std::size, std::ssize

 

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)

Metaprogramming library (C++11)

Diagnostics library

General 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)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

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_const_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++23)(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 (C++20)	projected (C++20)
Iterator adaptors
reverse_iterator move_iterator (C++11) move_sentinel (C++20) common_iterator (C++20) counted_iterator (C++20) basic_const_iterator (C++23) const_iterator (C++23) const_sentinel (C++23) 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) make_const_iterator (C++23) make_const_sentinel (C++23) 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 range.

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

3-4) Returns N.

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

size (1)
template <class C> constexpr auto size(const C& c) -> decltype(c.size()) { return c.size(); }
ssize (2)
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()); }
size (3)
template <class T, std::size_t N> constexpr std::size_t size(const T (&array)[N]) noexcept { return N; }
ssize (4)
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	Comment
__cpp_lib_nonmember_container_access	201411L	(C++17)	std::size(), std::data and std::empty
__cpp_lib_ssize	201902L	(C++20)	std::ssize() and unsigned std::span::size

Example

Run this code

#include <cassert>
#include <iostream>
#include <vector>
 
int main()
{
    // Works with containers
    std::vector<int> v{3, 1, 4};
    assert(std::size(v) == 3);
 
    // And works with built-in arrays too
    int a[]{-5, 10, 15};
    // Returns the number of elements (not bytes) as opposed to sizeof
    assert(std::size(a) == 3);
    std::cout << "size of a[]: " << sizeof a << '\n'; // 12, if sizeof(int) == 4
 
    // Provides a safe way (compared to sizeof) of getting string buffer size
    const char str[] = "12345";
    // These are fine and give the correct result
    assert(std::size(str) == 6);
    assert(sizeof(str) == 6);
 
    // But use of sizeof here is a common source of bugs
    const char* str_decayed = "12345";
    // std::cout << std::size(str_decayed) << '\n'; // Usefully fails to compile
    std::cout << sizeof(str_decayed) << '\n'; // Prints the size of the pointer!
 
    // Since C++20 the signed size (std::ssize) is available
    auto i = std::ssize(v);
    for (--i; i != -1; --i)
        std::cout << v[i] << (i ? ' ' : '\n');
    assert(i == -1);
}

Possible output:

size of a[]: 12
8
4 1 3

See also

ptrdiff_t	signed integer type returned when subtracting two pointers (typedef)
size_t	unsigned integer type returned by the sizeof operator (typedef)
ranges::size (C++20)	returns an integer equal to the size of a range (customization point object)
ranges::ssize (C++20)	returns a signed integer equal to the size of a range (customization point object)