std::span first() method

since C++20

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

// (2) Const version only
constexpr 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, };