关于map容器的迭代器深入学习

#include <iostream>
#include <map>
using namespace std;
int main() {
  std::map<int, std::string> myMap = {{1, "one"}, {2, "two"}, {3, "three"}};

  auto itBegin = myMap.begin();  // 迭代器指向开头位置
  std::cout << itBegin->first << ": " << itBegin->second << std::endl;
  --itBegin;
  if (itBegin == myMap.end()) {
    std::cout << "111" << std::endl;
  } else {
    std::cout << itBegin->first << ": " << itBegin->second << std::endl;
  }
  --itBegin;
  if (itBegin == myMap.end()) {
    std::cout << "222" << std::endl;
  } else {
    std::cout << itBegin->first << ": " << itBegin->second << std::endl;
  }
  --itBegin;
  if (itBegin == myMap.end()) {
    std::cout << "333" << std::endl;
  } else {
    std::cout << itBegin->first << ": " << itBegin->second << std::endl;
  }
  --itBegin;
  if (itBegin == myMap.end()) {
    std::cout << "444" << std::endl;
  } else {
    std::cout << itBegin->first << ": " << itBegin->second << std::endl;
  }
  --itBegin;
  if (itBegin == myMap.end()) {
    std::cout << "555" << std::endl;
  } else {
    std::cout << itBegin->first << ": " << itBegin->second << std::endl;
  }
  return 0;
}

1: one
111
3: three
2: two
1: one
555