std::size, std::ssize
From cppreference.com
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 ) |
(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
.Contents |
[edit] Parameters
c | - | a container or view with a size member function
|
array | - | an array of arbitrary type |
[edit] Return value
The size of c
or array
.
[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 |
(since C++20) |
[edit] 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; } |
[edit] Notes
Feature-test macro | Value | Std | Comment |
---|---|---|---|
__cpp_lib_nonmember_container_access |
201411L | (C++17) | for std::size()
|
__cpp_lib_ssize |
201902L | (C++20) | for std::ssize()
|
[edit] Example
Run this code
#include <iostream> #include <vector> 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) is available auto i = std::ssize(v); for (--i; i != -1; --i) { std::cout << v[i] << (i ? ' ' : '\n'); } std::cout << "i = " << i << '\n'; }
Output:
3 3 4 1 3 i = -1
[edit] See also
signed integer type returned when subtracting two pointers (typedef) | |
unsigned integer type returned by the sizeof operator (typedef) | |
(C++20) |
returns an integer equal to the size of a range (customization point object) |
(C++20) |
returns a signed integer equal to the size of a range (customization point object) |