Signature Description
struct MemUsage {
 size_t column_used_memory { 0 };
 size_t column_capacity_memory { 0 };
 size_t column_type_size { 0 };
 size_t index_used_memory { 0 };
 size_t index_capacity_memory { 0 };
 size_t index_type_size { 0 };

 template
 friend S &operator << (S &stream, const MemUsage &mu);
};
This struct holds the result of calling get_memory_usage() method on DataFrame.