Added a requirement for partial C++11 support, most importantly variadic templates and type traits, and dropped FMT_VARIADIC_* emulation macros. Variadic templates are available since GCC 4.4, Clang 2.9 and MSVC 18.0 (2013). For older compilers use {fmt} version 4.x which continues to be maintained and works with C++98 compilers.

Renamed symbols to follow standard C++ naming conventions and proposed a subset of the library for standardization in P0645R2 Text Formatting.

Implemented constexpr parsing of format strings and compile-time format string checks. For example

#include <fmt/format.h>

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

gives a compile-time error because d is an invalid specifier for strings (godbolt):

...
<source>:4:19: note: in instantiation of function template specialization 'fmt::v5::format<S, char [4]>' requested here
  std::string s = format(fmt("{:d}"), "foo");
                  ^
format.h:1337:13: note: non-constexpr function 'on_error' cannot be used in a constant expression
    handler.on_error("invalid type specifier");

Compile-time checks require relaxed constexpr (C++14 feature) support. If the latter is not available, checks will be performed at runtime.

Separated format string parsing and formatting in the extension API to enable compile-time format string processing. For example

struct Answer {};

namespace fmt {
template <>
struct formatter<Answer> {
  constexpr auto parse(parse_context& ctx) {
    auto it = ctx.begin();
    spec = *it;
    if (spec != 'd' && spec != 's')
      throw format_error("invalid specifier");
    return ++it;
  }

  template <typename FormatContext>
  auto format(Answer, FormatContext& ctx) {
    return spec == 's' ?
      format_to(ctx.begin(), "{}", "fourty-two") :
      format_to(ctx.begin(), "{}", 42);
  }

  char spec = 0;
};
}

std::string s = format(fmt("{:x}"), Answer());

gives a compile-time error due to invalid format specifier (godbolt):

...
<source>:12:45: error: expression '<throw-expression>' is not a constant expression
       throw format_error("invalid specifier");

Added iterator support:

#include <vector>
#include <fmt/format.h>

std::vector<char> out;
fmt::format_to(std::back_inserter(out), "{}", 42);

Added the format_to_n function that restricts the output to the specified number of characters (#298):

char out[4];
fmt::format_to_n(out, sizeof(out), "{}", 12345);
// out == "1234" (without terminating '\0')

Added the formatted_size function for computing the output size:

#include <fmt/format.h>

auto size = fmt::formatted_size("{}", 12345); // size == 5

Improved compile times by reducing dependencies on standard headers and providing a lightweight core API:

#include <fmt/core.h>

fmt::print("The answer is {}.", 42);

See Compile time and code bloat.

Added the make_format_args function for capturing formatting arguments:

// Prints formatted error message.
void vreport_error(const char *format, fmt::format_args args) {
  fmt::print("Error: ");
  fmt::vprint(format, args);
}
template <typename... Args>
void report_error(const char *format, const Args & ... args) {
  vreport_error(format, fmt::make_format_args(args...));
}

Added the make_printf_args function for capturing printf arguments ( #687, #694). Thanks @Kronuz (Germán Méndez Bravo).

Added prefix v to non-variadic functions taking format_args to distinguish them from variadic ones:

std::string vformat(string_view format_str, format_args args);

template <typename... Args>
std::string format(string_view format_str, const Args & ... args);

Added experimental support for formatting ranges, containers and tuple-like types in fmt/ranges.h (#735):

#include <fmt/ranges.h>

std::vector<int> v = {1, 2, 3};
fmt::print("{}", v); // prints {1, 2, 3}

Thanks @Remotion (Remo).

Implemented wchar_t date and time formatting (#712):

#include <fmt/time.h>

std::time_t t = std::time(nullptr);
auto s = fmt::format(L"The date is {:%Y-%m-%d}.", *std::localtime(&t));

Thanks @DanielaE (Daniela Engert).

Provided more wide string overloads (#724). Thanks @DanielaE (Daniela Engert).

Switched from a custom null-terminated string view class to string_view in the format API and provided fmt::string_view which implements a subset of std::string_view API for pre-C++17 systems.

Added support for std::experimental::string_view (#607):

#include <fmt/core.h>
#include <experimental/string_view>

fmt::print("{}", std::experimental::string_view("foo"));

Thanks @virgiliofornazin (Virgilio Alexandre Fornazin).

Allowed mixing named and automatic arguments:

fmt::format("{} {two}", 1, fmt::arg("two", 2));

Removed the write API in favor of the format API with compile-time handling of format strings.

Disallowed formatting of multibyte strings into a wide character target (#606).

Improved documentation ( #515, #614, #617, #661, #680). Thanks @ibell (Ian Bell), @mihaitodor (Mihai Todor), and @johnthagen.

Implemented more efficient handling of large number of format arguments.

Introduced an inline namespace for symbol versioning.

Added debug postfix d to the fmt library name (#636).

Removed unnecessary fmt/ prefix in includes (#397). Thanks @chronoxor (Ivan Shynkarenka).

Moved fmt/*.h to include/fmt/*.h to prevent irrelevant files and directories appearing on the include search paths when fmt is used as a subproject and moved source files to the src directory.

Added qmake project file support/fmt.pro (#641). Thanks @cowo78 (Giuseppe Corbelli).

Added Gradle build file support/build.gradle (#649). Thanks @luncliff (Park DongHa).

Removed FMT_CPPFORMAT CMake option.

Fixed a name conflict with the macro CHAR_WIDTH in glibc (#616). Thanks @aroig (Abdó Roig-Maranges).

Fixed handling of nested braces in fmt::join (#638).

Added SOURCELINK_SUFFIX for compatibility with Sphinx 1.5 (#497). Thanks @ginggs (Graham Inggs).

Added a missing inline in the header-only mode (#626). Thanks @aroig (Abdó Roig-Maranges).

Fixed various compiler warnings ( #640, #656, #679, #681, #705, #715, #717, #720, #723, #726, #730, #739). Thanks @peterbell10, @LarsGullik, @foonathan (Jonathan Müller), @eliaskosunen (Elias Kosunen), @christianparpart (Christian Parpart), @DanielaE (Daniela Engert), and @mwinterb.

Worked around an MSVC bug and fixed several warnings (#653). Thanks @alabuzhev (Alex Alabuzhev).

Worked around GCC bug 67371 (#682).

Fixed compilation with -fno-exceptions (#655). Thanks @chenxiaolong (Andrew Gunnerson).

Made constexpr remove_prefix gcc version check tighter (#648).

Renamed internal type enum constants to prevent collision with poorly written C libraries (#644).

Added detection of wostream operator<< (#650).

Fixed compilation on OpenBSD (#660). Thanks @hubslave.

Fixed compilation on FreeBSD 12 (#732). Thanks @dankm.

Fixed compilation when there is a mismatch between -std options between the library and user code (#664).

Fixed compilation with GCC 7 and -std=c++11 (#734).

Improved generated binary code on GCC 7 and older (#668).

Fixed handling of numeric alignment with no width (#675).

Fixed handling of empty strings in UTF8/16 converters (#676). Thanks @vgalka-sl (Vasili Galka).

Fixed formatting of an empty string_view (#689).

Fixed detection of string_view on libc++ (#686).

Fixed DLL issues (#696). Thanks @sebkoenig.

Fixed compile checks for mixing narrow and wide strings (#690).

Disabled unsafe implicit conversion to std::string (#729).

Fixed handling of reused format specs (as in fmt::join) for pointers (#725). Thanks @mwinterb.

Fixed installation of fmt/ranges.h (#738). Thanks @sv1990.