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Difference between revisions of "cpp/container/span/dynamic extent"

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< cpp‎ | container‎ | span
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m (add link to std::extents)
 
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{{cpp/container/span/navbar}}
 
{{cpp/container/span/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
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{{dcl header | span}}
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{{dcl header|span}}
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{{dcl | since=c++20 |1=
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{{dcl|since=c++20|1=
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inline constexpr std::ptrdiff_t dynamic_extent = -1;
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inline constexpr std::size_t dynamic_extent = std::numeric_limits<std::size_t>::max();
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
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{{tt|std::dynamic_extent}} is a constant of type {{lc|std::ptrdiff_t}} that is used to differentiate {{ltt|cpp/container/span|std::span}} of static and dynamic extent.
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{{tt|std::dynamic_extent}} is a constant of type {{lc|std::size_t}} that is generally used to indicate that any type using {{tt|std::dynamic_extent}} will ''dynamically'' store its value (e.g., size) rather than having the value ''statically'' known in the type.
 +
 
 +
It is being used in several contexts:
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* To differentiate {{ltt|cpp/container/span|std::span}} of static and dynamic extent.
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{{rrev|since=c++23|1=
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* To indicate that the extent at a certain rank index will be stored dynamically in {{ltt|cpp/container/mdspan/extents|std::extents}}.
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}}
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{{rrev|since=c++26|1=
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* To indicate that the padded layouts for {{ltt|cpp/container/mdspan|std::mdspan}} will dynamically store its padding value.
 +
}}
 +
 
 +
===Note===
 +
Since {{lc|std::size_t}} is an unsigned type, an equivalent definition is:
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{{source|1=
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inline constexpr std::size_t dynamic_extent = -1;
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}}
 +
 
 +
See [[cpp/language/implicit_conversion#Integral_conversions|integral conversions]].
 +
 
 +
===Example===
 +
{{example
 +
|code=
 +
#include <array>
 +
#include <cassert>
 +
#include <cstddef>
 +
#include <iostream>
 +
#include <span>
 +
#include <string_view>
 +
#include <vector>
 +
 
 +
int main()
 +
{
 +
    auto print = [](std::string_view const name, std::size_t ex)
 +
    {
 +
        std::cout << name << ", ";
 +
        if (std::dynamic_extent == ex)
 +
            std::cout << "dynamic extent\n";
 +
        else
 +
            std::cout << "static extent = " << ex << '\n';
 +
    };
 +
 
 +
    int a[]{1, 2, 3, 4, 5};
 +
 
 +
    std::span span1{a};
 +
    print("span1", span1.extent);
 +
 
 +
    std::span<int, std::dynamic_extent> span2{a};
 +
    print("span2", span2.extent);
 +
 
 +
    std::array ar{1, 2, 3, 4, 5};
 +
    std::span span3{ar};
 +
    print("span3", span3.extent);
 +
 
 +
    std::vector v{1, 2, 3, 4, 5};
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    std::span span4{v};
 +
    print("span4", span4.extent);
 +
}
 +
|output=
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span1, static extent = 5
 +
span2, dynamic extent
 +
span3, static extent = 5
 +
span4, dynamic extent
 +
}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/container/dsc span}}
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{{dsc inc|cpp/container/dsc span}}
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{{dsc inc|cpp/container/mdspan/dsc extents}}
 
{{dsc end}}
 
{{dsc end}}
  
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[[zh:cpp/container/span/dynamic_extent]]
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{{langlinks|de|es|ja|ru|zh}}

Latest revision as of 21:35, 12 April 2024

 
C++
 
Containers library
 
std::span
 
Defined in header <span>
inline constexpr std::size_t dynamic_extent = std::numeric_limits<std::size_t>::max();
 (since C++20)

std::dynamic_extent is a constant of type std::size_t that is generally used to indicate that any type using std::dynamic_extent will dynamically store its value (e.g., size) rather than having the value statically known in the type.

It is being used in several contexts:

  • To differentiate std::span of static and dynamic extent.
  • To indicate that the extent at a certain rank index will be stored dynamically in std::extents.
(since C++23)
  • To indicate that the padded layouts for std::mdspan will dynamically store its padding value.
(since C++26)

[edit] Note

Since std::size_t is an unsigned type, an equivalent definition is: 

inline constexpr std::size_t dynamic_extent = -1;

See integral conversions.

[edit] Example

Run this code
#include <array>
#include <cassert>
#include <cstddef>
#include <iostream>
#include <span>
#include <string_view>
#include <vector>
 
int main()
{
    auto print = [](std::string_view const name, std::size_t ex)
    {
        std::cout << name << ", ";
        if (std::dynamic_extent == ex)
            std::cout << "dynamic extent\n";
        else
            std::cout << "static extent = " << ex << '\n';
    };
 
    int a[]{1, 2, 3, 4, 5};
 
    std::span span1{a};
    print("span1", span1.extent);
 
    std::span<int, std::dynamic_extent> span2{a};
    print("span2", span2.extent);
 
    std::array ar{1, 2, 3, 4, 5};
    std::span span3{ar};
    print("span3", span3.extent);
 
    std::vector v{1, 2, 3, 4, 5};
    std::span span4{v};
    print("span4", span4.extent);
}

Output: 

span1, static extent = 5
span2, dynamic extent
span3, static extent = 5
span4, dynamic extent

[edit] See also

(C++20)
 a non-owning view over a contiguous sequence of objects
(class template) [edit]
(C++23)
 a descriptor of a multidimensional index space of some rank
(class template) [edit]
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