fmt() macro in format_to

[eglimi]

The fmt() macro is very useful to check if arguments are valid. As far as I can see, this is only possible for format. E.g. (from the documentation):

std::string s = fmt::format(fmt("{:d}"), "foo"); // Produces compiler error

I would like to be able to use the macro also in format_to, in order to avoid allocations from std::string. For example

fmt::memory_buffer buf;
fmt::format_to(buf, fmt("{:d}"), "foo");

Is this planned, or do I miss something?

Thank you for your support and the great work on fmtlib!

[vitaut]

Is this planned, or do I miss something?

It is planned, I just need to figure out how to do it in a clean way without proliferation of overloads.

[Milerius]

<3.

But let me tell you, there is worst from a microsoft library:

// The 'U' macro can be used to create a string or character literal of the platform type, i.e. utility::char_t.
// If you are using a library causing conflicts with 'U' macro, it can be turned off by defining the macro
// '_TURN_OFF_PLATFORM_STRING' before including the C++ REST SDK header files, and e.g. use '_XPLATSTR' instead.
#define U(x) _XPLATSTR(x)

use an internal token like underscore or something so people can name a variable fmt, or a member !

[vitaut]

@Milerius, you can easily disable the fmt macro by defining FMT_NO_FMT_STRING_ALIAS.

[Milerius]

@vitaut Thank's !

[vitaut]

Fixed in ffe414c. Now your example produces an expected compile-time error:

include/fmt/format.h:2312:27: error: constexpr variable 'invalid_format' must be initialized by a constant expression
  FMT_CONSTEXPR_DECL bool invalid_format =
                          ^
include/fmt/format.h:3532:13: note: in instantiation of function template specialization 'fmt::v5::internal::check_format_string<char [4], S>' requested here
  internal::check_format_string<Args...>(format_str);
            ^
test.cc:5:6: note: in instantiation of function template specialization 'fmt::v5::format_to<S, char [4], 500>' requested here
fmt::format_to(buf, fmt("{:d}"), "foo");
     ^
include/fmt/format.h:1418:13: note: non-constexpr function 'on_error' cannot be used in a constant expression
    handler.on_error("invalid type specifier");
            ^

Also refactored the code so that it's possible to handle both compile-time and dynamically checked format strings in one function template.