从继承的类实例中调用静态成员

Question

I have two classes, Y and X , and Y holds some static members -- which I'm trying to access via an X instance:

template <class T>
class Y {
public:
    Y() {
        i = 0;
        v = std::vector<int>(10, 10);
    }
    static int value() {
        return v[i];
    }
private:
    static int i;
    static std::vector<int> v;
};

class X : public Y<X> {
public:
    X() {
    }
};

int main() {
    X *x(new X());
    std::cout << x->value() << std::endl;
}

Even though it compiles, it does not link correctly:

$ g++ t.cpp
/tmp/ccB4ijzw.o: In function `Y<X>::Y()':
t.cpp:(.text._ZN1YI1XEC2Ev[Y<X>::Y()]+0x11): undefined reference to `Y<X>::i'
t.cpp:(.text._ZN1YI1XEC2Ev[Y<X>::Y()]+0x4a): undefined reference to `Y<X>::v'
/tmp/ccB4ijzw.o: In function `Y<X>::value()':
t.cpp:(.text._ZN1YI1XE5valueEv[Y<X>::value()]+0x6): undefined reference to `Y<X>::i'
t.cpp:(.text._ZN1YI1XE5valueEv[Y<X>::value()]+0x10): undefined reference to `Y<X>::v'
collect2: ld returned 1 exit status                                             

---

Context (if it matters):

I'm trying to write a memory manager -- class Y -- that statically holds a pool of memory, so that all the instances of a class X use memory blocks provided by Y .

I'm not sure if this is the best approach on what I'm trying to do, but was the somewhat most elegant way I thought of so far. Any ideas are very much welcome.

Answer 1

Static data members have to be defined as well as declared. If this was an ordinary class you'd do it like this:

// header:
class C {
    static int i;
};

// source:
int C::i = 3;

With a class template you don't ordinarily put the code in a source file, so you'd do this:

// header:
template <class T>
class C {
    static int i;
};

template <class T>
int C<T>::i = 3;

Answer 2

Since the function value is static it is common to all instances of class Y . The way to call a static member function is like

std::cout << Y<X>::value() << std::endl;

NOT (this is not illegal but it is just not a good practice. Why make the function static i you are going to do this anyway?.)

std::cout << x->value() << std::endl;

In addition you have to define all the static members. Something like

template <class T>
int Y<T>::i = 0;

template <class T>
std::vector<int> Y<T>::v(0);