How to return nullptr equivalent from a C++ function whose return type is pair?

Question

pair<int, int> findEmpty(int bo[9][9], int n){
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            if(bo[i][j]==0){
                return (make_pair(i, j));
            }
        }
    }
    //return nullptr; 
}

If nullptr can't be returned, what are the alternatives?

Answer 1

Use an optional<T> type to represent potentially non-existent state.

If you're using C++17 this can be done with std::optional<T>, if you use Boost then boost::optional<T>, etc. You can even implement your own version of optional. The point is to use a semantic type that indicates that there may, or may not, be a value.

Using std::optional from C++17, your code becomes:

optional<pair<int, int>> findEmpty(int bo[9][9], int n){
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            if(bo[i][j]==0){
                return make_pair(i, j);
            }
        }
    }
    return nullopt; 
}

---

Although you could solve this by returning sentinel-like values such as a pair of (-1, -1), this is not a recommended practice. This forces checking for specific sentinel values on the caller, which can easily be missed or forgotten -- and may cause the bad sentinel value to find its way into other arithmetic or indirections that may never expect a negative value.

Using a semantic type to convey the nullability helps to prevent these sorts of errors.