C++: Can I define vector within another vector?

Question

In C/C++ pure array, you can define an array within another array, for example:

int a[5];
int *p = &a[1];

So that if you change p[0] it would affect a[1] so on and so forth. Is there similar mechanism in vector?

vector<int> a(5, 0);
?

Answer 1

Your first example

int a[5];
int *p = &a[1];

does not define an array within another array, it only creates a pointer to an element inside the exisiting array. For standard containers like std::vector the same can be achieved by using iterators:

vector<int> a(5, 0);           // a = [0, 0, 0, 0, 0]
auto iterator = a.begin() + 1; // "points" to a[1]
*iterator = 2;                 // a = [0, 2, 0, 0, 0]

Iterators are widely used in the standard library and the go-to-solution to provide access to (sub-)ranges of containers.

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If you can use C++20 or later you might want to look at std::span instead.

Answer 2

The exact same code int *p = &a[1]; works if a is a vector too.

But it's really not recommended. it's even worse than with the array because a vector can reallocate, and your pointer will be invalidated.