Automatically expanding the message with runtime data

[CsatiZoltan]

There are already questions on how to add at runtime the value of a variable (in my case an int) to the log pattern, see e.g. #2546. It seems to be that it is not supported, and the recommended way is to add it to the message: #2303 (comment).

I don't want to enforce the user to pass this value manually. Instead, I was thinking of writing a function that makes this automatically. Example: instead of writing

spdlog::debug("{0} blabla {1} blabla", variable1, variable2);

the user writes

mylogger::debug("{0} blabla {1} blabla", variable1, variable2);

which in the background turns the logging into

spdlog::debug("[{2}]: {0} blabla {1} blabla", variable1, variable2, variable3); // `variable3` fetched at runtime

Here is my debug function:

namespace mylogger {
template <typename... Args>
inline void debug(spdlog::format_string_t<Args...> fmt, Args &&...args) {
    auto variable3 = fetch_variable();
    spdlog::debug(fmt, std::forward<Args>(args)...); // somehow add variable3 to `fmt` and `args`
};
}

Could you help me in expanding the argument list to put variable3 in it? Thank you.

[tt4g]

Sounds like you want to do the same thing with the following code

Compiler Explorer: https://godbolt.org/z/KM6dWEnzx

#include <utility>
#include <iostream>

int add(int lhs, int rhs)
{
    return lhs + rhs;
}

template <typename... Args>
int add(Args &&...args)
{
    int rhs = 100;
    return add(std::forward<Args>(args)..., rhs);
}

int main(void) {
    std::cout << add(10);
    return 0;
}

Isn't this enough?

 namespace mylogger {
 template <typename... Args>
 inline void debug(spdlog::format_string_t<Args...> fmt, Args &&...args) {
     auto variable3 = fetch_variable();
-    spdlog::debug(fmt, std::forward<Args>(args)...); // somehow add variable3 to `fmt` and `args`
+    spdlog::debug(fmt, fmt::runtime(std::forward<Args>(args))..., fmt::runtime(std::move(variable3))); // somehow add variable3 to `fmt` and `args`
 };
 }

See also (`fmt::runtime): #3215, #2430

[CsatiZoltan]

It doesn't compile

[tt4g]

Oh, that cannot be accomplished with a single template like the one you have provided.
I don't know how to achieve this with fmt.
You should ask in the fmt library repository.

[tt4g]

The macro allows string literals to be concatenated.
However, it is not an easy way to figure out.

https://godbolt.org/z/v7GP4oMMj

#include <string>
#include "spdlog/spdlog.h"


int fetch_var() {
    return 1;
}

namespace logger {

template <typename... Args>
inline void debug(spdlog::format_string_t<Args...> fmt, Args &&...args) {
    int variable3 = fetch_var();
    spdlog::debug(fmt, std::forward<Args>(args)..., variable3);
};

}

#define LOG_DEBUG(FORMAT, ...) logger::debug((FORMAT " {}"), __VA_ARGS__)


int main() {
    auto variable1 = "12";
    auto variable2 = "3";
    LOG_DEBUG("{}-{}", variable1, variable2);
}

[CsatiZoltan]

Thank you for the solution.
Some remarks:

• If I put numbers in the curly braces, I get the following error message

  [*** LOG ERROR #0001 ***] [2025-01-29 17:54:16] [] cannot switch from manual to automatic argument indexing
  

• The macro works not only for string literals, but for ints as well:

  auto variable1 = 12;
  auto variable2 = 3;

• Currently, it fails when I simply pass a string, e.g. LOG_DEBUG("No extra arguments passed")
• Can you contemplate a pure-template solution, i.e. avoiding macros?
• Is the __VA_ARGS__ variadic macro portable across compilers?
• Can be add coloring to the automatically passed variable variable3?

[tt4g]

If I put numbers in the curly braces, I get the following error message

Macros cannot support this.
Ask in the fmt repository if that is possible.

Currently, it fails when I simply pass a string, e.g. LOG_DEBUG("No extra arguments passed")

Add macros and template functions to match the number of arguments.

Can you contemplate a pure-template solution, i.e. avoiding macros?

Changing string literals is impossible with simple templates.

Is the VA_ARGS variadic macro portable across compilers?

Yes.
See: https://en.cppreference.com/w/cpp/preprocessor/replace

Can be add coloring to the automatically passed variable variable3?

Cannot (spdlog), spdlog only supports log formats (%^ and %$): https://github.com/gabime/spdlog/wiki/3.-Custom-formatting
fmt can add a color as an argument, but it is for print only: https://fmt.dev/11.1/api/#terminal-colors-and-text-styles

[tt4g]

fmtlib/fmt#4333

[CsatiZoltan]

fmtlib/fmt#4333

Yes, I opened that issue as you recommended asking it in the fmt project, not in spdlog.

Macros cannot support this. Ask in the fmt repository if that is possible.

No problem. In fact, I don't find it crippling to specify the arguments in the order of appearance.

fmt can add a color as an argument, but it is for print only: https://fmt.dev/11.1/api/#terminal-colors-and-text-styles

If fmt implements it in fmt::format, spdlog will be able to display in color?