std::{{{1}}}::at

 

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reference at( size_type pos );	(1)	(since {std})
const_reference at( size_type pos ) const;	(2)	(since {std})

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 of std::{{{1}}})