Can I get index directly from pointer of vector element in C++?

Question

I have a big vector container that holds around 300.000 object. Also I have pointers to these objects.

Are there any fast way to get index of object in vector with using pointer?

Answer 1

Since vectors are organized sequentially, you can get an index by subtracting pointer to initial element from the pointer to element in question:

std::vector<MyObject> vect;
MyObject *ptrX = ... // Pointer to element in question
ptrdiff_t index = ptrX - &vect[0];

Answer 2

Iterator header should be useful in that case.

Let's assume you have something like:

using Vector = std::vector<Foo>;
using Iterator = Vector::iterator;

Vector big_vector;

And now your have an iterator to an object:

Iterator p_obj = get_Theobj(big_vector);

The the index could be easily get with distance:

auto index = std::distance(big_vector.begin(), p_obj);
// Note: index's type is a `difference_type` aka ptrdiff_t (usually signed integer).

The powerful of using that approach is the versatility. Indeed, it works with "C-like vector", std::array, std::list, as well.