Question

I implemented a class filtered_ostream_iterator that helps to filter stream using predicate and I made it as a template class.

template<typename T, typename Pred>
class filtered_ostream_iterator
{
    ostream& os;
    Pred _filter;

public:
    filtered_ostream_iterator(ostream & o, Pred filter): os(o),
                                                        _filter(filter) {}

    filtered_ostream_iterator& operator++()
    {
        return *this;
    }

    filtered_ostream_iterator& operator*()
    {
        return *this;
    }

    filtered_ostream_iterator& operator=(T t)
    {

        if (_filter(t))
            os << t;
        return *this;
    }
};

It’s ok but now I have a problem. When I use filtered_ostream_iterator I have to define it in following way.

stringstream ss1;
auto filter = [](char t){ return (t >= 'a' && t <=  'z') || (t >= 'A' && t <=  'Z'); };
filtered_ostream_iterator<char, bool (*)(char)> it1(ss1, filter); // initialization

It looks not really good especially <char, bool (*)(char)>. Then I decided to make a special function that can automatically inference types.

template<typename Pred>
filtered_ostream_iterator<char, Pred> create_filtered_ostream_iterator(ostream& os, Pred pred)
{
    return filtered_ostream_iterator<char, Pred>(os, pred); // problem
}

And I use it in following way

auto it1 = create_filtered_ostream_iterator(ss1, filter);

You can see that I should specify type of elements in stream and now it’s not a template actually, but when I’m trying to replace the code below with some thing like this

template<typename Pred, typename T>
filtered_ostream_iterator<T, Pred> create_filtered_ostream_iterator(ostream& os, Pred pred)
{
    return filtered_ostream_iterator<T, Pred>(os, pred); // error
}

And when I’m using it in the same way

auto it1 = create_filtered_ostream_iterator(ss1, filter);

I’m getting the following error.

error: no matching function for call to 'create_filtered_ostream_iterator'

So how should I avoid all these problems? Or should I use my first variant of definition and don’t mind how hard it looks? What do you think about it?

Answer 1

Write it like this:

template <typename T, typename Pred>
filtered_ostream_iterator<T, Pred> create(std::ostream & os, Pred p)
{
    return filtered_ostream_iterator<T, Pred>(os, p);
}

Usage:

auto it = create<char>(ss1, filter);

Only trailing template arguments can be deduced, but you’re free to specify as many initial arguments as you like.

The alternative is to deduce the type from the stream:

template <typename TChar, typename TTraits, typename Pred>
filtered_ostream_iterator<typename TTraits::char_type, Pred>
create(std::basic_ostream<TChar, TTraits> & os, Pred & p)
{
    return filtered_ostream_iterator<typename TTraits::char_type, Pred>(os, p);
}

Usage:

auto it = create(ss, filter);