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Difference between revisions of "cpp/iterator/size"

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< cpp‎ | iterator
m (→‎Notes: ++)
m (→‎Notes: + feature test macro, extended.)
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===Notes===
 
===Notes===
−
{{feature test macro|__cpp_lib_ssize}}
+
{{feature test macro|__cpp_lib_nonmember_container_access|}} (for {{tt|std::size()}}).
 +
 
 +
{{feature test macro|__cpp_lib_ssize}} (for {{tt|std::ssize()}}).
  
 
===Example===
 
===Example===

Revision as of 03:24, 31 January 2022

 
C++
 
Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
(C++20)
(C++20)
(C++20)
Utilities
(C++20)
Iterator adaptors
Range access
(C++11)(C++14)
(C++14)(C++14)  
(C++11)(C++14)
(C++14)(C++14)  
sizessize
(C++17)(C++20)
(C++17)
(C++17)
 
Defined in header <array>
Defined in header <deque>
Defined in header <forward_list>
Defined in header <iterator>
Defined in header <list>
Defined in header <map>
Defined in header <regex>
Defined in header <set>
Defined in header <span>
(since C++20)
Defined in header <string>
Defined in header <string_view>
Defined in header <unordered_map>
Defined in header <unordered_set>
Defined in header <vector>
template <class C>
constexpr auto size(const C& c) -> decltype(c.size());
 (1) (since C++17)
template <class C>

constexpr auto ssize(const C& c)
    -> std::common_type_t<std::ptrdiff_t,

                          std::make_signed_t<decltype(c.size())>>;
 (2) (since C++20)
template <class T, std::size_t N>
constexpr std::size_t size(const T (&array)[N]) noexcept;
 (3) (since C++17)
template <class T, std::ptrdiff_t N>
constexpr std::ptrdiff_t ssize(const T (&array)[N]) noexcept;
 (4) (since C++20)

Returns the size of the given range.

1-2) Returns c.size(), converted to the return type if necessary.
3-4) Returns N.

Contents

Parameters

c - a container or view with a size member function
array - an array of arbitrary type

Return value

The size of c or array.

Exceptions

1-2) May throw implementation-defined exceptions.

Overloads

Custom overloads of size may be provided for classes and enumerations that do not expose a suitable size() member function, yet can be detected.

Overloads of size found by argument-dependent lookup can be used to customize the behavior of std::ranges::size, std::ranges::ssize, and std::ranges::empty.

(since C++20)

Possible implementation

First version 
template <class C> 
constexpr auto size(const C& c) -> decltype(c.size())
{
    return c.size();
}
Second version 
template <class C>
constexpr auto ssize(const C& c) 
    -> std::common_type_t<std::ptrdiff_t,
                          std::make_signed_t<decltype(c.size())>> 
{
    using R = std::common_type_t<std::ptrdiff_t,
                                 std::make_signed_t<decltype(c.size())>>;
    return static_cast<R>(c.size());
}
Third version 
template <class T, std::size_t N>
constexpr std::size_t size(const T (&array)[N]) noexcept
{
    return N;
}
Fourth version 
template <class T, std::ptrdiff_t N>
constexpr std::ptrdiff_t ssize(const T (&array)[N]) noexcept
{
    return N;
}

Notes

Feature-test macro Value Std Feature
__cpp_lib_nonmember_container_access  
(for std::size()).
Feature-test macro Value Std Feature
__cpp_lib_ssize  
(for std::ssize()).

Example

Run this code
#include <iostream>
#include <vector>
#include <iterator>
 
int main() 
{
    std::vector<int> v = { 3, 1, 4 };
    std::cout << std::size(v) << '\n'; 
 
    int a[] = { -5, 10, 15 };
    std::cout << std::size(a) << '\n';
 
    // since C++20 the signed size (ssize) can avail
    auto i = std::ssize(v);
    for (--i; i != -1; --i) {
        std::cout << v[i] << (i ? ' ' : '\n');
    }
    std::cout << "i = " << i << '\n';
}

Output: 

3
3
4 1 3 
i = -1

See also

 signed integer type returned when subtracting two pointers
(typedef) [edit]
unsigned integer type returned by the sizeof operator
(typedef) [edit]
(C++20)
 returns an integer equal to the size of a range
(customization point object)[edit]
(C++20)
 returns a signed integer equal to the size of a range
(customization point object)[edit]
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