Why isn't it an error if the arguments are more than required in std::format?

Question

The cppref page on std::format says:

It is not an error to provide more arguments than the format string requires:

// OK, produces "Hello world!"
std::format("{} {}!", "Hello", "world", "something"); 

Since std::format has a compile-time check to see if fmt and arguments mismatch, why isn't the example code above taken as an error?

What's the rationale behind?

Answer 1

A couple of reasons.

1. You can use the same argument any number of times in the format string:

    std::fomat("{1},{0},{1}", x, y);
   

   There is no reason to exclude mentioning it zero times.

2. It actually comes handy in providing localized strings:

    std::vformat(get_string("Preheat the oven to {} degrees"), temp, temp*9/5+32);
   

   The string returned by get_string would contain either {0} or {1}.

Note this has nothing to do with having or not having compile-time or run-time checks. It is (should be) either considered an error or not, regardless of which checks are performed when.