C + + How do your own Class A valid key type for Std::map?

In Java, if you want your own class to is a valid key type of the container, you just need to make it implement the Interf Ace "Comparable", and then it ' ll work properly.

How is the in C + +? I was wondering whether C + + had offered some kind of mechanism that acts like a interface in Java to enforce some "contra CT "to which a type must conform, such as Std::map, it's obvious its key type must is" comparable ".

How to ensure then the type indeed conform to the contract, through some kind of ' interface ' mechanism or what? Would the compiler does the work at compile time to check whether a type conform to the contract or not?

I looked up some C + + references in cppreference.com and cplusplus.com, I just couldn ' t find any useful information about W Hat contract std::map require on its key type.

By search on stackoverflow.com, I found this:http://stackoverflow.com/questions/19937260/ Use-user-defined-struct-as-the-key-for-map-c

So maybe I just need to define a operator< for that type?

Try it. Notice the main.cpp, if you remove the operator< on Box type. The compiler would complain when you use Box as a key type of std::map. So, the compiler does ensure that Box must conform to the contract defined by OPERATOR<.

Implement Std::map so it would offer the information to the Compiler:the Std::map::key_type must be a Typ E that have an overloaded operator< definition, which you must check for at compile time. (I ' m asking this question in stackoverflow.com, waiting for answer)

Main.cpp

1#include <iostream>2#include <map>3 4 using namespacestd;5 6 classBox {7Friend ostream&operator<< (Ostream &os,ConstBox &b);8FriendBOOL operator< (ConstBox &left,ConstBox &Right );9  Public:TenBox (intIDoubled); One~Box (); A Private: -     inti; -     DoubleD; the }; -  -Box::box (int_i,Double_d): I (_i), D (_d) {} -  +box::~Box () {} -  + BOOL operator< (ConstBox &left,ConstBox &Right ) A { at     return(Left.i <right.i); - } -  -ostream&operator<< (Ostream &os,ConstBox &b) - { -OS <<B.D; in     returnos; - } to  + intMain () - { theBox B1 (3,2), B2 (2,1), B3 (0,9); *Map<box,int>Bmap; $Bmap.insert (Pair<box,int> (B1,Ten));Panax NotoginsengBmap.insert (Pair<box,int> (B2,Ten)); -Bmap.insert (Pair<box,int> (B3,Ten)); the      for(Map<box,int>::iterator iter = Bmap.begin (); Iter! = Bmap.end (); ++iter) +     { Acout << Iter->first <<" "; the     } +cout <<Endl; -     return 0; $}

C + + How do your own Class A valid key type for Std::map?