程序调试小bug

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一段简单的代码：

std::ifstream input (fileName);   

while (input) {
  string s;
  input >> s;
  cout << s << endl;
};

编译报错：

error C2679: binary '>>' : no operator found which takes a right-hand operand of type 'std::string' (or there is no acceptable conversion)

原来是忘了#include <string>了