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Difference between revisions of "Template:cpp/container/begin"

From cppreference.com
(E: (+) for std::array::begin/cbegin)
 
(35 intermediate revisions by 6 users not shown)
Line 1: Line 1:
−
{{#switch:{{{1|}}}|span={{cpp/container/{{{1|}}}/title | begin }}
+
{{#vardefine:cont|{{{1|inplace_vector}}}}}<!--
−
|#default={{cpp/container/{{{1|}}}/title | begin | cbegin}}
+
-->{{cpp/container/{{#var:cont}}/title|begin|cbegin}}
−
}}
+
{{cpp/container/{{#var:cont}}/navbar}}
−
{{cpp/container/{{{1|}}}/navbar}}
+
−
{{#switch:{{{1|}}}|array=
+
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl rev begin}}
+
{{#switch:{{#var:cont}}
−
{{dcl | until=c++17|
+
|array=
 +
{{dcl|num=1|since=c++11|notes={{mark constexpr since c++17}}|
 
iterator begin() noexcept;
 
iterator begin() noexcept;
 
}}
 
}}
−
{{dcl | since=c++17|
+
{{dcl|num=2|since=c++11|notes={{mark constexpr since c++17}}|
−
constexpr iterator begin() noexcept;
+
−
}}
+
−
{{dcl rev end}}
+
−
{{dcl rev begin}}
+
−
{{dcl | until=c++17|
+
 
const_iterator begin() const noexcept;
 
const_iterator begin() const noexcept;
 
}}
 
}}
−
{{dcl | since=c++17|
+
{{dcl|num=3|since=c++11|notes={{mark constexpr since c++17}}|
−
constexpr const_iterator begin() const noexcept;
+
const_iterator cbegin() const noexcept;
 
}}
 
}}
−
{{dcl rev end}}
+
|vector=
−
{{dcl rev begin}}
+
{{dcl|num=1|notes={{mark noexcept since c++11}}<br>{{mark constexpr since c++20}}|
−
{{dcl | until=c++17|
+
iterator begin();
 +
}}
 +
{{dcl|num=2|notes={{mark noexcept since c++11}}<br>{{mark constexpr since c++20}}|
 +
const_iterator begin() const;
 +
}}
 +
{{dcl|num=3|since=c++11|notes={{mark constexpr since c++20}}|
 
const_iterator cbegin() const noexcept;
 
const_iterator cbegin() const noexcept;
 
}}
 
}}
−
{{dcl | since=c++17|
+
|inplace_vector=
 +
{{dcl|num=1|since=c++26|
 +
constexpr iterator begin() noexcept;
 +
}}
 +
{{dcl|num=2|since=c++26|
 +
constexpr const_iterator begin() const noexcept;
 +
}}
 +
{{dcl|num=3|since=c++26|
 
constexpr const_iterator cbegin() const noexcept;
 
constexpr const_iterator cbegin() const noexcept;
 
}}
 
}}
−
{{dcl rev end}}
 
−
{{dcl end}}
 
 
|span=
 
|span=
−
{{dcl begin}}
+
{{dcl|num=1|since=c++20|
−
{{dcl |1=  
+
 
constexpr iterator begin() const noexcept;
 
constexpr iterator begin() const noexcept;
 
}}
 
}}
−
{{dcl end}}
+
{{dcl|num=2|since=c++23|
−
|{{dcl begin}}
+
constexpr const_iterator cbegin() const noexcept;
−
{{cpp/container/noexcept dcl | {{{1|}}}|dcl=
+
}}
 +
|flat_map|flat_multimap|flat_set|flat_multiset=
 +
{{dcl|num=1|since=c++23|
 +
iterator begin() noexcept;
 +
}}
 +
{{dcl|num=2|since=c++23|
 +
const_iterator begin() const noexcept;
 +
}}
 +
{{dcl|num=3|since=c++23|
 +
const_iterator cbegin() const noexcept;
 +
}}
 +
|
 +
{{cpp/container/noexcept dcl|num=1|{{#var:cont}}|dcl=
 
iterator begin()
 
iterator begin()
 
}}
 
}}
−
{{cpp/container/noexcept dcl | {{{1|}}}|dcl=
+
{{cpp/container/noexcept dcl|num=2|{{#var:cont}}|dcl=
 
const_iterator begin() const
 
const_iterator begin() const
 
}}
 
}}
−
{{dcl | since={{cpp/std|{{{1|}}}|c++11}} |
+
{{dcl|num=3|since={{cpp/std|{{#var:cont}}|c++11}}|
 
const_iterator cbegin() const noexcept;
 
const_iterator cbegin() const noexcept;
 
}}
 
}}
−
{{dcl end}}}}
+
}}
 +
{{dcl end}}
  
−
Returns an iterator to the first element of the {{tt|{{{1}}}}}.  
+
Returns an iterator to the first element of the {{tt|{{#var:cont}}}}.
  
−
If the {{tt|{{{1}}}}} is empty, the returned iterator will be equal to {{lc|end()}}.
+
If the {{tt|{{#var:cont}}}} is empty, the returned iterator will be equal to {{lc|end()}}.
  
 
{{image|range-begin-end.svg}}
 
{{image|range-begin-end.svg}}
Line 62: Line 78:
 
===Complexity===
 
===Complexity===
 
Constant.
 
Constant.
−
{{#if:{{#pos:{{{1}}}|set}}|{{cpp/container/constant iterator note}}}}
+
<!---->
−
 
+
{{#ifexpr:{{cpp/container/if c++98|{{#var:cont}}|1|0}}or{{cpp/container/if set|{{#var:cont}}|1|0}}|
 +
===Notes===
 +
{{cpp/container/if set|{{#var:cont}}|{{cpp/container/constant iterator note}}}}
 +
{{cpp/container/if c++98|{{#var:cont}}|<p>libc++ backports {{tt|cbegin()}} to C++98 mode.</p>}}}}
 +
<!---->
 
===Example===
 
===Example===
−
{{#switch:{{{1|}}}|unordered_multiset=
+
{{#switch:{{#var:cont}}
−
{{example|code=
+
|deque|forward_list|list|vector={{cpp/container/begin/examples/sequence|{{#var:cont}}}}
−
#include <iostream>
+
|{{cpp/container/begin/examples/{{#var:cont}}}}
−
#include <iterator>
+
−
#include <string>
+
−
#include <unordered_set>
+
−
 
+
−
int main() {
+
−
    const std::unordered_multiset<std::string> words = {
+
−
        "some", "words", "to", "count",
+
−
        "count", "these", "words"
+
−
    };
+
−
 
+
−
    for (auto it = words.begin(); it != words.end(); ) {
+
−
        auto cnt = words.count(*it);
+
−
        std::cout << *it << ":\t" << cnt << '\n';
+
−
        std::advance(it, cnt); // all cnt elements have equivalent keys
+
−
    }
+
−
}
+
−
|p=true
+
−
|std=c++11
+
−
|output=
+
−
some:  1
+
−
words:  2
+
−
to:    1
+
−
count:  2
+
−
these:  1
+
−
}}
+
−
|multiset=
+
−
{{example|code=
+
−
#include <iostream>
+
−
#include <iterator>
+
−
#include <set>
+
−
#include <string>
+
−
 
+
−
int main()
+
−
{
+
−
    const std::multiset<std::string> words = {
+
−
        "some", "not", "sorted", "words",
+
−
        "will", "come", "out", "sorted",
+
−
    };
+
−
 
+
−
    for (auto it = words.begin(); it != words.end(); ) {
+
−
        auto cnt = words.count(*it);
+
−
        std::cout << *it << ":\t" << cnt << '\n';
+
−
        std::advance(it, cnt); // all cnt elements have equivalent keys
+
−
    }
+
−
}
+
−
|p=true
+
−
|std=c++11
+
−
|output=
+
−
come: 1
+
−
not: 1
+
−
out: 1
+
−
some: 1
+
−
sorted: 2
+
−
will: 1
+
−
words: 1
+
−
}}
+
−
|map=
+
−
{{example|code=
+
−
#include <iostream>
+
−
#include <map>
+
−
+
−
int main() {
+
−
  std::map<int, float> num_map;
+
−
  num_map[4] = 4.13;
+
−
  num_map[9] = 9.24;
+
−
  num_map[1] = 1.09;
+
−
  // calls a_map.begin() and a_map.end()
+
−
  for (auto it = num_map.begin(); it != num_map.end(); ++it) {
+
−
    std::cout << it->first << ", " << it->second << '\n';
+
−
  }
+
−
}
+
−
| output=
+
−
1, 1.09
+
−
4, 4.13
+
−
9, 9.24
+
−
}}
+
−
====Example using a custom comparison function====
+
−
{{example|code=
+
−
#include <cmath>
+
−
#include <iostream>
+
−
#include <map>
+
−
+
−
struct Point { double x, y; };
+
−
 
+
−
typedef Point * PointPtr;
+
−
//Compare the x-coordinates of two Point pointers
+
−
struct PointCmp {
+
−
    bool operator()(const PointPtr &lhs, const PointPtr &rhs) const {
+
−
        return lhs->x < rhs->x;
+
−
    }
+
−
};
+
−
+
−
int main() {
+
−
    //Note that although the x-coordinates are out of order, the
+
−
    // map will be iterated through by increasing x-coordinates
+
−
    Point points[3] = { {2, 0}, {1, 0}, {3, 0} };
+
−
 
+
−
    //mag is a map sending the address of node to its magnitude in the x-y plane
+
−
    //Although the keys are pointers-to-Point, we want to order the map by the
+
−
    // x-coordinates of the points and NOT by the addresses of the Points. This
+
−
    // is done by using the PointCmp class's comparison method.
+
−
    std::map<Point *, double, PointCmp> mag({
+
−
        { points,    2 },
+
−
        { points + 1, 1 },
+
−
        { points + 2, 3 }
+
−
    });
+
−
 
+
−
    //Change each y-coordinate from 0 to the magnitude
+
−
    for(auto iter = mag.begin(); iter != mag.end(); ++iter){
+
−
        auto cur = iter->first; // pointer to Node
+
−
        cur->y = mag[cur]; // could also have used  cur->y = iter->second;
+
−
    }
+
−
+
−
    //Update and print the magnitude of each node
+
−
    for(auto iter = mag.begin(); iter != mag.end(); ++iter){
+
−
        auto cur = iter->first;
+
−
        mag[cur] = std::hypot(cur->x, cur->y);
+
−
        std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
+
−
        std::cout << iter->second << '\n';
+
−
    }
+
−
+
−
    //Repeat the above with the range-based for loop
+
−
    for(auto i : mag) {
+
−
        auto cur = i.first;
+
−
        cur->y = i.second;
+
−
        mag[cur] = std::hypot(cur->x, cur->y);
+
−
        std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
+
−
        std::cout << mag[cur] << '\n';
+
−
        //Note that in contrast to std::cout << iter->second << '\n'; above,
+
−
        // std::cout << i.second << '\n'; will NOT print the updated magnitude
+
−
    }
+
−
}
+
−
| output=
+
−
The magnitude of (1, 1) is 1.41421
+
−
The magnitude of (2, 2) is 2.82843
+
−
The magnitude of (3, 3) is 4.24264
+
−
The magnitude of (1, 1.41421) is 1.73205
+
−
The magnitude of (2, 2.82843) is 3.4641
+
−
The magnitude of (3, 4.24264) is 5.19615
+
−
}}
+
−
|set=
+
−
{{example|code=
+
−
#include <set>
+
−
#include <iostream>
+
−
+
−
int main() {
+
−
  std::set<int> set = { 6, 1, 3, 4, 2, 5 };
+
−
  for (auto it = set.begin(); it != set.end(); ++it)
+
−
    std::cout << *it << "\n";
+
−
}
+
−
|p=true
+
−
|std=c++11
+
−
|output=
+
−
1
+
−
2
+
−
3
+
−
4
+
−
5
+
−
6
+
−
}}
+
−
|unordered_map=
+
−
{{example|code=
+
−
#include <cmath>
+
−
#include <iostream>
+
−
#include <unordered_map>
+
−
 
+
−
struct Node { double x, y; };
+
−
 
+
−
int main() {
+
−
    Node nodes[3] = { {1, 0}, {2, 0}, {3, 0} };
+
−
   
+
−
    //mag is a map mapping the address of a Node to its magnitude in the plane
+
−
    std::unordered_map<Node *, double> mag = {
+
−
        { nodes,    1 },
+
−
        { nodes + 1, 2 },
+
−
        { nodes + 2, 3 }
+
−
    };
+
−
   
+
−
    //Change each y-coordinate from 0 to the magnitude
+
−
    for(auto iter = mag.begin(); iter != mag.end(); ++iter){
+
−
        auto cur = iter->first; // pointer to Node
+
−
        cur->y = mag[cur]; // could also have used  cur->y = iter->second;
+
−
    }
+
−
+
−
    //Update and print the magnitude of each node
+
−
    for(auto iter = mag.begin(); iter != mag.end(); ++iter){
+
−
        auto cur = iter->first;
+
−
        mag[cur] = std::hypot(cur->x, cur->y);
+
−
        std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
+
−
        std::cout << iter->second << '\n';
+
−
    }
+
−
 
+
−
    //Repeat the above with the range-based for loop
+
−
    for(auto i : mag) {
+
−
        auto cur = i.first;
+
−
        cur->y = i.second;
+
−
        mag[cur] = std::hypot(cur->x, cur->y);
+
−
        std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
+
−
        std::cout << mag[cur] << '\n';
+
−
        //Note that in contrast to std::cout << iter->second << '\n'; above,
+
−
        // std::cout << i.second << '\n'; will NOT print the updated magnitude
+
−
    }
+
−
}
+
−
|p=true
+
−
|std=c++11
+
−
|output=
+
−
The magnitude of (3, 3) is 4.24264
+
−
The magnitude of (1, 1) is 1.41421
+
−
The magnitude of (2, 2) is 2.82843
+
−
The magnitude of (3, 4.24264) is 5.19615
+
−
The magnitude of (1, 1.41421) is 1.73205
+
−
The magnitude of (2, 2.82843) is 3.4641
+
−
 
+
−
}}
+
−
|unordered_set=
+
−
{{example|code=
+
−
#include <iostream>
+
−
#include <unordered_set>
+
−
 
+
−
struct Point { double x, y; };
+
−
 
+
−
int main() {
+
−
    Point pts[3] = { {1, 0}, {2, 0}, {3, 0} };
+
−
   
+
−
    //points is a set containing the addresses of points
+
−
    std::unordered_set<Point *> points = { pts, pts + 1, pts + 2 };
+
−
   
+
−
    //Change each y-coordinate of (i, 0) from 0 into i^2 and print the point
+
−
    for(auto iter = points.begin(); iter != points.end(); ++iter){
+
−
        (*iter)->y = ((*iter)->x) * ((*iter)->x); //iter is a pointer-to-Point*
+
−
        std::cout << "(" << (*iter)->x << ", " << (*iter)->y << ") ";
+
−
    }
+
−
    std::cout << '\n';
+
−
   
+
−
    //Now using the range-based for loop, we increase each y-coordinate by 10
+
−
    for(Point * i : points) {
+
−
        i->y += 10;
+
−
        std::cout << "(" << i->x << ", " << i->y << ") ";
+
−
    }
+
−
}
+
−
|p=true
+
−
|std=c++11
+
−
|output=
+
−
(3, 9) (1, 1) (2, 4)
+
−
(3, 19) (1, 11) (2, 14)
+
−
}}
+
−
|vector|list|forward_list|deque=
+
−
{{example|code=
+
−
#include <iostream>
+
−
#include <{{{1}}}>
+
−
#include <string>
+
−
 
+
−
int main()
+
−
{
+
−
std::{{{1}}}<int> ints {1, 2, 4, 8, 16};
+
−
std::{{{1}}}<std::string> fruits {"orange", "apple", "raspberry"};
+
−
std::{{{1}}}<char> empty;
+
−
 
+
−
// Sums all integers in the {{{1}}} ints (if any), printing only the result.
+
−
int sum = 0;
+
−
for (auto it = ints.cbegin(); it != ints.cend(); it++)
+
−
sum += *it;
+
−
std::cout << "Sum of ints: " << sum << "\n";
+
−
 
+
−
// Prints the first fruit in the {{{1}}} fruits, without checking if there is one.
+
−
std::cout << "First fruit: " << *fruits.begin() << "\n";
+
−
 
+
−
if (empty.begin() == empty.end())
+
−
std::cout << "{{{1}}} 'empty' is indeed empty.\n";
+
−
}
+
−
|output=
+
−
Sum of ints: 31
+
−
First fruit: orange
+
−
{{{1}}} 'empty' is indeed empty.
+
−
}}
+
−
|array=
+
−
{{example
+
−
|code=
+
−
#include <array>
+
−
#include <iostream>
+
−
#include <algorithm>
+
−
#include <iomanip>
+
−
 
+
−
int main()
+
−
{
+
−
    std::cout << std::boolalpha;
+
−
 
+
−
    std::array<int, 0> empty;
+
−
    std::cout << "1) "
+
−
              << (empty.begin() == empty.end()) << ' '    // true
+
−
              << (empty.cbegin() == empty.cend()) << '\n'; // true
+
−
    // *(empty.begin()) = 42; // => undefined behaviour at run-time
+
−
 
+
−
 
+
−
    std::array<int, 4> numbers{5, 2, 3, 4};
+
−
    std::cout << "2) "
+
−
              << (numbers.begin() == numbers.end()) << ' '    // false
+
−
              << (numbers.cbegin() == numbers.cend()) << '\n' // false
+
−
              << "3) "
+
−
              << *(numbers.begin()) << ' '    // 5
+
−
              << *(numbers.cbegin()) << '\n'; // 5
+
−
 
+
−
    *numbers.begin() = 1;
+
−
    std::cout << "4) " << *(numbers.begin()) << '\n'; // 1
+
−
    // *(numbers.cbegin()) = 42; // compile-time error:
+
−
                                // read-only variable is not assignable
+
−
 
+
−
    // print out all elements
+
−
    std::cout << "5) ";
+
−
    std::for_each(numbers.cbegin(), numbers.cend(), [](int x) {
+
−
      std::cout << x << ' ';
+
−
    });
+
−
    std::cout << '\n';
+
−
 
+
−
 
+
−
    constexpr std::array constants{'A', 'B', 'C'};
+
−
    static_assert(constants.begin() != constants.end());  // OK
+
−
    static_assert(constants.cbegin() != constants.cend()); // OK
+
−
    static_assert(*constants.begin() == 'A');              // OK
+
−
    static_assert(*constants.cbegin() == 'A');            // OK
+
−
    // *constants.begin() = 'Z'; // compile-time error:
+
−
                                // read-only variable is not assignable
+
−
}
+
−
|output=
+
−
1) true true
+
−
2) false false
+
−
3) 5 5
+
−
4) 1
+
−
5) 1 2 3 4
+
−
}}
+
−
|{{example}}
+
 
}}
 
}}
  
 
===See also===
 
===See also===
−
 
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/container/dsc end |{{{1|}}}}}
+
{{dsc inc|cpp/container/dsc end|{{#var:cont}}}}
 +
{{dsc inc|cpp/iterator/dsc begin}}
 
{{dsc end}}
 
{{dsc end}}

Latest revision as of 05:07, 16 August 2024

 
C++
 
Containers library
 
std::inplace_vector
Member types
Member functions
Non-member functions
 
constexpr iterator begin() noexcept;
 (1) (since C++26)
constexpr const_iterator begin() const noexcept;
 (2) (since C++26)
constexpr const_iterator cbegin() const noexcept;
 (3) (since C++26)

Returns an iterator to the first element of the inplace_vector.

If the inplace_vector is empty, the returned iterator will be equal to end().

range-begin-end.svg

Contents

[edit] Parameters

(none)

[edit] Return value

Iterator to the first element.

[edit] Complexity

Constant.

[edit] Example

Template:cpp/container/begin/examples/inplace vector

[edit] See also

 returns an iterator to the end
(public member function of std::inplace_vector<T,N>) [edit]
(C++11)(C++14)
 returns an iterator to the beginning of a container or array
(function template) [edit]
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