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

 

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std::span

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span::beginspan::cbegin (C++20)(C++23)

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span::rendspan::crend (C++20)(C++23)

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span::first (C++20)

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as_bytesas_writable_bytes (C++20)(C++20)

Non-member constant

dynamic_extent (C++20)

Deduction guides(C++20)

 

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

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)
{
    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, };

See also

last (C++20)	obtains a subspan consisting of the last N elements of the sequence (public member function)
subspan (C++20)	obtains a subspan (public member function)