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std::span::at(3)	       C++ Standard Libary		std::span::at(3)

NAME
     std::span::at - std::span::at

Synopsis
	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 <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

See also
	operator[] access specified element
		   (public member function)

http://cppreference.com 	   2024.06.10			std::span::at(3)
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