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std::size, std::ssize

From cppreference.com
< cpp‎ | iterator
 
 
Iterator library
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(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 <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 >
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.

[edit] Parameters

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

[edit] 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

[edit] Exceptions

1,2) May throw implementation-defined exceptions.

[edit] 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)

[edit] 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;
}

[edit] 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()

[edit] 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

[edit] See also

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

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