C++ standard library forward_list issue

Question

I need some help solving the exercise 9.28 from the C++ Primer 5th edition. Here is the task:

Write a function that takes a forward_list and two additional string arguments. The function should find the first string and insert the second immediately following the first. If the first string is not found, then insert the second string at the end of the list.

I can't get the idea written in the book as to how forward_list actually works and why we indeed need it, so I'm asking for help. Below is the code I wrote which is incomplete and I need someone help me finish it:

#include <iostream>
#include <forward_list>

using namespace std;

void find_and_insert(forward_list<string>& flstr, const string& needle, const string& rplc) {

    auto curr = flstr.begin();
    auto last = flstr.end();
    bool found = false;

    while (curr != last) {

        if (*curr == needle) {

            found = true;
            // what to put here?!
            break;
        }

        // what to put here?!... 
        ++curr;
    }

    for (const auto& elem : flstr) {

        cout << elem << endl;
    }

    return;
}

int main(int argc, char *argv[]) {

    cout << endl;

    forward_list<string> flstring = {"Foo", "Bar", "Baz", "Tomcat"};

    find_and_insert(flstring, "Bar", "Insertion");

    cout << endl;

    return EXIT_SUCCESS;
}

Any suggestion to refactor the code is welcome!

Answer 1

You can write the function the following way using member function before_begin

#include <iostream>
#include <forward_list>
#include <string>

void find_and_insert( std::forward_list<std::string> &lst, 
                      const std::string &src, 
                      const std::string &dest ) 
{
    auto before = lst.before_begin();
    auto first  = lst.begin();

    while ( first != lst.end() && *first != src ) ++before, ++first;

    lst.insert_after( first == lst.end() ? before : first, dest );
}

int main()
{
    std::forward_list<std::string> lst;

    find_and_insert( lst, "one", "zero" );

    for ( const std::string &s : lst ) std::cout << s << ' ';
    std::cout << std::endl;

    find_and_insert( lst, "zero", "two");

    for ( const std::string &s : lst ) std::cout << s << ' ';
    std::cout << std::endl;

    find_and_insert( lst, "zero", "one");

    for ( const std::string &s : lst ) std::cout << s << ' ';
    std::cout << std::endl;

    find_and_insert( lst, "two", "three");

    for ( const std::string &s : lst ) std::cout << s << ' ';
    std::cout << std::endl;

    find_and_insert( lst, "four", "four");

    for ( const std::string &s : lst ) std::cout << s << ' ';
    std::cout << std::endl;
}    

The program output is

zero 
zero two 
zero one two 
zero one two three 
zero one two three four 

Answer 2

For the first part you need to use insert_after. You will pass the current iterator to it and the string to insert and it will insert that string after the current element.

As far as as inserting an element at the end that gets a little more complicated as std::forward_list::iterator is a ForwardIterator. Luckily we have the before_begin which will return an iterator 1 before the beginning. You can get capture that iterator and every time you increment curr you increment that iterator as well. Once the loop is over check to see found is true. If it is then you can use that 1 before the beginning iterator as it now points to the last element. You will pass that to insert_after with the string just like you do in the while loop.