std::size, std::ssize

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

Iterator library

Iterator concepts
indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) input_or_output_iterator (C++20) sentinel_for (C++20) sized_sentinel_for (C++20)	input_iterator (C++20) output_iterator (C++20) forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
iterator_traits input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20) iterator (deprecated in C++17)	incrementable_traits (C++20) indirectly_readable_traits (C++20) iter_value_titer_difference_titer_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++20)(C++20)
Algorithm concepts and utilities
Indirect callable concepts
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20) indirect_unary_predicate (C++20)	indirect_binary_predicate (C++20) indirect_equivalence_relation (C++20) indirect_strict_weak_order (C++20)
Common algorithm requirements
indirectly_movable (C++20) indirectly_movable_storable (C++20) indirectly_copyable (C++20) indirectly_copyable_storable (C++20)	indirectly_swappable (C++20) indirectly_comparable (C++20) permutable (C++20) mergeable (C++20) sortable (C++20)
Utilities
indirect_result_t	projected
Iterator adaptors
reverse_iterator move_iterator (C++11) move_sentinel (C++20) common_iterator (C++20) counted_iterator (C++20) back_insert_iterator front_insert_iterator insert_iterator	make_reverse_iterator (C++14) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20) back_inserter front_inserter inserter
Stream iterators
istream_iterator ostream_iterator	istreambuf_iterator ostreambuf_iterator
Iterator customization points
ranges::iter_move (C++20)
ranges::iter_swap (C++20)
Iterator operations
advance distance prev (C++11) next (C++11)	ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)
Range access
begincbegin (C++11)(C++14) endcend (C++11)(C++14) sizessize (C++17)(C++20)	rbegincrbegin (C++14)(C++14) rendcrend (C++14)(C++14) empty (C++17) data (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 container c or array array.

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

3-4) Returns N.

Parameters

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

Return value

The size of c or array

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; }

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] << ' ';
    }
    std::cout << "\n" "i = " << i << '\n';
}

Output:

3
3
4 1 3 
i = -1

See also

ptrdiff_t	signed integer type returned when subtracting two pointers (typedef)
size_t	unsigned integer type returned by the sizeof operator (typedef)