class BaseStage
{
//Interface of stage
};
class Stage1: BaseStage
{
    int operator()(SomeType data);
};
class Stage2: BaseStage
{
    float operator()(SomeType data);
};

class Line
{
    std::vector<BaseStage> _line;
    std::vector<BaseStage>::iterator _it;
    void func()
    {
        _it = _line.begin();
        SomeType oldData, newData;
        while (_it != _line.end())
        {
            newData = (*it)(oldData);
            oldData = newData;
        }
    }
};