NAME¶

std::span::first - std::span::first

Synopsis¶

template< std::size_t Count > (since C++20)
constexpr std::span<element_type, Count> first() const;
constexpr std::span<element_type, std::dynamic_extent> first( (since C++20)
size_type Count ) const;

Obtains a span that is a view over the first Count elements of this span. The
program is ill-formed if Count > Extent. The behavior is undefined if Count >
size().

Return value¶

A span r that is a view over the first Count elements of *this, such that r.data()
== this->data() && r.size() == Count.

Example¶

// Run this code

#include <iostream>
#include <ranges>
#include <span>
#include <string_view>

void print(std::string_view const title,
std::ranges::forward_range auto const& container)
{
auto size{std::size(container)};
std::cout << title << '[' << size << "]{";
for (auto const& elem : container)
std::cout << elem << (--size ? ", " : "");
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};

See also¶

last obtains a subspan consisting of the last N elements of the sequence
(public member function)
subspan obtains a subspan
(public member function)