NAME¶

std::vector::at - std::vector::at

Synopsis¶

reference at( size_type pos ); (1) (constexpr since C++20)
const_reference at( size_type pos ) const; (2) (constexpr since C++20)

Returns a reference to the element at specified location pos, with bounds checking.

If pos is not within the range of the container, an exception of type
std::out_of_range is thrown.

Parameters¶

pos - position of the element to return

Return value¶

Reference to the requested element.

Exceptions¶

std::out_of_range if pos >= size().

Complexity¶

Constant.

Example¶

// Run this code

#include <iostream>
#include <vector>
#include <stdexcept>

#ifdef __GNUG__
[[gnu::noinline]]
#endif
unsigned int runtime_six() // Emulate runtime input
{
return 6u;
}

int main()
{
std::vector<int> data = {1, 2, 4, 5, 5, 6};

// Set element 1
data.at(1) = 88;

// Read element 2
std::cout << "Element at index 2 has value " << data.at(2) << '\n';

std::cout << "data size = " << data.size() << '\n';

try
{
// Set element 6, where the index is determined at runtime
data.at(runtime_six()) = 666;
}
catch (std::out_of_range const& exc)
{
std::cout << exc.what() << '\n';
}

// Print final values
std::cout << "data:";
for (int elem : data)
std::cout << ' ' << elem;
std::cout << '\n';
}

Possible output:¶

Element at index 2 has value 4
data size = 6
vector::_M_range_check: __n (which is 6) >= this->size() (which is 6)
data: 1 88 4 5 5 6

See also¶

operator[] access specified element
(public member function)