std::size, std::ssize - cppreference.com (original) (raw)
| Defined in header | ||
|---|---|---|
| Defined in header | ||
| Defined in header <flat_map> | ||
| Defined in header <flat_set> | ||
| Defined in header <forward_list> | ||
| Defined in header <inplace_vector> | ||
| Defined in header | ||
| Defined in header | ||
| Defined in header | ||
| Defined in header | ||
| Defined in header | ||
| Defined in header | ||
| Defined in header | ||
| Defined in header <string_view> | ||
| Defined in header <unordered_map> | ||
| Defined in header <unordered_set> | ||
| Defined in header | ||
| 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.
Contents
- 1 Parameters
- 2 Return value
- 3 Exceptions
- 4 Overloads
- 5 Possible implementation
- 6 Notes
- 7 Example
- 8 See also
[edit] Parameters
| c | - | a container or view with a size member function |
|---|---|---|
| array | - | an array of arbitrary type |
[edit] Return value
- 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.
[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
#include #include #include #include int main() { // Works with containers std::vector 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:🧵: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:
[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] |