Corrade/Containers/StringStl.h file new in Git master

STL std::string compatibility for Corrade::Containers::String and Corrade::Containers::StringView.

Including this header allows you to convert a Corrade::Containers::String / Corrade::Containers::StringView from and to std::string. A separate Corrade/Containers/StringStlView.h header provides compatibility with std::string_view from C++17, Corrade/Containers/StringStlHash.h then provides a std::hash specialization. See String STL compatibility and StringView STL compatibility for more information.