有行代码看不懂,EventRecord.substr(12,4)=="1997"

有行代码看不懂,请指教EventRecord.substr(12,4)=="1997";
#include <string> 
#include <list> 
#include <algorithm> 
#include<iostream>
using namespace std;
 
class EventIsIn1997 { 
public: 
  bool operator () (string& EventRecord) { 
  // year field is at position 12 for 4 characters in EventRecord 
  return EventRecord.substr(12,4)=="1997"; //这行看不懂,请指教
  } 
}; 
 
int main (void) { 
  list<string> Events; 
 
  // string positions 0123456789012345678901234567890123456789012345 
  Events.push_back("07 January 1995 Draft plan of house prepared"); 
  Events.push_back("07 February 1996 Detailed plan of house prepared"); 
  Events.push_back("10 January 1997 Client agrees to job"); 
  Events.push_back("15 January 1997 Builder starts work on bedroom"); 
  Events.push_back("30 April 1997 Builder finishes work"); 
 
  list<string>::iterator EventIterator = find_if (Events.begin(), Events.end(), EventIsIn1997()); 
 
  // find_if completes the first time EventIsIn1997()() returns true 
  // for any object. It returns an iterator to that object which we 
  // can dereference to get the object, or if EventIsIn1997()() never 
  // returned true, find_if returns end() 
  if (EventIterator==Events.end()) { 
  cout << "Event not found in list" << endl; 
  } 
  else { 
  cout << *EventIterator << endl; 
  } 
} 


------解决方案--------------------
return EventRecord.substr(12,4)=="1997"; //这行看不懂,请指教 

取得字串 从第12个位置开始取得长度为4.

== 是判断是否等于 “1997”

不知道解释的是否清晰？？