no-mock.cpp

#include <iostream>
using namespace std;

class DBClient { // DBClient for production
    public: 
    virtual bool connect() { // should be virtual, this may be the disadvantage.
        return true;
    }
};

class FakeDBClient : public DBClient { // DBClient for testing
    public: 
    bool connect() {
        return false;
    }
};

class Model { // Production code, the base of ORM.
    public:
    void foo(DBClient *client) {
        if ( client->connect() ) {
            cout << "client ok" << endl;
        } else {
            cout << "client fail" << endl;
        }
    }
};

int main() {
    Model *model = new Model();

    DBClient *dbc = new DBClient();
    model->foo(dbc);

    FakeDBClient *fdbc = new FakeDBClient();
    model->foo(fdbc);
    
    return 0;
}