open函数是否对传入的字符串值有某种限制？
ifstream file;
string filename = "output.txt";
file.open(filename);
我尝试使用字符串变量传递字符串值，但是当它尝试编译时，结果是......
agent.cpp:2:20: error: ofstream: No such file or directory
agent.cpp: In function ‘std::string readingline(std::string)’:
agent.cpp:11: error: aggregate ‘std::ifstream file’ has incomplete type and cannot be defined
agent.cpp: In function ‘int main()’:
agent.cpp:44: error: aggregate ‘std::ofstream writefile’ has incomplete type and cannot be defined
另一方面，当我只用“filename.txt”这样的引号传递一个字符串值时，它编译得很好并且运行正常。
ifstream file;
file.open("output.txt");
为什么会这样？
有没有办法解决这个问题？