Feature request: support %s with numeric arguments

[avikivity]

We're (seastar, scylla) switching from boost::format to libfmt; but many of our format strings have %s for numeric arguments; this is handled by boost.

Could fmtlib be changed to tolerate %s for numeric arguments?

[vitaut]

Thanks for using the library!

It is possible to make fmt::printf and friends accept %s for numeric arguments, but I'd rather not diverge from std::printf too much unless there are strong reasons to do so. However, it is easy to define a custom printf argument formatter that implements the desired behavior:

#include "fmt/printf.h"

// A custom argument formatter that allows %s for integer values.
class CustomPrintfArgFormatter :
  public fmt::internal::BasicPrintfArgFormatter<CustomPrintfArgFormatter, char> {
public:
  typedef fmt::internal::BasicPrintfArgFormatter<CustomPrintfArgFormatter, char> Base;

  CustomPrintfArgFormatter(fmt::BasicWriter<char> &w, fmt::FormatSpec &s) : Base(w, s) {}

  template <typename T>
  void visit_any_int(T value) {
    if (this->spec().type_ == 's')
      this->spec().type_ = 'd';
    Base::visit_any_int(value); // The same can be done for floating-point values.
  }
};

FMT_FUNC int custom_printf(fmt::CStringRef format, fmt::ArgList args) {
  fmt::MemoryWriter w;
  fmt::internal::PrintfFormatter<char, CustomPrintfArgFormatter>(args).format(w, format);
  std::size_t size = w.size();
  return std::fwrite(w.data(), 1, size, stdout) < size ? -1 : static_cast<int>(size);
}
FMT_VARIADIC(int, custom_printf, fmt::CStringRef)

int main() {
  custom_printf("%s", 42); // Prints 42 instead of throwing an exception.
}

Currently this relies on internal APIs, but I can make them public. Will it work for you?

[avikivity]

Yes, it will. Perhaps an easier alternative for people migrating from boost is to have a variant compatible with boost::format, accessible via a different name.

(we had a very east migration because you boost::format calls were wrapped in a variadic function that generated the silly fmt % arg % arg syntax, and the only thing missing was %s for numbers, which I added as a local patch)

[vitaut]

PrintfFormatter and BasicPrintfArgFormatter are now public and documented in http://fmtlib.net/dev/api.html#_CPPv2N3fmt15PrintfFormatterE.

I'd be happy to accept a custom formatter that implements Boost Format compatibility if anyone is interested in contributing it. Unfortunately I don't have time to work on it myself.