C++: Making a global ofstream variable

Question

I want to open an output file in main.cpp, then write to it in another file calculate.cpp.

main.cpp:

#include main.hpp

using namespace std;

int main() {
    outputfile.open("output.txt");
}

using the global variable from the header file main.hpp

extern std::ofstream outputfile;

Then write to it in another file calculate.cpp

#include main.hpp

void calculate() {
    outputfile << "write this to the external file" << endl;
}

When I do this, I get the error

undefined reference to 'outputfile' in main.cpp
undefined reference to 'outputfile' in calculate.cpp

I'm working on a large code that has a premade make file, so I don't think proper linking is the issue.

Answer 1

You haven't defined outputfile anywhere. The line extern std::ofstream outputfile; declares the variable, but nothing actually defines storage for it. You need the following line in either main.cpp or calculate.cpp

std::ofstream outputfile;