Can't create std::ofstream variable despite including <iostream>

I am getting an ofstream error in C++, here is my code

#include <iostream>

int main() {
    std::ofstream myfile;
    myfile.open("example.txt");
    myfile << "Writing this to a file.\n";
    myfile.close();
    return 0;
}

error from Dev-C++ 10

C:\devp\main.cpp aggregate `std::ofstream OutStream' has incomplete type and cannot be defined