std::size, std::ssize
From cppreference.com
Defined in header <array>
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Defined in header <deque>
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Defined in header <flat_map>
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Defined in header <flat_set>
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Defined in header <forward_list>
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Defined in header <inplace_vector>
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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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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 |
[edit] Parameters
c | - | a container or view with a size member function
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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())
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 |
(since C++20) |
[edit] Possible implementation
size (1) |
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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
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__cpp_lib_ssize |
201902L |
(C++20) | std::ssize() (2,4) and unsigned std::span::size()
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[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) | |
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) |