Difference between revisions of "cpp/container/span/first"
From cppreference.com
(E: simplification (now span::last() has its own Example)) |
m (→Example: -',') |
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{{cpp/container/span/navbar}} | {{cpp/container/span/navbar}} | ||
{{dcl begin}} | {{dcl begin}} | ||
| − | {{dcl | 1= | + | {{dcl|since=c++20|1= |
template< std::size_t Count > | template< std::size_t Count > | ||
constexpr std::span<element_type, Count> first() const; | constexpr std::span<element_type, Count> first() const; | ||
}} | }} | ||
| − | {{dcl | 1= | + | {{dcl|since=c++20|1= |
| − | constexpr std::span<element_type, std::dynamic_extent> first( | + | constexpr std::span<element_type, std::dynamic_extent> first( size_type Count ) const; |
}} | }} | ||
{{dcl end}} | {{dcl end}} | ||
| Line 13: | Line 13: | ||
Obtains a span that is a view over the first {{tt|Count}} elements of this span. The program is ill-formed if {{c|Count > Extent}}. The behavior is undefined if {{c|Count > size()}}. | Obtains a span that is a view over the first {{tt|Count}} elements of this span. The program is ill-formed if {{c|Count > Extent}}. The behavior is undefined if {{c|Count > size()}}. | ||
| − | === Return value === | + | ===Return value=== |
A span {{tt|r}} that is a view over the first {{tt|Count}} elements of {{c|*this}}, such that {{c|1=r.data() == this->data() && r.size() == Count}}. | A span {{tt|r}} that is a view over the first {{tt|Count}} elements of {{c|*this}}, such that {{c|1=r.data() == this->data() && r.size() == Count}}. | ||
===Example=== | ===Example=== | ||
| − | {{example | + | {{example |
| − | + | |code= | |
#include <iostream> | #include <iostream> | ||
| + | #include <ranges> | ||
#include <span> | #include <span> | ||
#include <string_view> | #include <string_view> | ||
| − | void print(std::string_view const title, | + | void print(std::string_view const title, |
| − | + | std::ranges::forward_range auto const& container) | |
| − | std::cout << title << | + | { |
| + | auto size{std::size(container)}; | ||
| + | std::cout << title << '[' << size << "]{"; | ||
for (auto const& elem : container) | for (auto const& elem : container) | ||
| − | std::cout << elem << ", "; | + | std::cout << elem << (--size ? ", " : ""); |
std::cout << "};\n"; | std::cout << "};\n"; | ||
} | } | ||
| Line 39: | Line 42: | ||
std::span<const int, std::dynamic_extent> span_first_dynamic = span.first(4); | std::span<const int, std::dynamic_extent> span_first_dynamic = span.first(4); | ||
| − | print("span.first(4): | + | print("span.first(4): ", span_first_dynamic); |
} | } | ||
int main() | int main() | ||
{ | { | ||
| − | int a[8]{ 1,2,3,4,5,6,7,8 }; | + | int a[8]{1, 2, 3, 4, 5, 6, 7, 8}; |
| − | print("int | + | print("int a", a); |
run_game(a); | run_game(a); | ||
} | } | ||
| − | + | |output= | |
| − | int | + | int a[8]{1, 2, 3, 4, 5, 6, 7, 8}; |
| − | span: [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<5>(): [5]{1, 2, 3, 4, 5}; |
| − | span.first(4): | + | span.first(4): [4]{1, 2, 3, 4}; |
}} | }} | ||
| − | === See also === | + | ===See also=== |
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/container/span/dsc last}} | + | {{dsc inc|cpp/container/span/dsc last}} |
| − | {{dsc inc | cpp/container/span/dsc subspan}} | + | {{dsc inc|cpp/container/span/dsc subspan}} |
{{dsc end}} | {{dsc end}} | ||
| − | {{langlinks|ja|zh}} | + | {{langlinks|de|es|ja|ru|zh}} |
Latest revision as of 02:44, 2 October 2023
| template< std::size_t Count > constexpr std::span<element_type, Count> first() const; |
(since C++20) | |
| constexpr std::span<element_type, std::dynamic_extent> first( size_type Count ) const; |
(since C++20) | |
Obtains a span that is a view over the first Count elements of this span. The program is ill-formed if Count > Extent. The behavior is undefined if Count > size().
[edit] Return value
A span r that is a view over the first Count elements of *this, such that r.data() == this->data() && r.size() == Count.
[edit] Example
Run this code
#include <iostream> #include <ranges> #include <span> #include <string_view> void print(std::string_view const title, std::ranges::forward_range auto const& container) { auto size{std::size(container)}; std::cout << title << '[' << size << "]{"; for (auto const& elem : container) std::cout << elem << (--size ? ", " : ""); std::cout << "};\n"; } void run_game(std::span<const int> 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) | |
| obtains a subspan (public member function) |
