Yet another Fibonacci number generator
Assuming you're using the current c++ standard, since you don't specify in your question.


Prefer stoi to atoi. atoi has issues.
Prefer standard int types: std::uint128_t instead of unsigned long long int
Create a type alias for your map type since you use it a lot.
Don't include unnecessary headers in the fibbonacci.h file. Include them in the implementation.
It would make more sense to implement your memoizing map statically in the function.
You should use the map count method rather than relying on [] returning zero for a missing key


Putting these together

using fibmap = std::map&lt;int, std::uint128_t&gt;;

std::uint128_t fibonacci(int seq) {

    static fibmap m = {{0,0}, {0,1}};
    if (!m.count(seq)){
        m[seq] = fibonacci(seq - 2) + fibonacci(seq - 1);
    }
    return m[seq];
}