How to add a variable to spdlog's flag

Question

I want to use spdlog for my code's logging. In my code, there is a important variable for the step in simulation, and I want it to be always displayed in my logs. Here is the format I wants.

[log_level][the_special_variable][logger_name] messages

So how could format the logger? Or there isn't any way to do that?

Edited: Sorry I am not good at asking a question in English.

I've read the Readme.md in spdlog's github, and i saw this

// Log patterns can contain custom flags.
// the following example will add new flag '%*' - which will be bound to a <my_formatter_flag> instance.
#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";
        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>();
    }
};

void custom_flags_example()
{    
    auto formatter = std::make_unique<spdlog::pattern_formatter>();
    formatter->add_flag<my_formatter_flag>('*').set_pattern("[%n] [%*] [%^%l%$] %v");
    spdlog::set_formatter(std::move(formatter));
}

but I can't understand the usage of it. It seems like it can only add a string for the custom flag. I would like to kwon if it is OK to display a int variable.

Answer 1

Yes, it is okay to add an int to the log message, you just have to stringify it. For example, in the format method:

auto str = std::to_string(my_special_int_variable);
dest.append(...);

The only question is how you make your int var available in the formatter. The example above assumes it's a global variable.