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

 

C++

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Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

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Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

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Numerics library

Date and time library

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Filesystem library(C++17)

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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

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set

multiset

map

multimap

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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)

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std::span

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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 Offset,           std::size_t Count =std::dynamic_extent> constexprstd::span<element_type, /* see below */>     subspan()const;	(1)	(since C++20)
constexprstd::span<element_type, std::dynamic_extent>     subspan( size_type offset,              size_type count =std::dynamic_extent)const;	(2)	(since C++20)

Obtains a subview over some consecutive elements of this span, the elements to be included are determined by an element count and an offset.

1) The element count and offset are provided as template arguments, and the subview has a dynamic extent only if both Count and Offset are std::dynamic_extent.

• If Count is std::dynamic_extent, the subview contains all elements starting from the Offset^th.
• Otherwise, the subview contains Count elements starting from the Offset^th.

Denote the second template argument of the return type as FinalExtent, it is defined as Count !=std::dynamic_extent
    ? Count
    :(Extent !=std::dynamic_extent
        ? Extent - Offset
        :std::dynamic_extent).

If Offset <= Extent &&(Count ==std::dynamic_extent|| Count <= Extent - Offset) is false, the program is ill-formed.

If Offset <= size()&&(Count ==std::dynamic_extent|| Count <= size()- Offset) is false, the behavior is undefined.	(until C++26)
If Offset <= size()&&(Count ==std::dynamic_extent|| Count <= size()- Offset) is false: 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)

2) The element count and offset are provided as function arguments, and the subview always has a dynamic extent.

• If count is std::dynamic_extent, the subview contains all elements starting from the offset^th.
• Otherwise, the subview contains count elements starting from the offset^th.

If offset <= size()&&(count ==std::dynamic_extent|| count <= size()- offset) is false, the behavior is undefined.	(until C++26)
If offset <= size()&&(count ==std::dynamic_extent|| count <= size()- offset) is false: 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)

Return value

1)std::span<element_type, FinalExtent>
    (data()+ Offset, Count !=std::dynamic_extent? Count : size()- Offset))

2)std::span<element_type, std::dynamic_extent>
    (data()+ offset, count !=std::dynamic_extent? count : size()- offset))

Example

Run this code

#include <algorithm>#include <cstdio>#include <numeric>#include <ranges>#include <span>   void display(std::span<constchar> abc){constauto columns{20U};constauto rows{abc.size()- columns +1};   for(auto offset{0U}; offset < rows;++offset){ std::ranges::for_each(abc.subspan(offset, columns), std::putchar);std::puts("");}}   int main(){char abc[26]; std::ranges::iota(abc, 'A'); display(abc);}

Output:

ABCDEFGHIJKLMNOPQRST BCDEFGHIJKLMNOPQRSTU CDEFGHIJKLMNOPQRSTUV DEFGHIJKLMNOPQRSTUVW EFGHIJKLMNOPQRSTUVWX FGHIJKLMNOPQRSTUVWXY GHIJKLMNOPQRSTUVWXYZ

See also

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