std::span<T,Extent>::at

constexpr reference at( size_type pos ) const;

(since C++26)

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

If pos is not within the range of the span, 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, i.e. *(data() + pos).

Exceptions

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

Complexity

Constant.

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_span`	202311L	(C++26)	`std::span::at`

Example

#include <iostream>
#include <span>
#include <stdexcept>
 
#ifdef __GNUG__ 
[[gnu::noinline]]
#endif
unsigned int runtime_six() // Emulate runtime input
{
    return 6u;
}
 
int main()
{
    int x[]{1, 2, 4, 5, 5, 6};
    std::span data(x);
 
    // 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
std::out_of_range: pos >= size()
data: 1 88 4 5 5 6