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  1. #include <iostream>
  2. #include <map>
  3. #include <string>
  4. #include <string.h>
  5. #include <functional>
  6.  
  7. using namespace std;
  8.  
  9. // Define comparator  functor:
  10. struct functor_test
  11. {
  12. 	bool operator()(string const & lhs, string const & rhs)
  13. 	{
  14. 		return strncmp(lhs.c_str(), rhs.c_str(), 4) < 0;
  15. 	}
  16. };
  17.  
  18. // Define comparator  function:
  19. bool function_test(string const & lhs, string const & rhs)
  20. {
  21. 	return strncmp(lhs.c_str(), rhs.c_str(), 4) < 0;
  22. }
  23.  
  24. int main() {
  25. 	// Define comparator  lambda:
  26. 	auto lambda_test = [](string const & lhs, string const & rhs)
  27. 	{
  28. 		return strncmp(lhs.c_str(), rhs.c_str(), 4) < 0;
  29. 	};
  30.  
  31. 	// These are all valid ways to declare the key comparator :
  32.  
  33. 	//As a functor:
  34. 	//  map<string, string, functor_test>                                 map;
  35.  
  36. 	//As a function using a function reference type:
  37. 	//  map<string, string, bool(&)(string const&, string const&)>        map(function_test);
  38.  
  39. 	//As a function using a function pointer type:
  40. 	//  map<string, string, bool(*)(string const&, string const&)>        map(function_test);
  41.  
  42. 	//As a function using a function class type wrapper:
  43. 	//  map<string, string, function<bool(string const&, string const&)>> map(function_test);
  44.  
  45. 	//As a predefined lambda:
  46. 	//  map<string, string, decltype(lambda_test)>                        map(lambda_test);
  47.  
  48. 	//As a predefined lambda using a function class type wrapper:
  49. 	//  map<string, string, function<bool(string const&, string const&)>> map(lambda_test);
  50.  
  51. 	//As a lambda using a function class type wrapper:
  52. 	map<string, string, function<bool(string const&, string const&)>>   map(
  53. 		[](string const & lhs, string const & rhs)
  54. 		{
  55. 			return strncmp(lhs.c_str(), rhs.c_str(), 4) < 0;
  56. 		});
  57.  
  58. 	map["test"] = "Foo";
  59. 	map["blah"] = "Drei";
  60. 	map["fayh"] = "Najh";
  61.  
  62. 	std::cout << map.find("test123")->second << endl;    // Should output 'Foo'
  63. 	std::cout << map.find("test_2165")->second << endl;  // Should output 'Foo' as well
  64. 	if (map.find("tes") == map.end())
  65. 	{
  66. 		cout << "Not found" << endl;
  67. 	}// Key not found
  68. 	std::cout << map.find("fayh_TK_Ka")->second << endl; // 'Najh'
  69.  
  70. 	return 0;
  71. }
Success #stdin #stdout 0s 3476KB
comments (?)
stdin 
Standard input is empty
stdout 
Foo
Foo
Not found
Najh
https://ideone.com/sg4sET
How to define comparator for STL containers
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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