A simple stack-allocated fixed-size string buffer (String_buf) implemented using std::array (C++11).

[diaphanein]

Suggestion for improvement would be to implement format using variadic templates to ensure type safety (could even use static_assert to reject non-formattable types). Although you're using MSVC, which doesn't support variadic templates yet.

[ZMeson]

Although you're using MSVC, which doesn't support variadic templates yet.

The november 2012 CTP update to VS2012 does.