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

 

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std::span

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span::span

span::operator=

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span::beginspan::cbegin (C++23)

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span::empty

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as_bytesas_writable_bytes

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dynamic_extent

Deduction guides

 

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

Run this code

#include <chrono>
#include <cstddef>
#include <iostream>
#include <span>
#include <stdexcept>
 
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
    {
        // Try to set an element at random position >= size()
        auto moon_phase = []
        {
            return std::chrono::system_clock::now().time_since_epoch().count() % 8;
        };
        data.at(data.size() + moon_phase()) = 13;
    }
    catch(const std::out_of_range& ex)
    {
        std::cout << ex.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 (which is 8) >= size() (which is 6)
data: 1 88 4 5 5 6

See also

operator[]

access specified element (public member function)