Question

So I’m having a rather tumultuous conversion to C++ from Java/C#. Even though I feel like I understand most of the basics, there are some big fat gaping holes in my understanding.

For instance, consider the following function:

Fruit&
FruitBasket::getFruitByName(std::string fruitName)
{
    std::map<std::string,Fruit>::iterator it = _fruitInTheBascit.find(fruitName);
    if(it != _fruitInTheBascit.end()) 
    {
        return (*it).second;
    }
    else
    {
           //I would so love to just return null here
    }

}

Where _fruitsInTheBascit is a std::map<std::string,Fruit>. If I query getFruitByName("kumquat") you know it’s not going to be there – who eats kumquats? But I don’t want my program to crash. What should be done in these cases?

P.S. tell me of any other stupidity that I haven’t already identified.

Answer 1

There is no such thing in C++ as a null reference, so if the function returns a reference, you can’t return null. You have several options:

1. Change the return type so that the function returns a pointer; return null if the element is not found.

2. Keep the reference return type but have some sort of “sentinel” fruit object and a return a reference to it if the object is not found.

3. Keep the reference return type and throw an exception (e.g., FruitNotFoundException) if the fruit is not found in the map.

I tend to use (1) if a failure is likely and (3) if a failure is unlikely, where “likely” is a completely subjective measure. I think (2) is a bit of a hack, but I’ve seen it used neatly in some circumstances.

As an example of an “unlikely” failure: in my current project, I have a class that manages objects and has a function is_object_present that returns whether an object is present and a function get_object that returns the object. I always expect that a caller will have verified the existence of an object by calling is_object_present before calling get_object, so a failure in this case is quite unlikely.