Return the index of the next-unused argument. If the indexing mode is unset, the indexing mode is set to e_AUTOMATIC. If the indexing mode is e_MANUAL, caused by a previous call to check_arg_id then an exception of type format_error is thrown. If there are no remaining unused arguments then an exception of type format_error is thrown.
Declared in <bslfmt_formatparsecontext.h>
constexpr
size_t
next_arg_id();