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

18 Jan 2017

      Ok. here's my second attempt.

It has 2 improvements:

1. allows appending to an existing string by specifiying std::string&
join(onto(s), [optional separator("xxx"), parts...);
2. replaces use of std::ostringstream with a (very simple!) version that
does string appending.

#include <sstream>
#include <iostream>
#include <iomanip>

struct string_ref_buffer
        :  std::streambuf
{
    using inherited = std::streambuf;
    using char_type = inherited::char_type;
    using char_traits = std::char_traits;

    int overflow(int c) override
    {
        if (c != char_traits::eof()) {
            buffer_.push_back(c);
        }
        return char_traits::not_eof(c);
    }

    string_ref_buffer(std::string& buffer)
            : buffer_(buffer)
    {

    }

    const std::string& str() const & { return buffer_; }
    std::string&& str()&& { return std::move(buffer_); }

    std::string& buffer_;
    std::size_t inpos_ = 0;
    std::size_t outpos_ = 0;
};

namespace detail {
    template<class SepStr>
    struct separator_object
    {
        template<class T>
        std::ostream& operator ()(std::ostream& s, T&& t) const
        {
            return s << sep << t;
        }

        //
        // other iomanp specialisations here
        //
        std::ostream& operator ()(std::ostream& s,
std::ios_base&(*t)(std::ios_base&)) const
        {
            t(s);
            return s;
        }

        SepStr const& sep;
    };

    struct no_separator_object
    {
        template<class T>
        std::ostream& operator ()(std::ostream& s, T&& t) const
        {
            return s << t;
        }
    };

    template
    auto& join_onto(Target&& target, Separator&& sep, Rest&&...rest)
    {
        string_ref_buffer sbuf { target.str() };
        std::ostream ss(std::addressof(sbuf));
        using expand = int [];
        void(expand{0,
                    ((sep(ss, rest)), 0)...
        });
        return target;
    };

    template
    auto join(Separator&& sep, String&& s, Rest&&...rest)
    {
        std::string result {};
        string_ref_buffer sbuf { result };
        std::ostream ss(std::addressof(sbuf));
        ss << s;
        using expand = int [];
        void(expand{0,
                    ((sep(ss, rest)), 0)...
        });
        return result;
    };

    template<class String>
    struct onto_type
    {
        String& str() { return target_.get(); }
        std::reference_wrapper<String> target_;
    };

}

template<class String>
auto onto(String& target)
{
    return detail::onto_type<String> { target };
}

template<class Sep>
static constexpr auto separator(Sep const& sep)
{
    using sep_type = std::remove_const_t;
    return detail::separator_object { sep };
}

template
decltype(auto) join(detail::separator_object<SepObject> sep, String&&
s, Rest&&...rest)
{
    return detail::join(sep,
                        std::forward<String>(s),
                        std::forward<Rest>(rest)...);
};

template
decltype(auto) join(String&& s, Rest&&...rest)
{
    return detail::join(detail::no_separator_object(),
                        std::forward<String>(s),
                        std::forward<Rest>(rest)...);
};

template
decltype(auto) join(detail::onto_type<Target> target,
detail::separator_object<SepObject> sep, String&& s, Rest&&...rest)
{
    return detail::join_onto(target,
                        sep,
                        std::forward<String>(s),
                        std::forward<Rest>(rest)...);
};

template
decltype(auto) join(detail::onto_type<Target> target, String&& s, Rest&&...rest)
{
    return detail::join_onto(target,
                        detail::no_separator_object(),
                        std::forward<String>(s),
                        std::forward<Rest>(rest)...);
};

int main()
{
    auto s= std::string("foo");

    s = join("Hello ", ", World.", " The hex for ", 58, " is ", std::hex, 58);
    std::cout << s << std::endl;

    s = join(separator(" : "), "a", "b", std::hex, 200 , std::quoted("banana"));
    std::cout << s << std::endl;

    join(onto(s), separator(", "), "funky", "chicken");
    join(onto(s), "=====");
    std::cout << s << std::endl;
}

expected output:

Hello , World. The hex for 58 is 3a
a : b : c8 : "banana"
a : b : c8 : "banana", funky, chicken=====

On 18 January 2017 at 09:53, Richard Hodges  wrote:
...
That's pretty straightforward with another overload:
auto& s = join(to(y), separator(", "), "A", "b", 42);
where to(y) is something like
template<String>
struct to_existing_type<String> {
    String& get() { return s_; }
    String s_;
};
template<class String>
auto to(String& s)
{
  return to_existing_type<S>(s);
}
With a bit of template unwrapping, we could imagine something like this:
join(to(x), 2, 3, to(y), "foo", "bar", create(), "baz", 42);
which would return a tuple:
std::tuple
in c++17 this would allow:
auto&& [x, y, z] = join(to(x), 2, 3, to(y), "foo", "bar", create(), "baz",
42);
But this maybe taking it a bit far... What do you think?
On 18 January 2017 at 09:06, Olaf van der Spek  wrote:
...
On Mon, Jan 16, 2017 at 11:41 AM, Richard Hodges 
wrote:
...
Sorry to chime in so late in the discussion.
What about a syntax similar to this?
int main()
{
    auto s = join("Hello ", ", World.", " The hex for ", 58, " is ",
std::hex, 58);
    std::cout << s << std::endl;
s = join(separator(" : "), "a", "b", std::hex, 200 ,
std::quoted("banana"));
    std::cout << s << std::endl;
}
Which would produce the following output:
Hello , World. The hex for 58 is 3a
a : b : c8 : “banana"
The syntax is fine but it's missing an appending variant, like append(s,
"A", "B", 42);
This variant is important as it (also) allows you to reuse existing
storage.
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