Write Then Read in One Object

Write a function int writeAndSum(const string& filename, const vector<int>& v) that uses ONE fstream object for both directions:
1. Open filename in ios::in | ios::out | ios::trunc mode.
2. Write every number in v to the file separated by single spaces, followed by a newline.
3. Move the read position back to the start with seekg(0).
4. Read the numbers back from the file with >> until reading fails, adding them up as you go.
5. Return the sum you read from the file.

The test prints the returned sum on its own line. For v = {1, 2, 3, 4, 5} it prints the output shown in Expected Output.

Do NOT write a main function.