std::span first() method

od C++20

// (1) Const version onlytemplate< std::size_t Count >constexpr std::span<element_type, Count> first() const;// (2) Const version onlyconstexpr std::span<element_type, std::dynamic_extent> first( size_type Count ) const;

Obtains a span that is a view over the first 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 first Count elements of *this, with the following properties:

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

Complexity

Constant - O(1).

Exceptions

(none)

Example

Main.cpp

#include <iostream>#include <span>#include <string_view> void print(std::string_view const title,            /* std::ranges::forward_range */ auto const& container) {    std::cout << title << "[" << std::size(container) << "]{ ";    for (auto const& elem : container)        std::cout << elem << ", ";    std::cout << "};\n";} void run_game(std::span<const int> span){    print("span: ", span);     std::span<const int, 5> span_first = span.first<5>();    print("span.first<5>(): ", span_first);     std::span<const int, std::dynamic_extent> span_first_dynamic = span.first(4);    print("span.first(4):   ", span_first_dynamic);} int main(){    int a[8]{ 1, 2, 3, 4, 5, 6, 7, 8, };    print("int  a", a);    run_game(a);}

Output

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