Apply the specified visitor to this variant by passing the value this variant currently holds to the visitor object's operator(), and return the value returned by the visitor. If this variant is unset, a default constructed bslmf::Nil is passed to the visitor. Note that this method is selected only if the template parameter type VISITOR defines a typedef of ResultType in its public interface. Also note that this method is defined inline to work around a Windows compiler bug with SFINAE functions.
Declared in <bdlb_variant.h>
template<class VISITOR>
VISITOR::ResultType
apply(VISITOR& visitor) const
requires Variant_ReturnValueHelper<VISITOR>::value == 1;