std::transform 转换大小写

#include <algorithm>
#include <iostream>

int main() {
  std::string a = "abc";
  std::string b = "XYZ";

  // 小写转大写，源和目的不同
  std::string c;
  std::transform(a.begin(), a.end(), std::back_inserter(c), ::toupper);
  
  // 小写转大写，源和目的相同
  std::transform(a.begin(), a.end(), a.begin(), ::toupper);

  // 大写转小写
  std::transform(b.begin(), b.end(), b.begin(), ::tolower);

  std::cout << a << std::endl;
  std::cout << b << std::endl;

  return 0;
}