I ran into an error yesterday and, while it’s easy to get around, I wanted to make sure that I’m understanding C++ right.
I have a base class with a protected member:
class Base
{
protected:
int b;
public:
void DoSomething(const Base& that)
{
b+=that.b;
}
};
This compiles and works just fine. Now I extend Base but still want to use b:
class Derived : public Base
{
protected:
int d;
public:
void DoSomething(const Base& that)
{
b+=that.b;
d=0;
}
};
Note that in this case DoSomething is still taking a reference to a Base, not Derived. I would expect that I can still have access to that.b inside of Derived, but I get a cannot access protected member error (MSVC 8.0 – haven’t tried gcc yet).
Obviously, adding a public getter on b solved the problem, but I was wondering why I couldn’t have access directly to b. I though that when you use public inheritance the protected variables are still visible to the derived class.
1
A class can only access protected members of instances of this class or a derived class. It cannot access protected members of instances of a parent class or cousin class.
In your case, the Derived class can only access the b protected member of Derived instances, not that of Base instances.
Changing the constructor to take a Derived instance will solve the problem.
9
protected members can be accessed:
- through
thispointer - or to the same type protected members even if declared in base
- or from friend classes, functions
To solve your case you can use one of last two options.
Accept Derived in Derived::DoSomething or declare Derived friend to Base:
class Derived;
class Base
{
friend class Derived;
protected:
int b;
public:
void DoSomething(const Base& that)
{
b+=that.b;
}
};
class Derived : public Base
{
protected:
int d;
public:
void DoSomething(const Base& that)
{
b+=that.b;
d=0;
}
};
You may also consider public getters in some cases.
1
As mentioned, it’s just the way the language works.
Another solution is to exploit the inheritance and pass to the parent method:
class Derived : public Base
{
protected:
int d;
public:
void DoSomething(const Base& that)
{
Base::DoSomething(that);
d=0;
}
};
1
You have access to the protected members of Derived, but not those of Base (even if the only reason it’s a protected member of Derived is because it’s inherited from Base)
1
You can try with static_cast< const Derived*>(pBase)->Base::protected_member …
class Base
{
protected:
int b;
public:
...
};
class Derived : public Base
{
protected:
int d;
public:
void DoSomething(const Base& that)
{
b += static_cast<const Derived*>(&that)->Base::b;
d=0;
}
void DoSomething(const Base* that)
{
b += static_cast<const Derived*>(that)->Base::b;
d=0;
}
};
3
Following the hack for stl I wrote a small code which seems to solve the problem of accessing the protected members in derived class
#include <iostream>
class B
{
protected:
int a;
public:
void dosmth()
{
a = 4;
}
void print() {std::cout<<"a="<<a<<std::endl;}
};
class D: private B
{
public:
void dosmth(B &b)
{
b.*&D::a = 5;
}
};
int main(int argc, const char * argv[]) {
B b;
D d;
b.dosmth();
b.print();
d.dosmth(b);
b.print();
return 0;
}
Prints
a=4
a=5
1
class Derived : public Base
{
protected:
int d;
public:
void DoSomething()
{
b+=this->b;
d=0;
}
};
//this will work
Use this pointer to access protected members
class Derived : public Base
{
protected:
int d;
public:
void DoSomething(const Base& that)
{
this->b+=that.b;
d=0;
}
};
4