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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Containers library

node-handle (C++17)

Sequence

array (C++11)

vector

vector<bool>

inplace_vector (C++26)

hive (C++26)

deque

forward_list (C++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

Adaptors

stack

queue

priority_queue

flat_set (C++23)

flat_multiset (C++23)

flat_map (C++23)

flat_multimap (C++23)

Views

span (C++20)

mdspan (C++23)

Tables

Iterator invalidation

Member function table

Non-member function table

 

std::span

Member functions

span::span

span::operator=

Element access

span::front

span::back

span::at (C++26)

span::operator[]

span::data

Iterators

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

span::endspan::cend (C++23)

span::rbeginspan::crbegin (C++23)

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

Observers

span::empty

span::size

span::size_bytes

Subviews

span::first

span::last

span::subspan

Non-member functions

as_bytesas_writable_bytes

Non-member constant

dynamic_extent

Deduction guides

 

template< std::size_t Count > constexpr std::span<element_type, Count> first() const;	(1)	(since C++20)
constexpr std::span<element_type, std::dynamic_extent>     first( size_type count ) const;	(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 Count > size() or count > size() is true, the behavior is undefined.	(until C++26)
If Count > size() or count > size() is true: If the implementation is hardened, a contract violation occurs. Moreover, if the contract-violation handler returns under “observe” evaluation semantic, the behavior is undefined. If the implementation is not hardened, the behavior is undefined.	(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};

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)