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Showing content from https://github.com/fmtlib/fmt/issues/4307 below:

Can't access context args() directly in 11.1.0 · Issue #4307 · fmtlib/fmt · GitHub

I have a use case where my custom formatter needs to format to a buffer and then remove either the front or the back of the buffer.

Example on 11.0.0 where this works: https://godbolt.org/z/96KeTG9bG

In order to do this, I create a new format_context using the original context's args() and locale() in the format function, but with a temporary append buffer. I then can output just a chunk of the buffer.

With 11.1.0 there was some code cleanup in 1416eda that removed the accessor for args_ which breaks my use case. In order for my code to work I either need:

• An accessor to args_ and locale_ (locale() is still available)
• A method to replace out_
• A constructor to make a new context from a const context& and a new appender

Sample found on godbolt:

#include <fmt/core.h>
#include <string_view>

template<class T>
class truncator
{
    const T& t;
    int c;
  public:
    constexpr truncator(const T& data_, const int count_) noexcept
        : t{data_}, c{count_}
    {}

    constexpr const T& data() const noexcept { return t; }
    constexpr int count() const noexcept { return c; }
};

template<class T>
constexpr truncator<T> trunc(const T& data, int count) noexcept
{
    return {data, count};
}

namespace fmt
{
template<typename T, typename Char>
struct formatter<truncator<T>, Char> : public formatter<T, Char>
{
  public:
    template<typename FormatContext>
    auto format(const truncator<T>& v, FormatContext& ctx) const
    {
        basic_memory_buffer<Char> buffer;
        format_context ctx2(appender{buffer}, ctx.args(), ctx.locale());
        formatter<T, Char>::format(v.data(), ctx2);
		auto beg = buffer.begin();
		auto end = buffer.end();
        const auto size = std::distance(beg, end);
        const auto abs_count = v.count() < 0 ? -v.count() : v.count();
        if(v.count() != 0 && size > abs_count) {
            const auto size_remove = size - abs_count;
			if(v.count() > 0) {
				beg = std::next(beg, size_remove);
			} else {
				end = std::prev(end, size_remove);
			}
        }
        ctx.advance_to(std::copy(beg, end, ctx.out()));
		return ctx.out();
    }
};
}

int main() {
  fmt::print(">{0}<\n", trunc(1234, -2));
  fmt::print(">{0}<\n", trunc(1234, 2));
}

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