Thread: STL map comparing keys

Code:

#include <iostream>
#include <string>
#include <map>
#include <cstdio>
using namespace std;

   struct CStrCmp : public binary_function <const char*, const char*, bool>
   {
      bool operator()(
         const char* Left, 
         const char* Right
      ) const
      {
          return (strcmp(Left, Right) < 0);
      }
   };

int main(void)
{
	map<char*, string, CStrCmp> str_map;

	char hello_str[] = "Hello";
	char test_str[]  = "Test";
	char boo_str[]   = "boo!!";

	str_map["Hello"] = string("Goodbye");
	str_map["Test"] = string("Another Test");
	str_map[boo_str] = string("You have been booed!");

	cout << str_map["Hello"] << endl;
	cout << str_map["Test"]  << endl;
	cout << str_map["boo!!"] << endl;

	cout << str_map[hello_str] << endl;
	cout << str_map[test_str]  << endl;
	cout << str_map[boo_str]   << endl;

	cout << str_map.count(hello_str) << endl;
	cout << str_map.count(test_str)  << endl;
	cout << str_map.count("boo!!")   << endl;

	cout << str_map.count("Failure Test") << endl;

	cin.get();
	return 0;
}

I figured this out by noting that the default compare function is documented as less<Key>, then looking up the less class and basically copying it.

>> You can also pass a predicate as the first constructor argument. <<

Could you elaborate on that? I looked up what a "predicate" is, but I can't figure out where one would use it in this context. The first template argument to map seems to expect a type. Thanks.