std::span<T,Extent>::first
From cppreference.com
template< std::size_t Count >
constexpr std::span<element_type, Count> first() const;
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(1) | (since C++20) |
constexpr std::span<element_type, std::dynamic_extent>
first( size_type count ) const;
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(2) | (since C++20) |
Obtains a subview over the first 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 |
(until C++26) |
|
If
|
(since C++26) |
Parameters
| count | - | the number of the elements of the subview |
Return value
1)
std::span<element_type, Count>{data(), Count}2)
std::span<element_type, std::dynamic_extent>{data(), count}Example
Run this code
#include <iostream>
#include <ranges>
#include <span>
#include <string_view>
void print(const std::string_view title,
const std::ranges::forward_range auto& container)
{
auto size{std::size(container)};
std::cout << title << '[' << size << "]{";
for (const auto& 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};