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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../container/vector/back.html below:

std::vector<T,Allocator>::back - cppreference.com

reference back();

(1) (constexpr since C++20)

const_reference back() const;

(2) (constexpr since C++20)

Returns a reference to the last element in the container.

If empty() is true, the behavior is undefined.

(until C++26)

If empty() is true:

If the implementation is hardened, a contract violation occurs. Moreover, if the contract-violation handler returns under âobserveâ evaluation semantic, the behavior is undefined.
If the implementation is not hardened, the behavior is undefined.

(since C++26) [edit] Return value

Reference to the last element.

[edit] Complexity

Constant.

[edit] Notes

For a non-empty container c, the expression c.back() is equivalent to *std::prev(c.end()).

[edit] Example

#include <cassert>
#include <vector>
 
int main()
{
    std::vector<char> letters{'a', 'b', 'c', 'd'};
    assert(letters.back() == 'd');
}

[edit] See also access the first element
(public member function) [edit] returns a reverse iterator to the beginning
(public member function) [edit] returns an iterator to the end
(public member function) [edit]

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