Question

I wrote this function in C++ as part of a bigger program:

Object Single_list<Object>::pop_front() {
    //Single_node<Object> *tmp_front;  
    //Object hold;
    if (empty()) {
        throw underflow();
    } 

    Single_node<Object> first_node = front();
    Single_node<Object> *ptr = list_head;                                                            
    list_head = list_head->next();  
    delete ptr;
    return first_node.retrieve();
}

However, when I try and use this function I get the following message:

WARNING: calling delete twice on the same memory location: 0x100100160

I am really confused, I am not deleting the pointer (assuming that’s what is causing the problem?) twice.

Any advice will be appreciated.

As requested, here is the constructor and retrieve function for the Single_Node, although I am not sure how helpful this will be to the error regarding deletion that I am getting:

template <typename Object>
Single_node<Object>::Single_node( const Object &e, Single_node<Object> *n ):element( e ), next_node( n )
{
    // empty constructor
}

template <typename Object>
Object Single_node<Object>::retrieve() const
{
    return element;
}

template <typename Object>
Single_node<Object> *Single_node<Object>::next() const
{ 
    return next_node;
}

Answer 1

Building on Mankarse’s comment, try the following:

Object Single_list<Object>::pop_front() {

    if (empty()) {
        throw underflow();
    } 

    Object result = front().retrieve();

    Single_node<Object> *ptr = list_head;
    list_head = list_head->next();  
    delete ptr;
    return result;
}

This grabs the result from the first node before it gets deleted.