How to access a private member in Google Mock

I am attempting to write a mock for a class with a private vector that will insert data into the private vector. However, I am not seeing a way to do it with Google Mock. Ideally, I would like to not have anything related to testing in my interface. Also, I would not like to make the private vector protected and subclass the class and add an accessor method, as this would cause my code to leak its implementation.

Here's what I have so far. What I'm trying to accomplish is to insert data with the Fake class, and use the Mock class to Invoke Real::first() on the pointer to the class of Fake (so that I can use Fake's vector instead of Real's). When this program is compiled, -1 is returned instead of 4.

#include <iostream>
#include <vector>
#include <gmock/gmock.h>

using namespace std;
//using ::testing::_;
using ::testing::Invoke;

class A {
protected:
    vector<int> a;

public:
    virtual int first() = 0;
    virtual ~A() {}
};

class Real : public A {
public:
    virtual int first() {
        cout << "size: " << a.size() << endl;
        return a[0];
    }
};

class Fake : public A {
public:
    virtual void insert(int b) {
        a.push_back(b);
        cout << a.size() << endl;
    }

private:
    virtual int first() { return -1; }
};

class Mock : public A {
public:
    Mock(Fake* c) :
        c_(c) {}

    MOCK_METHOD0(first, int());

    void delegate() {
        ON_CALL(*this, first())
            .WillByDefault(Invoke((Real*)c_, &Real::first));
    }

private:
    Fake* c_;
};

int main(void) {
    Fake c;
    c.insert(4);

    Mock z(&c);

    z.delegate();

    cout << z.first() << endl;
    return 0;
}

Does anyone know what I'm doing wrong? Or is there an easier way to accomplish this?