Question

If I have the following:

vector<int> v(4,0);
vector<int>* p = &v;
int element = p[0];

Will element be the same value as v[0]? I’m getting confused here about the [] operator.

Answer 1

Short answer: no.

What you have is not even valid C++. p[0] is the same as *p, which is of type vector<int> and thus not convertible to int.

The []-notation merely suggests that you think of p as an array of vectors, and you are accessing the array’s first element, which is v. More generally, for any pointer p, the notation p[k] is identical to *(p + k) (in fact to a fault, as you can say a[5] and 5[a] interchangeably).

So if you really wanted to, you could write p[0][i] for the ith element of the vector, though it is more customary to just write (*p)[i] (parentheses needed for the correct precedence).

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_{When I first skimmed over the question, I thought you might be looking for some clever hack and wanted to know whether **(int**)(p) was equal to v[0]. That is indeed plausible, as the first element of the vector’s data structure is often the pointer to the vector’s internal buffer. Don’t use this at home.}