Question

I have a vector and it is going to hold 10 objects maximum. When I create the vector I believe it creates 10 “blank” objects of the class it will hold. Therefore I have a problem because my code relies on checking that a vector element is null and obviously it is never null.

How can I check whether a vector object contains an element I inserted, or one of the default constructor “blank” objects upon initialisation?

Is there a technique round this?

(I need to check for null because I am writing a recursive algorithm and the termination point, when the recursive function returns is when an object in the vector is null)

Answer 1

An instance of a class cannot be null. Only a pointer.

You do, however, have size() that you can use.

typedef stdd::vector<SomeClass> vec;
//define some vec, v
for (vec::size_type i = 0, s = vec.size(); i < s; ++i) {
    //do something with v[i]
}

With a recursive function you could use this idea by passing along a maximum index.

void recursiveFunc(vec& v, vec::size_type s);

Then when checking your condition to recurse, you would need to check “am I at the end of the vector?”

Alternatively, instead of working on indexes, you could use iterators:

template <typename Iterator>
void recursiveFunc(Iterator begin, const Iterator& end);

If done correctly (and if possible in your situation), this could decouple your manipulation from being aware of the underlying data being stored in a vector.

The loop to go over the vector would then look like:

while (begin != end) {
    //do something with *begin
    ++begin;
}