std::span last() method

od C++20

// (1) Const version only
template< std::size_t Count >
constexpr std::span<element_type, Count> last() const;

// (2) Const version only
constexpr std::span<element_type, std::dynamic_extent> last( size_type Count ) const;

Obtains a span that is a view over the last Count elements of this span.

important

The program is ill-formed

if Count > Extent.

Undefined Behaviour

The behavior is undefined

if Count > size().

Parameters

• (2) - Count - the number of the elements to make a span of

Return value

A span s that is a view over the last Count elements of *this, with the following properties:

• s.data() == this->data() + (this->size() - Count)
• s.size() == Count

Complexity

Constant - O(1).

Exceptions

(none)

Example

Main.cpp

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