#include <iostream>
#include <vector>
 
class Dataset {
public:
   virtual void doSomething(int& data) = 0;
};
 
template<typename T1, typename T2> 
class Dataset_impl : public Dataset
{
protected:
   std::vector<std::pair<T1, T2> > _data_buffer;
   void doSomething(int &x) {};
};
 
template <typename T1=std::string, typename T2=std::string>
class OnlineDataset : public Dataset
{
public:
    void doSomething(int& data) override {};
};
 
template<typename T1, typename T2>
void foo(Dataset_impl<T1, T2> x) {}
 
int main() {
	Dataset_impl<int, int> y;
	foo(y);
	return 0;
}

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