what does vector.size mean?

Question

I wonder about this code

vector<pair<int,int>> map;

std::cout << "hello"<< std::endl;

map.push_back(make_pair(1,2));
map.push_back(make_pair(3,4));
map.push_back(make_pair(5,6));

map.resize(0);

std::cout << map[0].first
            << map[0].second << std::endl;
std::cout << map[2].first << std::endl;

std::cout << map.size() << std::endl;
std::cout << map.capacity() << std::endl;

I resize the map to size 0, but the result shows like this:

hello
12
5
0
4

Why do I get this?

The [`[]` operator](http://en.cppreference.com/w/cpp/container/vector/operator_at) have no bounds checking. After setting the size to zero all indexes are out of bounds and you will have [*undefined behavior*](http://en.cppreference.com/w/cpp/language/ub). — Some programmer dude, Feb 20 '18 at 07:05
Possible duplicate of [Vector going out of bounds without giving error](https://stackoverflow.com/questions/16620222/vector-going-out-of-bounds-without-giving-error) — Tadeusz Kopec for Ukraine, Feb 20 '18 at 08:03
you set the size to 0, the you print the size, it is 0... I would understand if you are surprised by the rest of the code, but whats wrong with the size? — 463035818_is_not_an_ai, Feb 20 '18 at 08:09
btw calling a `std::vector` as `map` is rather confusing, as there is also `std::map` — 463035818_is_not_an_ai, Feb 20 '18 at 08:10
Possible duplicate of [size vs capacity of a vector?](https://stackoverflow.com/questions/6296945/size-vs-capacity-of-a-vector) — eesiraed, Feb 21 '18 at 01:22

score 0 · Answer 1 · answered Feb 20 '18 at 08:20

Size of the vector (objects it contains) is not necessarily equal to its capacity (storage space allocated for it)

Looking at http://www.cplusplus.com/reference/vector/vector/size/, you can notice this statement: "This is the number of actual objects held in the vector, which is not necessarily equal to its storage capacity."

If you check you can see the following: http://www.cplusplus.com/reference/vector/vector/capacity/ "This capacity is not necessarily equal to the vector size. It can be equal or greater, with the extra space allowing to accommodate for growth without the need to reallocate on each insertion."

I hope this answers your question

score 0 · Answer 2 · answered Feb 21 '18 at 01:17

Besides the thing about the vector capacity in the other answer, accessing out of bounds indexes on vectors with the bracket operator (instead of at(), which provides bound checking) produces undefined behavior.

In other words, the behavior is not defined in the standard and can change based on things like your compiler. In your case, it apparently did not crash and outputted the values even after they're no longer in the vector.

Needless to say, you want to make sure your program is free of undefined behavior.

warashi nguyen · Answer 3 · 2023-07-07T04:06:50.403


#include <iostream>
#include <vector>
#include <utility>

int main() {
    std::vector<std::pair<int, int>> map;

    std::cout << "hello" << std::endl;

    map.emplace_back(1, 2);
    map.emplace_back(3, 4);
    map.emplace_back(5, 6);

    map.resize(0);

    if (!map.empty()) {
        std::cout << map[0].first << map[0].second << std::endl;
        std::cout << map[2].first << std::endl;
    } else {
        std::cout << "Vector is empty" << std::endl;
    }

    std::cout << map.size() << std::endl;
    std::cout << map.capacity() << std::endl;

    return 0;
}

declare a vector of pair<int,int> named map.
push three pairs of integers into the vector using push_back. resize the vector to size 0 using resize(0).
Try to access the elements of the vector using the [] operator: map[0] and map[2].
Output the size of the vector using size() and its capacity using capacity().

As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers [in the help center](/help/how-to-answer). — Community, Jul 04 '23 at 08:15

what does vector.size mean?

3 Answers3