Question

The problem is passing lists/vectors by reference

   int main(){
        list<int> arr;
        //Adding few ints here to arr
        func1(&arr);   
        return 0; 
    }

    void func1(list<int> * arr){
     // How Can I print the values here ?

     //I tried all the below , but it is erroring out.
     cout<<arr[0]; // error
     cout<<*arr[0];// error
     cout<<(*arr)[0];//error

     //How do I modify the value at the index 0 ?

     func2(arr);// Since it is already a pointer, I am passing just the address
     }

     void func2(list<int> *arr){

     //How do I print and modify the values here ? I believe it should be the same as above but 
     // just in case.
     }

Is the vectors any different from the lists ? Thanks in advance.

Any links where these things are explained elaborately will be of great help. Thanks again.

Answer 1

You aren’t passing the list by reference, but by a pointer. In “C talk” the two are equal, but since there is a reference type in C++, the distinction is clear.

To pass by reference, use & instead of * – and access “normally”, i.e.

void func(list<int>& a) {
    std::cout << a.size() << "\n";
}

To pass by pointer, you need to derefer the pointer with an asterisk (and do take note of operator presedence), i.e.

void func(list<int>* arr) {
    std::cout << (*a).size() << "\n"; // preferably a->size();
}

There is no operator[] in std::list.