forwardlist_name1.merge(forward_list& forwardlist_name2)
                  or
forwardlist_name1.merge(forward_list& forwardlist_name2, Compare comp)

// CPP program to illustrate the
// forward_list::merge() function
#include <bits/stdc++.h>
using namespace std;
  
int main()
{
    forward_list<int> fl1 = { 12, 25, 31, 41 };
    forward_list<int> fl2 = { 10, 20, 30 };
  
    // 合并两个forward_list
    fl1.merge(fl2);
  
    // 输出forward_list的内容
    cout << "List contains following elements" << endl;
    for (auto it = fl1.begin(); it != fl1.end(); ++it)
        cout << *it << " ";
  
    return 0;
}

List contains following elements
10 12 20 25 30 31 41

#include <bits/stdc++.h>
using namespace std;
  
// 比较函数
bool cmp_fun(int a, int b)
{
    return a > b;
}
  
int main()
{
    forward_list<int> fl1 = { 41, 31, 25, 12 };
    forward_list<int> fl2 = { 30, 20, 10 };
  
    // 合并两个forward_list
    fl1.merge(fl2, cmp_fun);
  
    // 输出forward_list的内容
    cout << "List contains following elements" << endl;
    for (auto it = fl1.begin(); it != fl1.end(); ++it)
        cout << *it << " ";
  
    return 0;
}

List contains following elements
41 31 30 25 20 12 10