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I have a std::vector with n elements. Now I need to pass a pointer to a vector that has the last n-1 elements to a function.
For example, my vector<int> foo contains (5,2,6,87,251). A function takes vector<int>* and I want to pass it a pointer to (2,6,87,251).
Can I just (safely) take the iterator ++foo.begin(), convert it to a pointer and pass that to the function? Or use &foo[1]?
UPDATE: People suggest that I change my function to take an iterator rather than a pointer. That seems not possible in my situation, since the function I mentioned is the find function of unordered_set<std::vector*>. So in that case, is copying the n-1 elements from foo into a new vector and calling find with a pointer to that the only option? Very inefficient! It's like Shlemiel the painter, especially since i have to query many subsets: the last n-1, then n-2, etc. elements and see if they are in the unordered_set.
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Unfortunately, there is no way to convert from an iterator directly to a pointer without dereferencing the iterator ( it[0] and &*it both dereference the iterator). As a result, the only way to convert iterators to pointers that is able to handle all valid iterators requires having access to the vector in question.
An iterator is an object (like a pointer) that points to an element inside the container. We can use iterators to move through the contents of the container. They can be visualized as something similar to a pointer pointing to some location and we can access the content at that particular location using them.
Use an iteratorAn iterator can be generated to traverse through a vector. vector<int>::iterator iter; An iterator is used as a pointer to iterate through a sequence such as a string or vector . The pointer can then be incremented to access the next element in the sequence.
here it is, obtaining a reference to the coresponding pointer of an iterator use :
example:
string my_str= "hello world"; string::iterator it(my_str.begin()); char* pointer_inside_buffer=&(*it); //<-- [notice operator * returns a reference so doing & on a reference will give you the address].
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That seems not possible in my situation, since the function I mentioned is the find function of
unordered_set<std::vector*>.
Are you using custom hash/predicate function objects? If not, then you must pass unordered_set<std::vector<int>*>::find() the pointer to the exact vector that you want to find. A pointer to another vector with the same contents will not work. This is not very useful for lookups, to say the least.
Using unordered_set<std::vector<int> > would be better, because then you could perform lookups by value. I think that would also require a custom hash function object because hash does not to my knowledge have a specialization for vector<int>.
Either way, a pointer into the middle of a vector is not itself a vector, as others have explained. You cannot convert an iterator into a pointer to vector without copying its contents.
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