Declared but not defined. Meta-function use only. The tag can be used to recover meta-information from an expression. Example: sizeof(f(expr).tag()) returns a different compile-time value depending on the type of the result of calling the f function but does not actually call the f function or the tag method at run-time. Note that f(expr)::VALUE or sizeof(f(expr)::Type) would be ill-formed and that f(expr).value is not a compile-time expression.