C++学习笔记STL(Standard Template Library)标准模板库-顺序容器（forward_list单链表）

std::forward_list

是支持从容器中的任何位置快速插入和移除元素的容器。不支持快速随机访问。它实现为单链表，且实质上与其在C中实现相比无任何开销。与，std::list相比，此容器在不需要双向迭代时提供更有效地利用空间的存储。

一，元素访问

1.front 访问第一个元素；

二，迭代器

1.before_begin/cbefore_begin 返回指向第一个元素之前迭代器

2.begin/cbegin 返回指向起始的迭代器

3.end/cend 返回指向未尾的迭代器

    forward_list<char> letters{ 'o', 'm', 'g', 'w', 't', 'f' };

    if (!letters.empty()) {
        cout << "The first character is: " << letters.front() << '
';
    }


    forward_list<int> nums{ 1, 2, 4, 8, 16 };
    forward_list<std::string> fruits{ "orange", "apple", "raspberry" };
    forward_list<char> empty;

    // 求和 forward_list nums 中的所有整数（若存在），仅打印结果。
    cout << "Sum of nums: " <<
       accumulate(nums.begin(), nums.end(), 0) << "
";

    // 打印 forward_list fruits 中的首个 fruis ，不检查是否有一个。
    cout << "First fruit: " << *fruits.begin() << "
";

    if (empty.begin() == empty.end())
       cout << "forward_list 'empty' is indeed empty.
";

三，容量

1.empty 检查容器是否为空

2.max_size 返回可容纳的最大元素数

四，修改器

1.clear 清除内容

2.insert_after 在某个元素后插入新元素

template<typename T>
std::ostream& operator<<(std::ostream& s, const std::forward_list<T>& v)
{
    s.put('[');
    char comma[3] = { '', ' ', '' };
    for (const auto& e : v) {
        s << comma << e;
        comma[0] = ',';
    }
    return s << ']';
}

int main(){
    std::forward_list<std::string> words{ "the", "frogurt", "is", "also", "cursed" };
    std::cout << "words: " << words << '
';
    // insert_after (2)                                                         
    auto beginIt = words.begin();
    words.insert_after(beginIt, "strawberry");
    std::cout << "words: " << words << '
';
    // insert_after (3)                                                         
    auto anotherIt = beginIt;
    ++anotherIt;
    anotherIt = words.insert_after(anotherIt, 2, "strawberry");
    std::cout << "words: " << words << '
';
    // insert_after (4)
    vector<std::string> V = { "apple", "banana", "cherry" };
    anotherIt = words.insert_after(anotherIt, V.begin(), V.end());
    std::cout << "words: " << words << '
';
    // insert_after (5)                                                         
    words.insert_after(anotherIt, { "jackfruit", "kiwifruit", "lime", "mango" });
    std::cout << "words: " << words << '
';

}

3.emplace_after 在元素后原位置构造元素

struct Sum {
    std::string remark;
    int sum;

    Sum(std::string remark, int sum)
        : remark{ std::move(remark) }, sum{ sum } {}

    void print() const {
        std::cout << remark << " = " << sum << '
';
    }
};
    std::forward_list<Sum> list;
    auto iter = list.before_begin();
    std::string str{ "1" };
    for (int i{ 1 }, sum{ 1 }; i != 10; sum += i) {
        iter = list.emplace_after(iter, str, sum);
        ++i;
        str += " + " + std::to_string(i);
    }

    for (const Sum& s : list) s.print();

4.erase_after 擦除元素后的元素

5.push_front 插入元素到容器起始

6.emplace_front在容器头部就地构造元素

7.pop_front 移除首元素

8.resize 改变容器中可存储元素的个数

9.swap 交换内容

五，操作

1.merge　　合并两个已排序列表

std::ostream& operator<<(std::ostream& ostr, const std::forward_list<int>& list)
{
    for (auto& i : list) {
        ostr << " " << i;
    }
    return ostr;
}
    std::forward_list<int> list1 = { 5,9,0,1,3 };
    std::forward_list<int> list2 = { 8,7,2,6,4 };
    list1.sort();
    list2.sort();
    std::cout << "list1:  " << list1 << "
";
    std::cout << "list2:  " << list2 << "
";
    list1.merge(list2);
    std::cout << "merged: " << list1 << "
";

2.splice_after　　从另一forward_list移动元素

3.remove/remove_if　　移除满足特定标准的元素

    std::forward_list<int> l = { 1,100,2,3,10,1,11,-1,12 };
    l.remove(1); // 移除两个等于 1 的元素
    l.remove_if([](int n) { return n > 10; }); // 移除全部大于 10 的元素
    for (int n : l) {
        std::cout << n << ' ';
    }
    std::cout << '
';

4.reverse　　将该链表的所有元素的顺序反转

   std::forward_list<int> list = { 8,7,5,9,0,1,3,2,6,4 };
    std::cout << "before:     " << list << "
";
    list.sort();
    std::cout << "ascending:  " << list << "
";
    list.reverse();
    std::cout << "descending: " << list << "
";

5.unique　　删除连续的重复元素

  std::forward_list<int> x = {1, 2, 2, 3, 3, 2, 1, 1, 2};
 
  std::cout << "contents before:";
  for (auto val : x)
    std::cout << ' ' << val;
  std::cout << '
';
 
  x.unique();
  std::cout << "contents after unique():";
  for (auto val : x)
    std::cout << ' ' << val;
  std::cout << '
';
 

6.sort　　对元素进行排序

    std::forward_list<int> list = { 8,7,5,9,0,1,3,2,6,4 };

    std::cout << "before:     " << list << "
";
    list.sort();
    std::cout << "ascending:  " << list << "
";
    list.sort(std::greater<int>());
    std::cout << "descending: " << list << "
";