Unnecessary virtual specifier on destructor in final class hll_sketch_alloc

[devillove084]

Description When compiling code that includes and uses the hll_sketch class with the Clang compiler, a -Wunnecessary-virtual-specifier warning is generated. The warning points to the destructor of the hll_sketch_alloc class, which is marked as virtual while the class itself is declared as final.

Virtual method '~hll_sketch_alloc' is inside a 'final' class and can never be overridden [-Wunnecessary-virtual-specifier]