CODEWITHSUNDEEP
c++pointersvector
Vim
Vim

Reputation: 1518

C++ pointers to consecutive vector elements are paradoxical

I want to find out the "distances" between locations of consecutive vector elements in the memory. So I try the following code

#include <cstdlib>
#include <iostream>
#include <vector>
using namespace std;

int main()
{
    vector<int> vec(3);
    for (int i = 0; i <= 1; i++)
    {
        cout << &vec[i+1] - &vec[i] << endl;
    }
    return 0;
}

The result are unsurprisingly two 1s. However, when I try printing the individual pointers:

vector<int> vec(3);
for (int i = 0; i <= 2; i++)
{
    cout << &vec[i] << endl;
}

The result is

01485048
0148504C
01485050

So the difference between two consecutive elements should be 4. Seemingly a contradiction to the previous result. What goes wrong here?

Upvotes: 0

Views: 139

Answers (3)

MSalters
MSalters

Reputation: 179981

Nothing particular to vector. C++ pointer arithmetic (and iterator arithmetic) is done in terms of elements. And indeed, vec[i+1] and vec[i] are one element apart.

One element here happens to be 4 bytes, which is what you see when printing the bit pattern of the pointers. You can double-check this with sizeof(vec[0])

Upvotes: 4

user7860670
user7860670

Reputation: 37600

You should cast pointers to uintptr_t so substraction will yield amount of bytes in between them:

cout << (reinterpret_cast< uintptr_t >(&vec[i+1]) - reinterpret_cast< uintptr_t >(&vec[i])) << endl;

Upvotes: 1

Pontifex
Pontifex

Reputation: 136

Calculating a difference between two pointers returns number of objects between them - that's why you cannot perform arithmetic on two pointers of different types. If you want to know the distance of bytes between them, either cast both pointers to char* or multiply the result of substraction by sizeof(your_type).

Upvotes: 1

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