01.explicit map ( const Compare& comp = Compare(),
02. const Allocator& = Allocator() );
//in the book the instruction said that it is creat an empty map, but I don't understand what is the " const Allocator&" means, especially the "&" means.
Also,the instruction did't tell me what is the comp means.
Generally, the map requires a compare and an allocator. There is a default but that can be overridden when the object is instantiated. Compare is used to ensure proper ordering of key/value pairs using the key. Allocator is what is used to dynamically allocate memory as the container grows.