Is there any way to pass a potentially null C-string?

[persona94]

I know we should sanitize whatever pointers we hand over to the library but is there any way to mitigate this?

const char* badPtr = nullptr;
QUILL_LOG_INFO(logger,"{msg}", badPtr);

This will segV and that's not a surprise.

Since our code is full of char* ptrs and some could be null I wrote a wrapper around this

   // Concept to identify C-style string pointer types
    template <typename T>
    concept CStringPointer = std::same_as<T, const char*> || std::same_as<T, char*>;

    // Safe format helper that handles different types appropriately
    template <typename T>
    auto safe_fmt_arg(T&& arg)
    {
        if constexpr (CStringPointer<std::remove_cvref_t<T>>)
        {
            return arg ? arg : "null";
        }
        else
        {
            return std::forward<T>(arg);
        }
    }

    template <typename... Args>
    std::string logUtil(const std::string& fmt_str, Args&&... args)
    {
        return fmt::format(fmt::runtime(fmt_str), safe_fmt_arg(std::forward<Args>(args))...);
     }

   
#define FIXED_LOG_INFO(fmt_str, ...) \
{\
auto  msg = logUtil(fmt_str, ##__VA_ARGS__); \
QUILL_LOG_INFO(logger, "{message}", msg); \
}

I've also derived and overriden quill::ConsoleSink to format my logs in a certain way.

All this works very well (and I love your library - it's fast, super configurable and well maintained!)

So now I can do

FIXED_LOG_INFO("This is int {} float {} ptr {} bad ptr {} string {}", 10, 5.0, fmt::ptr(p), badPtr, str);
and it prints as
"timestamp":"2025-03-28T19:25:46.724654Z","file_name":"main.cpp","function":"main","line":"24","log_level":"INFO","message":"This is int 10 float 5 ptr 0x16d103468 bad ptr null string Hello"}

However I want to implement 1 "extra credit" feature.

FIXED_LOG_INFO("Debug with tags {} and {}", "LA12", 10, "{thisID}", "TS1234", "{exe}", "Nono");

I'd expect it to show up as

{"timestamp":"2025-03-28T19:27:41.769253Z","file_name":"main.cpp","function":"main","line":"35","log_level":"INFO","message":"Debug with tags LA12 and 10","thisID":"TS1234","exe":"Nono"}

That will work if I define FIXED_LOG_INFO as

#define FIXED_LOG_INFO(fmt_str, ...) \
{\
auto  msg = logUtil(fmt_str, ##__VA_ARGS__); \
QUILL_LOG_INFO(logger, "{message}-{someId}-{odherId}", vcsLogMsg, 10, 20, ##__VA_ARGS__); \
}

But it will segV if I give it a nullptr for a char* because VA_ARGS go to the Quill macro interface. Any ways you think I can avoid the segfault?

I can't scrub the codebase for all potential null char* ptrs unfortunately, there are simply too many.

[odygrd]

hey, thanks for reporting. It is not expected to try to log a nullptr but I do not mind adding a check for nullptr for char* and char const* so you can log those without having to check on your side

#737

[persona94]

Wow yeah that would be amazing. Thank you!!

[odygrd]

your are welcome! I have merged it to master. I am planning to do a release once libfmt 11.1.5 is released

[persona94]

Amazing! Thank you!!