Using the fmt Library in the MongoDB Server.

The fmt library is extensible, but to avoid user extensions interfering with each other we want to be disciplined in how we use the extension points. For now, please discuss extension use cases with the #server-programmability team.

Our mongo::StringData is usable wherever a string-like type can be used in fmt library calls. This was done by adding a special function to string_data.h. More types will be added as we figure out which to support.

Basic Usage:

s = fmt::format("{}, {}, {}, ... {}!", 1, '2', "three", kInf);

Note also that the functions in fmt/printf.h (printf, sprintf, fprintf) can replace most calls to the printf family of functions, with a safer and often faster alternative. These will throw if the argument types don't match the format specifiers, where printf would have just suffered undefined behavior.

The fmt::formatter<T> template can be specialized to support custom types, but this is advanced and not easy to get right. In any case, such hooks must be defined in the same place as the T to avoid one-definition-rule violations. Any type that can be put to a std::ostream can be written out with fmt via #include <fmt/ostream.h>, so many potential fmt::formatter specializations will be unnecessary.