass() {
        protected_member = 42; // 现在可以访问了
    }
};

class BaseClass {
public:
    // ...
protected:
    int protected_member;

public:
    int get_protected_member() { return protected_member; }
    void set_protected_member(int value) { protected_member = value; }
};

class DerivedClass : public BaseClass {
public:
    DerivedClass() {
        set_protected_member(42); // 现在可以访问并修改了
    }
};