打印列表列表 C++ STL 列表

Question

I have a top list that stores inner lists. I'm using the standard template library list template.

I am attempting to print the values of the inner lists. The top list is "L" and the inner list is "I".

void ListofLists::dump()
{
    list<list<IntObj>>::iterator itr;
    for (itr = L.begin(); itr != L.end(); itr++)
    {
        list<IntObj>::iterator it;
        for (it = I.begin(); it != I.end(); it++)
        {
            cout << *it << "  ";
        } 
        cout << endl << "End" << endl;
    }
}

My IDE doesn't like the line cout << *it << " "; and I'm not really sure how to change it while having the program do what I want it to do, which is print the data inside of the lists. It red underlined the “<<“ operator and says “no operator “<<“ matches these operands.”

Can someone help me as to why? I've looked and can't really find what I'm looking for. I'm not understanding something correctly. I know it is adding the data to the data structure correctly because my IDE enables me to view my locals.

Thanks to anyone who helps! Means a lot.

Answer 1

Try to use :

list<IntObj>::const_iterator i;

instead the one you are using to avoid compiling error.

Answer 2

The inner loop does not make sense.

If you want to use iterators then the function can be defined like

void ListofLists::dump() /* const */
{
    for (list<list<IntObj>>::iterator itr = L.begin(); itr != L.end(); itr++)
    {
        for ( list<IntObj>::iterator it = itr->begin(); it != itr->end(); it++)
        {
            cout << *it << "  ";
        } 
        cout << endl << "End" << endl;
    }
}

However it will be simpler to use the range-based for loop. For example

void ListofLists::dump() /* const */
{
    for ( const auto &inner_list : L )
    {
        for ( const auto &item : inner_list )
        {
            cout << item << "  ";
        } 
        cout << endl << "End" << endl;
    }
}

Take into account that you have to define the operator << for the class IntObj . Its declaration should look like

std::ostream & operator <<( std::ostream &, const IntObj & );