Странное поведение строки

int main() {
    list a1("bca");
    list a2("gty");
    char* b1 = a1 + a2;
    cout << b1;
 
 
    return 0;
}

#include "stdafx.h"
 
struct list_item {
    char item;
    list_item* nextptr;
};
 
ostream& operator<< (ostream& output, const list_item& printing_list) {
    output << printing_list.item;
    return output;
}
 
 
class list {
    friend ostream& operator<< (ostream&, list&);
public:
    list();                         
    list(const char*);
    list(const list&);
 
    ~list();
 
    void interface_list();
    list_item* insert_item(char = 0, int = 0);
    void delete_item(char = 0, int = 0);
    int isempty() const;
    void print();
 
    list_item& operator[] (int) const;
    const list& operator=(const list&);
    char* operator+ (const list&) const;
    list operator++ ();
    list& operator+= (const list&);
 
private:
    list_item* first_itemptr;
    int count;
};
 
//////MAIN////////////////MAIN/////////MAIN//////////
int main() {
    list a1("bca");
    list a2("gty");
    char* b1 = a1 + a2;
    cout << b1;
 
 
    return 0;
}
////////////////////////////////////////////////////
 
list::list()
{
    first_itemptr = NULL;
    count = 0;
}
 
list::list(const char * line)
{
    int length = strlen(line); 
    first_itemptr = NULL; count = 0;
    for (int i = 0; i < length; i++) {
        insert_item(line[i], 0);
    }
}
 
list::list(const list& copy) {
    list_item* counting = copy.first_itemptr;
    first_itemptr = NULL;
    count = 0;
    while (counting != NULL) {
        insert_item(counting->item, 0);
        counting = counting->nextptr;
    }
}
 
ostream& operator<< (ostream& out,  list& printing_list) {
    printing_list.print();
    return out;
}
 
 
 
void list::interface_list()
{
 
    //INTERFACE PRINTING---------------------------------------------
    cout << "\n\nInterface of list class\n"
        << "1 to see the options\n0 to exit\n\n";
    if (getch() == '0')
         return;
    cout << "Options:\n"
        << "1) Insert letter\n"
        << "2) Delete item";
    if (!(this->isempty()))
        cout << "\n";
    else cout << "(unvailable)\n";
    cout << "3) Print list\n"
        << "4) Is list empty\n?"
        << "?\n";
    //INTERFACE REALISATION---------------------------------------------
    switch (getche() - '0') {
    case(1) : {
        this->insert_item(0, 1);
        this->interface_list();
        break; }
    case(2) : {
        if (!(this->isempty())) {
            this->delete_item(0, 1);
            this->interface_list();
            break;
        }
        else {
            cout << "This option is unvailable, try again please\n\n";
            this->interface_list();
            break;
        }   }
    case(3) : {
        if (this->isempty()) {
            cout << "Sorry, there is nothing in list try again please\n\n";
            this->interface_list();
            break;
        }
        else {
            this->print();
            this->interface_list();
        }
    }
    case(4) : {
        cout << (this->isempty() ? "\nList is empty\n" : "List isn't empty\n");
        this->interface_list();
        break; }
    default: {
        cout << "Wrong type, try again please\n\n";
        this->interface_list();
        break;
    }
    }
}
 
 
list_item * list::insert_item(char inserting_char, int choice)
{
    list_item* prevptr = NULL, *newptr = NULL, *next_goingptr = first_itemptr;
    if (choice == 1) {
        cout << "What symbol do you want to insert?";
        cin >> inserting_char;
    }
    newptr = new list_item;
    newptr->item = inserting_char;
    newptr->nextptr = NULL;
    while (next_goingptr != NULL && inserting_char > next_goingptr->item) {
        prevptr = next_goingptr;
        next_goingptr = next_goingptr->nextptr;
    }
    if (prevptr == NULL && next_goingptr==NULL) {
        newptr->nextptr = first_itemptr;
        first_itemptr = newptr;
        cout << "Symbol ""<< inserting_char << ""succusfully have insetred\n";
        count++;
    }
    else {
        if (next_goingptr!= NULL && inserting_char == next_goingptr->item ) {
            cout << "Sorry, list already have this symbol\n";
            return NULL;
        }
        else {
            newptr->nextptr = next_goingptr;
            if (prevptr == NULL) {
                first_itemptr = newptr;
            }
            else
                prevptr->nextptr = newptr;
            cout << "Symbol "" << inserting_char << ""succusfully have insetred\n";
            count++;
        }
    }
}
 
void list::delete_item(char deleting_char, int choice) {
    list_item* prevptr = NULL, *next_goingptr = first_itemptr;
    if (choice == 1) {
        cout << "What symbol do you want to delete?\n";
        cin >> deleting_char;
    }
    if ((first_itemptr)->item == deleting_char) {
        first_itemptr = (first_itemptr)->nextptr;
        delete next_goingptr;
        cout << "Symbol "" << deleting_char << ""succusfully have deleted\n";
        count--;
        return;
    }
    else {
        while (next_goingptr != NULL && deleting_char != next_goingptr->item) {
            prevptr = next_goingptr;
            next_goingptr = next_goingptr->nextptr;
        }
        if (next_goingptr == NULL)
            cout << "Sorry, there is no "" << deleting_char << ""symbol in list\n";
        else {
            prevptr->nextptr = next_goingptr->nextptr;
            delete next_goingptr;
            cout << "Symbol "" << deleting_char << ""succusfully have deleted\n";
            count--;
        }
    }
 
}
 
void list::print() {
    list_item* currentptr = first_itemptr;
    while (currentptr != NULL) {
        cout << currentptr->item << " --> ";
        currentptr = currentptr->nextptr;
    }
    cout << "NULL\n";
}
 
int list::isempty() const {
    return count ? 0 : 1;
}
 
 
 
list::~list() {
    list_item* timely = first_itemptr, *deleting = first_itemptr;
    while (timely != NULL) {
        timely = timely->nextptr;
        delete deleting;
        deleting = timely;
    }
    first_itemptr = NULL;
    count = 0;
}
 
 
 
list_item& list::operator[](int i) const {
    int a = 0;
    if (i <1 || i>count) {
        cout << "There is no item with this number";
        //return a;
    }
    list_item* current_item = first_itemptr;
    for (int j = 1; j != i; j++) {
        current_item = current_item->nextptr;
    }
    return *current_item;
}
 
const list& list::operator=(const list& right) {
    this->~list();
    list_item* counting = (right.first_itemptr), *current = NULL, *prev = NULL; int i = 0;
    while (counting != NULL) {
        current = new(list_item);
        current->item = counting->item;
        current->nextptr = NULL;
        if (i == 0)
            first_itemptr = current;
        if (prev != NULL)
            prev->nextptr = current;
        prev = current;
        counting = counting->nextptr;
        i++;
    }
    return *this;
}
 
char* list::operator+(const list& right) const {
    char string[100]; int i = 0;
    list_item *current=first_itemptr;
    for (; current != NULL; i++, current = current->nextptr)
        string[i] = current->item;
    current = right.first_itemptr;
    for (; current != NULL; i++, current = current->nextptr)
        string[i] = current->item;
    string[i] = '\0';
    return string;
 
}

char * f()
{
    char  x[] = "Hello";
    return x;
}
int main()
{
    char * x = f();
    cout<<x<<endl;
 
    system("pause");
    return 0;
}

#include <string>
string list::operator+(const list& right) const 
{
    string ss;
    list_item *current=first_itemptr;
    for (; current != NULL;current = current->nextptr)
        ss+= current->item;
    current = right.first_itemptr;
    for (; current != NULL;current = current->nextptr)
        ss += current->item;
    return ss;
}

string b1 = a1 + a2;