Difference between revisions of "cpp/iterator/size"
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m (→Notes: + feature test macro, extended.) |
Alexis wilke (Talk | contribs) m (Fixed comment.) |
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std::cout << std::size(a) << '\n'; | std::cout << std::size(a) << '\n'; | ||
| − | // since C++20 the signed size (ssize) | + | // since C++20 the signed size (ssize) is available |
auto i = std::ssize(v); | auto i = std::ssize(v); | ||
for (--i; i != -1; --i) { | for (--i; i != -1; --i) { | ||
Revision as of 08:52, 29 May 2022
| Defined in header <array>
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| Defined in header <deque>
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| Defined in header <forward_list>
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| Defined in header <iterator>
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| Defined in header <list>
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| Defined in header <map>
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| Defined in header <regex>
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| Defined in header <set>
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| Defined in header <span>
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(since C++20) |
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| Defined in header <string>
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| Defined in header <string_view>
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| Defined in header <unordered_map>
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| Defined in header <unordered_set>
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| Defined in header <vector>
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| 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 |
Parameters
| c | - | a container or view with a size member function
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| 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 |
(since C++20) |
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; } |
Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_nonmember_container_access |
std::size()).
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_ssize |
std::ssize()).
Example
Run this code
#include <iostream> #include <vector> #include <iterator> 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
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) |
