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

From cppreference.com
< cpp‎ | container‎ | span
 
C++
 
Containers library
 
std::span
 
template< std::size_t Count >
constexpr std::span<element_type, Count> last() const;
 (1) (since C++20)
constexpr std::span<element_type, std::dynamic_extent>
    last( size_type count ) const;
 (2) (since C++20)

Obtains a subview over the last Count or count elements of this span.

1) The element count is provided as a template argument, and the subview has a static extent.
If Count > Extent is true, the program is ill-formed.
2) The element count is provided as a function argument, and the subview has a dynamic extent.

If Count > size() or count > size() is true, the behavior is undefined.

(until C++26)

If Count > size() or count > size() is true:

  • If the implementation is hardened, a contract violation occurs. Moreover, if the contract-violation handler returns under “observe” evaluation semantic, the behavior is undefined.
  • If the implementation is not hardened, the behavior is undefined.
(since C++26)

Contents

[edit] Parameters

count - the number of the elements of the subview

[edit] Return value

1) std::span<element_type, Count>{data() + (size() - Count), Count}
2) std::span<element_type, std::dynamic_extent>{data() + (size() - count), count}

[edit] Example

Run this code
#include <iostream>
#include <span>
#include <string_view>
 
void println(const std::string_view title, const auto& container)
{
    std::cout << title << '[' << std::size(container) << "]{ ";
    for (const auto& elem : container)
        std::cout << elem << ", ";
    std::cout << "};\n";
};
 
void run(std::span<const int> span)
{
    println("span: ", span);
 
    std::span<const int, 3> span_last = span.last<3>();
    println("span.last<3>(): ", span_last);
 
    std::span<const int, std::dynamic_extent> span_last_dynamic = span.last(2);
    println("span.last(2): ", span_last_dynamic);
}
 
int main()
{
    int a[8]{1, 2, 3, 4, 5, 6, 7, 8};
    println("int a", a);
    run(a);
}

Output: 

int a[8]{ 1, 2, 3, 4, 5, 6, 7, 8, };
span: [8]{ 1, 2, 3, 4, 5, 6, 7, 8, };
span.last<3>(): [3]{ 6, 7, 8, };
span.last(2): [2]{ 7, 8, };

[edit] See also

 obtains a subspan consisting of the first N elements of the sequence
(public member function) [edit]
obtains a subspan
(public member function) [edit]
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/container/span/last&oldid=182292"

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