In C and C++, you can't assign an array to another array: you have to individually copy each element of the array from one to another. As a C-style string (which is what you're using) is simply an array of characters, you can't do this:
file_in_name = "test.txt";
Instead, you have to do this:
strncpy(file_in_name, "test.txt", 79);
You may have been confused because the first way of doing this is allowed when you declare the variable. It's a quirk of C - you can only do it then.