class DataHandler
{
public:
    void doSomethingWithData() {}
protected:
private:
};

class SqlDatabaseDataRetriever
{
public:
    Data getData() { }
protected:
private:
};

class DataHandler
{
public:
    DataHandler(SqlDatabaseDataRetriever& dataRetriever)
    {
        _dataRetriever = dataRetriever;
    }

    void doSomethingWithData()
    {
        Data data = this->_dataRetriever.getData();
    }
protected:
    SqlDatabaseDataRetriever _dataRetriever;
private:
};

class DataRetriever
{
public:
    virtual Data getData() = 0;
protected:
private;
}

class SqlDatabaseDataRetriever : public DataRetriever
{
public:
    virtual Data getData() { }
protected:
private;
}

class ApiDataRetriever : public DataRetriever
{
public:
    virtual Data getData() { }
protected:
private;
}

class DataHandler
{
public:
    DataHandler(DataRetriever& dataRetriever)
    {
        _dataRetriever = dataRetriever;
    }

    void doSomethingWithData()
    {
        Data data = this->_dataRetriever.getData();
    }
protected:
    DataRetriever _dataRetriever;
private:
};