Custom flag aligning (padding)

[agil899]

I've implemented my own custom flag y following the README guidelines and I'd like to add aligning so that the length of the text is constant.
I tried %-10y but this doesn't work, is there a different way to do it for custom flags?

[tt4g]

Adding padding seems to be the role of the formatter.
As far as I can tell from the source of the short_level_formatter, it passes padding_info to ScopedPadder to add padding.
The ScopedPadder is passed as a template argument, either scoped_padder or null_scoped_padder.

spdlog/include/spdlog/pattern_formatter-inl.h

Lines 138 to 153 in 76fb40d

	// short log level appender
	template<typename ScopedPadder>
	class short_level_formatter final : public flag_formatter
	{
	public:
	explicit short_level_formatter(padding_info padinfo)
	: flag_formatter(padinfo)
	{}
	void format(const details::log_msg &msg, const std::tm &, memory_buf_t &dest) override
	{
	string_view_t level_name{level::to_short_c_str(msg.level)};
	ScopedPadder p(level_name.size(), padinfo_, dest);
	fmt_helper::append_string_view(level_name, dest);
	}
	};

spdlog/include/spdlog/pattern_formatter-inl.h

Lines 1397 to 1404 in 76fb40d

	if (padding.enabled())
	{
	handle_flag_<details::scoped_padder>(*it, padding);
	}
	else
	{
	handle_flag_<details::null_scoped_padder>(*it, padding);
	}

For custom formatters, it appears that set_padding_info() is called and padding_info is passed to the padinfo_ member variable.

spdlog/include/spdlog/pattern_formatter-inl.h

Lines 1096 to 1104 in 76fb40d

	// process custom flags
	auto it = custom_handlers_.find(flag);
	if (it != custom_handlers_.end())
	{
	auto custom_handler = it->second->clone();
	custom_handler->set_padding_info(padding);
	formatters_.push_back(std::move(custom_handler));
	return;
	}

spdlog/include/spdlog/pattern_formatter.h

Lines 71 to 75 in 76fb40d

	void set_padding_info(const details::padding_info &padding)
	{
	flag_formatter::padinfo_ = padding;
	}
	};

So adding padding in the custom formatting function should work.

Example (NOTE has not been tested):

#include "spdlog/pattern_formatter.h"
class my_formatter_flag : public spdlog::custom_flag_formatter
{
public:
    void format(const spdlog::details::log_msg &, const std::tm &, spdlog::memory_buf_t &dest) override
    {
        std::string some_txt = "custom-flag";
        if (this->padinfo_.enabled()) 
        { 
            spdlog::details::scoped_padder(some_txt.size(), this->padinfo_, dest); 
        }
        dest.append(some_txt.data(), some_txt.data() + some_txt.size());
    }

    std::unique_ptr<custom_flag_formatter> clone() const override
    {
        return spdlog::details::make_unique<my_formatter_flag>();
    }
};

[agil899]

Thanks a lot for your help!
You were right except for a small detail that dest.append() needs to be in the same scope as scopped_padder for left aligning to work. Also there needs to be an instance of scopped_padder so that the destructor doesn't get called immediately.
Final result:

#include "spdlog/pattern_formatter.h"
class my_formatter_flag : public spdlog::custom_flag_formatter
{
public:
    void format(const spdlog::details::log_msg &, const std::tm &, spdlog::memory_buf_t &dest) override
    {
        std::string some_txt = "custom-flag";
        if (this->padinfo_.enabled()) 
        { 
            spdlog::details::scoped_padder p(some_txt.size(), this->padinfo_, dest); 
            dest.append(some_txt.data(), some_txt.data() + some_txt.size());
            return;
        }
        dest.append(some_txt.data(), some_txt.data() + some_txt.size());
    }

    std::unique_ptr<custom_flag_formatter> clone() const override
    {
        return spdlog::details::make_unique<my_formatter_flag>();
    }
};