Re: [boost] Variadic append for std::string

Subject: Re: [boost] Variadic append for std::string
From: Christof Donat (cd_at_[hidden])
Date: 2017-01-25 12:19:22

Hi,

Am 24.01.2017 14:00, schrieb Richard Hodges:
> It's almost looking like conversion to utf8, wide strings, strings or
> string_views should be a filter rather than an option.

A filter always needs an input encoding and an output encoding. If we 
want to have filters to chose the output encoding between UTF-8, Latin 
1, etc. what is going to be the input encoding?

> Note that I am still resisting the idea of .str() as a member function. 
> If
> the joiner or concatenation object exports begin() and end(), it's
> un-necessary, because the object returned by create_string() (or 
> similar)
> can use the iterators.

I like that idea, though I still don't like the name to_string().

How about this:

auto s = generate<std::sting>(concat(...));
auto ws = generate<std::wsting>(concat(...));

generate<std::sting>(s, concat(...)); // returns reference to s for 
consistency

auto v = generate<std::vector<int>>(any_other_range); // actually will 
just copy

A very naive Implementation might look like this:

template<typename ReturnT, typename Sequence>
auto generate(ReturnT& r, Sequence& seq) -> ReturnT& {
     std::copy(std::begin(seq), std::end(seq), std::begin(r));
     return r;
}
template<typename ReturnT, typename Sequence>
auto generate(Sequence& seq) -> ReturnT {
     ReturnT r{seq.size()};
     generate(r, seq);
     return r;
}

The word "generate" is very generic, therefore I'm still not really 
happy with it.

The string factories, of course need more than only begin() and end(). 
They also have to provide a way to estimate the size of the output, 
because we want to be able to allocate the complete buffer upfront. In 
the naive implementation I called that size().

> Having iterators also means that the attributed factory can be used as 
> a
> source in std::copy, std::transform, std::for_each etc.

.. and in range expressions :-)

Christof