std::span<T,Extent>::first - cppreference.com (original) (raw)
Obtains a subview over the first Count or count elements of this span.
- The element count is provided as a template argument, and the subview has a static extent.
If Count > Extent is true, the program is ill-formed.
- The element count is provided as a function argument, and the subview has a dynamic extent.
| If Count > size() or count > size() is true, the behavior is undefined. | (until C++26) |
|---|---|
| If Count > size() or count > size() is true: If the implementation is hardened, a contract violation occurs. Moreover, if the contract-violation handler returns under “observe” evaluation semantic, the behavior is undefined. If the implementation is not hardened, the behavior is undefined. | (since C++26) |
[edit] Parameters
| count | - | the number of the elements of the subview |
|---|
[edit] Return value
- std::span<element_type, Count>{data(), Count}
[edit] Example
#include #include #include #include void print(const std::string_view title, const std::ranges::forward_range auto& container) { auto size{std::size(container)}; std::cout << title << '[' << size << "]{"; for (const auto& elem : container) std::cout << elem << (--size ? ", " : ""); std::cout << "};\n"; } void run_game(std::span span) { print("span: ", span); std::span<const int, 5> span_first = span.first<5>(); print("span.first<5>(): ", span_first); std::span<const int, std::dynamic_extent> span_first_dynamic = span.first(4); print("span.first(4): ", span_first_dynamic); } int main() { int a[8]{1, 2, 3, 4, 5, 6, 7, 8}; print("int a", a); run_game(a); }
Output:
int a[8]{1, 2, 3, 4, 5, 6, 7, 8}; span: [8]{1, 2, 3, 4, 5, 6, 7, 8}; span.first<5>(): [5]{1, 2, 3, 4, 5}; span.first(4): [4]{1, 2, 3, 4};
[edit] See also
| | obtains a subspan consisting of the last N elements of the sequence (public member function) [edit] | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | obtains a subspan (public member function) [edit] |