在C++ STL中使用forward_list::cbefore_begin()

为了忠实地解释在C++ STL中使用forward_list::cbefore_begin()的概念，我们先讨论一下C++编程语言中的List代码和输出。forward_list c before begin函数是STL（标准模板库）中的一个内置功能。它返回一个指向forward_list第一个元素前面位置的常量随机访问迭代器。

C++ STL中的List

C++代码

// CPP program to show the implementation of List
// Here we are writing down the C++ programming language code to 
// demonstrate the concept of List in C++ programming language code
// in the Standard Template Library (STL)
#include 
#include 
#include 
using namespace std;

// function for printing the elements in a list
// the below few lines of code help us with displaying the elements that
// we have inserted into it which is a part of the C++ Standard Template 
// Library (STL)
void showlist(list g)
{
    list::iterator it;
    for (it = g.begin(); it != g.end(); ++it)
        cout << '\t' << *it;
    cout << '\n';
}

// the main driver code functionality starts from here
int main()
{

// then below code helps us with creating a list in C++ STL dynamically
    list gqlist1, gqlist2;
// loop to push elements into the list which we have just created
    for (int i = 0; i < 10; ++i) {
        gqlist1.push_back(i * 2);
        gqlist2.push_front(i * 3);
    }
    // cout statement helping us to print gqlist1 elements on console
    cout << "\nThe List 1 has elements  : ";
    showlist(gqlist1);
    // cout statement helping us to print gqlist2 elements on console
    cout << "\nThe List 2 has elements : ";
    showlist(gqlist2);
    // cout statement helping us to print gqlist1 front elements on console
    cout << "\n list1.front() : " << gqlist1.front();
    // cout statement helping us to print gqlist1 back elements on console
    cout << "\n list1.back() : " << gqlist1.back();
//cout statement helping us to print gqlist after poping elements on console
    cout << "\n list1.pop_front() : ";
    gqlist1.pop_front();
    showlist(gqlist1);
//cout statement helping us to print list after poping elements on console
    cout << "\n list2.pop_back() : ";
    gqlist2.pop_back();
    showlist(gqlist2);
//cout statement helping us to print gqlist after reverse elements on 
// console
    cout << "\n list1.reverse() : ";
    gqlist1.reverse();
    showlist(gqlist1);
//cout statement helping us to print gqlist after sort elements on console
    cout << "\n list2.sort(): ";
    gqlist2.sort();
    showlist(gqlist2);

    return 0;
    // end of the C++ programming language code explaining 
    // forward_list::cbefore_begin() in C++ STL

}

输出：

The List 1 has elements  :  0   2   4   6   8   10  12  14  16  18

List 2 has elements :   27  24  21  18  15  12  9   6   3   0

 list1.front() : 0
 list1.back() : 18
 list1.pop_front() :    2   4   6   8   10  12  14  16  18

 list2.pop_back() :     27  24  21  18  15  12  9   6   3

 list1.reverse() :  18  16  14  12  10  8   6   4   2

list2.sort():   3   6   9   12  15  18  21  24  27

forward_list::cbefore_begin()的语法

forwardlist_name.cbefore_begin()

C++代码

// Here we are writing down the c++ programming language code to 
// demonstrate the concept of forward_list::cbefore_begin() in C++ STL
// where it illustrates the function cbefore_begin() in C++ programming code
#include 
using namespace std;
// the main driver code functionality starts from here
int main()
{
    // the below code snippet helps us with creating a list with few
    // initial values like 20, 30, 40 and ends with 50
    forward_list fl = { 20, 30, 40, 50 };

    // here, we are trying to perform the function
    // cbefore_begin helping us to use the stl library in the C++ language
    auto it = fl.cbefore_begin();

    // the below code snippet helps us with the insertion operation 
    // where the element will be added to the first place
    fl.insert_after(it, 10);

    cout << "the elements which have gotten inserted are as follows" << endl;

    // the below for loop helps us with printing down the
    // elements, which we have created in the list on our output screens
    for (auto it = fl.begin(); it != fl.end(); ++it)
        cout << *it << " ";

    return 0;
    // end of the C++ programming language code explaining 
    // forward_list::cbefore_begin() in C++ STL
}

输出：

the elements which have gotten inserted are as follows
10 20 30 40 50