c++ string formatting function requiring constant expression to be used inside a logging function

Question

I'm trying to create a logging function, that will write formatted string to a buffer. The function should automatically add source code location to the formatted string using std::source_location. The formatting function format_to requires the formating string is constant expression. The signature of the format_to function is simplified in the example.

#include <source_location>
#include <string_view>
#include <cstdint>

struct format_string_with_location {
  consteval format_string_with_location(const char* s, const std::source_location& l = std::source_location::current()) :
      str(s), loc(l)
      {}

  std::string_view str;
  std::source_location loc;
};

struct format_string {
  template<typename T>
  consteval format_string(const T& t) : str{t} {}

  std::string_view str;
};

template<typename...Args>
void format_to(format_string str, Args&&...) {}

template<typename... Args>
void debug(format_string_with_location format, const Args&... args)
{
    format_to("{} {}:", format.loc.file_name(), format.loc.line());
    format_to(format.str, args...);
}

int main() {
    debug("error: {}", 42);
}

Demo

format_to("error {}", 42);

works as expected, but

debug("error: {}", 42);

fails to compile with following error:

<source>: In instantiation of 'void debug(format_string_with_location, const Args& ...) [with Args = {int}]':
<source>:31:10:   required from here
<source>:27:14:   in 'constexpr' expansion of 'format_string(format.format_string_with_location::str)'
<source>:27:14: error: 'format' is not a constant expression
   27 |     format_to(format.str, args...);
      |     ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~

How to fix it?

Answer 1

Parameters are not constexpr (even in consteval functions, or with consteval type).

You might reorganize your code as follow though:

struct format_string {
  template<typename T>
  consteval format_string(const T& t) : str{t} {}

  std::string_view str;
};

struct format_string_with_location {
  consteval format_string_with_location(
     const char* s,
     const std::source_location& l = std::source_location::current()) :
      str(s), loc(l)
      {}

  format_string str;
  std::source_location loc;
};

Demo

You no longer construct a format_string with parameter.

Answer 2

consteval - "every potentially-evaluated call to the function must (directly or indirectly) produce a compile time constant expression."

You could change the format_string constructor to constexpr:

struct format_string {
    template <typename T>
    constexpr format_string(const T& t) : str{t} {}

    std::string_view str;
};

Then the following would work:

template <typename... Args>
void format_to(format_string str, Args&&... args) {
    std::cout << std::vformat(str.str, std::make_format_args(args...));
}

template <typename... Args>
void debug(format_string_with_location format, const Args&... args) {
    format_to("{} {}:", format.loc.file_name(), format.loc.line());
    format_to(format.str, args...);
}