NAME¶

std::size,std::ssize - std::size,std::ssize

Synopsis¶

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 <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 > (1) (since C++17)
constexpr auto size( const C& c ) -> decltype(c.size());
template< class C >

constexpr auto ssize( const C& c ) (2) (since C++20)
-> std::common_type_t<std::ptrdiff_t,

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

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 (since C++20)
std::ranges::empty.

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 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 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);

// 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:
constexpr char 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 returns an integer equal to the size of a range
(C++20) (customization point object)
ranges::ssize returns a signed integer equal to the size of a range
(C++20) (customization point object)