清除字符串中非字母数字字符[C++]

 //http://www.cplusplus.com/forum/beginner/148876/ #include <iostream> #include <string>  #include <cctype>#include <algorithm>int main() {     const std::string str = "it. is? awesome!";    {         // simple         std::string a;         for (char c : str) if (std::isalnum(c)) a += c;         std::cout << a << 'n';     }     {         // complicated: after erasing, stay at the same position for the next char         // but cater to the erase having reduced the size of the string         std::string b = str;         for (std::size_t i = 0; i < b.size(); ++i)             while (!std::isalnum(b[i]) && i < b.size()) b.erase(i, 1);         std::cout << b << 'n';      }      {          // erase(iterator) returns the iterator following the erased character.          // http://en.cppreference.com/w/cpp/string/basic_string/erase          std::string c = str;          for (std::string::iterator iter = c.begin(); iter != c.end();)              if (!std::isalnum(*iter)) iter = c.erase(iter);              else ++iter; // not erased, increment iterator              std::cout << c << 'n';      }      {          // erase-remove idiom https://en.wikipedia.org/wiki/Erase-remove_idiom          std::string d = str;          d.erase(std::remove_if(d.begin(), d.end(), [](char c) { return !std::isalnum(c); }), d.end());          std::cout << d << 'n';       } }