当其中一个是函数参数时，在编译时连接 const char

Question

I'm trying to add a filename prefix to each log string in spdlog. The Spdlog formatting string looks as this:

Test log {}

Logs are written as this: spdlog::error("Test log {}", value)

I'm trying to wrap this call and concatenate additional {} before formatting string, so I can pass the prefix of the file there.

static constexpr char * prefixHolder("{} ");

template<typename... Args>
void critical(const char fmt[], Args&&... args) const
{
    constexpr auto fullFmt = prefixHolder + fmt; //I can't find any solution for this

    spdlog::critical(fullFmt, m_prefix, std::forward<Args>(args)...);
}


Log log("MyClassOrLogger");

log.critical("My format {}", value);

Is it possible to solve this at compile time? I've tried some approaches, but I haven't found any way to make input fmt argument constexpr for the compiler.

C++ 17

Any suggestions or solutions?

Thanks

Answer 1

Parameter value cannot be used for constexpr.

You might turn:

template<typename... Args>
constexpr void critical(const char fmt[], Args&&... args)

into

template <char... cs, typename... Args>
void critical(std::integer_sequence<char, cs...>, Args&&... args)
{
    constexpr char fullFmt[] = {'{', '}', ' ', cs... , '\0'};

    spdlog::critical(fullFmt, m_prefix, std::forward<Args>(args)...);
}

to allow to have constexpr char array.

ensure-that-char-pointers-always-point-to-the-same-string-literal shows way to create similar sequences from literal string. In your case, it would be:

template <typename Char, Char... Cs>
constexpr auto operator"" _cs() -> std::integer_sequence<Char, Cs...> {
    return {};
}

Usage is then something like:

log.critical("My format {}"_cs, value); // gcc extension
log.critical(MAKE_SEQUENCE("My format {}"), value);

Demo with gcc extension.
Demo with MACRO.