Expected unqualified-id before '{' [closed]

Currently getting an 'Expected unqualified-id' error before '{' when trying to overload the + operator in the implementation file. I'm guessing this is also causing the 'Function 'operator+' is not implemented' error I'm getting in the header as well. I'm not quite sure what's gone wrong here and how I would fix it.

Header file:

#ifndef BIGINT_BIGINT_H
#define BIGINT_BIGINT_H

#include <iostream>

class Bigint {

    public:
        // the input operator
        friend std::istream& operator>> (std::istream& in, Bigint& n);

        // the output operator
        friend std::ostream& operator<< (std::ostream& out, const Bigint& n);

        // the + operator
        friend Bigint operator+ (const Bigint& n1, const Bigint& n2);


    private:
        int digits [256]; // array number will be stored in
        int size; // size of number

};


#endif //BIGINT_BIGINT_H

Implementation file:

#include "Bigint.h"
#include <iostream>


std::istream& operator>> (std::istream& in, Bigint& n) {

    char number[256];

    in.get(number, 256, ' ');

    int size = 0;

    while(number[size] != '\0'){
        size++;
    }

    for(int i = 0; i < size; i++){
        n.digits[i] = number[size - i - 1] - '0';
    }

    n.size = size;

    return in;
}


std::ostream& operator<< (std::ostream& out, const Bigint& n) {

    for(int i = 0; i < n.size; i++){
        out << (n.digits[n.size - i -1]);
    }

    return out;
}


Bigint operator+ (const Bigint& n1, const Bigint& n2); {

    for (int i = 0; i < n2.size; i++){
        int sum = n1.digits[i] + n2.digits[i];

        if (sum > 9) {
            sum = sum % 10;
            n1.digits[i] = sum;
            n1.digits[i + 1] += 1;
        }
        else {
            n1.digits[i] = sum;
        }
    }

    return n1;
}

Thanks.