explicit specifier — Explicit constructor specifier
function-specifier := explicit
The explicit
specifier can be
used with a constructor to prevent implicit type conversions. It is
permitted for any constructor but makes sense only for constructors
that can be called with a single argument. An explicit constructor can
be invoked from a declaration that uses function-like initialization
or from an explicit type cast but not from a declaration that uses
assignment-like initialization, nor from an implicit type cast.