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)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

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) is-integer-like is-signed-integer-like (C++20)(C++20)	sentinel_for (C++20) sized_sentinel_for (C++20) input_iterator (C++20) output_iterator (C++20) input_or_output_iterator (C++20)	forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (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)	iterator (deprecated in C++17) iterator_traits incrementable_traits (C++20) indirectly_readable_traits (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)	projected_value_t (C++26)
Iterator adaptors
reverse_iterator make_reverse_iterator (C++14) move_iterator (C++11) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20)	front_insert_iterator back_insert_iterator inserter insert_iterator front_inserter back_inserter 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) make_const_iterator (C++23) make_const_sentinel (C++23)
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) rbegincrbegin (C++14)(C++14)   endcend (C++11)(C++14) rendcrend (C++14)(C++14)   sizessize (C++17)(C++20) empty (C++17) data (C++17)

 

Defined in header <array>
Defined in header <deque>
Defined in header <flat_map>
Defined in header <flat_set>
Defined in header <forward_list>
Defined in header <inplace_vector>
Defined in header <iterator>
Defined in header <list>
Defined in header <map>
Defined in header <regex>
Defined in header <set>
Defined in header <span>
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 > constexprauto size(const C& c )-> decltype(c.size());	(1)	(since C++17)
template<class C > constexprauto 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 > constexprstd::size_t size(const T (&array)[N])noexcept;	(3)	(since C++17)
template<class T, std::ptrdiff_t N > constexprstd::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

1)c.size()

2)static_cast<std::common_type_t<std::ptrdiff_t,
                               std::make_signed_t<decltype(c.size())>>>(c.size())

3,4)N

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>constexprauto size(const C& c)-> decltype(c.size()){return c.size();}
ssize (2)
template<class C>constexprauto 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())>>;returnstatic_cast<R>(c.size());}
size (3)
template<class T, std::size_t N>constexprstd::size_t size(const T (&array)[N])noexcept{return N;}
ssize (4)
template<class T, std::ptrdiff_t N>constexprstd::ptrdiff_t ssize(const T (&array)[N])noexcept{return N;}

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_nonmember_container_access	201411L	(C++17)	std::size(), std::data and std::empty
__cpp_lib_ssize	201902L	(C++20)	std::ssize()(2,4) and unsigned std::span::size()

Example

Run this code

#include <cassert>#include <cstring>#include <iostream>#include <vector>   int main(){// Works with containersstd::vector<int> v{3, 1, 4};assert(std::size(v)==3);   // And works with built-in arrays tooint a[]{-5, 10, 15};// Returns the number of elements (not bytes) as opposed to sizeofassert(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 sizeconstchar str[]="12345";// These are fine and give the correct resultassert(std::size(str)==6);assert(sizeof(str)==6);   // But use of sizeof here is a common source of bugsconstchar* str_decayed ="12345";// std::cout << std::size(str_decayed) << '\n'; // Usefully fails to compilestd::cout<< sizeof(str_decayed)<<'\n';// Prints the size of the pointer!   // Since C++20 the signed size (std::ssize) is availableauto i = std::ssize(v);for(--i; i !=-1;--i)std::cout<< v[i]<<(i ?' ':'\n');assert(i ==-1);   // Note that the string literal includes the ending null character, which// will be part of the constructed characters array. This makes std::size// behave differently from std::strlen and std::string::size:constexprchar symbols[]="0123456789";   static_assert(std::size(symbols)==11); static_assert(std::string(symbols).size()==10);assert(std::strlen(symbols)==10);}

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)