write to ostream inside of function with c++

consider the following function:

void w开发者_如何学JAVAriter(ofstream &output) {
output << "a string to write" << endl;

}

but when i try to call this, i get an error that '<<' is not defined. how can I fix this?

Add #include <fstream> or change ofstream to ostream

Most likely you lack an include, so that ofstream is only forward declared.