Create a variant object holding the 0th alternative, which is value-initialized. If this variant is allocator-aware, the specified allocator is used to supply memory. This constructor participates in overload resolution only if the 0th alternative is default constructible.
Declared in <bslstl_variant.h>
template<class t_FIRST = t_HEAD>
requires std::is_default_constructible<t_FIRST>::value
variant(
std::allocator_arg_t,
allocator_type allocator);