CODEWITHSUNDEEP
c++pointersnullpointerexception
Ammar
Ammar

Reputation: 102

How to return null pointer for list member c++?

I need to return pointer for a member in a list, and if the required member is not there I get a null pointer (or any other indication)

list<LINKS>::iterator find_link(list<LINKS> &link, char* ID)
{
    list<LINKS>::iterator link_index;
    LINKS link_i;

    link_index = link.begin();
    for (int i = 0; i < link.size(), i++)
    {
        link_i = *link_index;

        if (!strcmp(link_i.ID, ID)) { return link_index; };

        link_index++;

    };
    cout << "Node outsession does not exist " << ID;
    return nullptr; // ERROR (nullptr is not the same type as list<LINKS>::iterator)
};

I added the error message (cout << "Node outsession does not exist " << ID;) to indicate if the link is not found, but I need to indicate that for the caller function.

How can I do that ? Thanks

Upvotes: 0

Views: 658

Answers (2)

Jay Miller
Jay Miller

Reputation: 2234

nullptr is not an iterator, so you can't return it in a function that returns an iterator value

std::find(begin(), end(), value) returns an iterator and returns end() if the value is not found. To check it you say:

std::list<int> l;
...
auto found = std::find(l.begin(), l.end(), 6); // returns an std::list::iterator
if (found == l.end())
{
    std::cout << "Bogus!" << std::endl;
}
...

There are other examples in which the container find returns a value, and uses a special value for "not found"

std::string s = "go to the store";
auto found = s.find('x'); // returns a size_t index into the string
if (found == std::string::npos)
{
    std::cout << "Bogus!" << std::endl;
}

Returning nullptr would make sense if your function returned a pointer. In that case, nullptr would be a reasonable sentinel value for not found.

Upvotes: 2

OfNothing
OfNothing

Reputation: 600

The normal accepted way to return 'value not found' when searching a list is to return an iterator pointing to the end of the list. In this case link.end()

This is exactly how the std::find algorithm from <algorithm> behaves.

In your calling function test the returned iterator as follows:

list<LINKS>::iterator itr = find_link(list, "Funyuns");
if( itr == list.end() ) {
   std::cout << "You are all out of Funyons :-(" << std::endl;
}

Upvotes: 1

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