Question

I’m trying to make a little custom vector…

element class, should point to the adress of the next class but with this code

class foo {
private:
    int attr;
public:
    foo(){attr = 10;}
    int get_attr(){return attr;}
    void set_attr(int a){attr =a;}
};

class element_foo {
private:
    foo data;
    element_foo *ptr_next;
public:
    element_foo(){ptr_next = NULL;}
    element_foo(int dat){data.set_attr(dat); ptr_next = NULL;}
    element_foo(int dat, element_foo next){
        data.set_attr(dat);
        ptr_next = &next;
    }
    foo get_data(){return data;}

    element_foo get_next(){return *ptr_next;}

    void print_array(){
        if (ptr_next == NULL) {
            std::cout<< data.get_attr()<<std::endl;
        }
        else {
            std::cout<< data.get_attr()<<std::endl;
            this->get_next().print_array();
        }

    }
};



int main (int argc, char * const argv[]) {
    // insert code here...  
    element_foo a1(10);
    element_foo a2(15,a1);
    element_foo a3(20,a2);

    a3.print_array();

    std::cout << "Hello, World!\n";
    return 0;
}

when I print a3, it get to segmentation fault… why? where is my mistake?

Answer 1

The error is in this constructor:

element_foo(int dat, element_foo next){
    data.set_attr(dat);
    ptr_next = &next;
}

You are taking the address of a local. ptr_next = &next; Once the function ends, the address is invalid.

What you need to do is to pass next in as a pointer:

element_foo(int dat, element_foo *next){
    data.set_attr(dat);
    ptr_next = next;
}

And change your main to this:

element_foo a1(10);
element_foo a2(15,&a1);
element_foo a3(20,&a2);

EDIT:

Alternatively, you can just pass it by reference:

element_foo(int dat, element_foo &next){
    data.set_attr(dat);
    ptr_next = &next;
}