“required from here” error declaring a List of Lists

Question

I'm trying to declare a list of lists in this way:

List_vector<List_vector<int> > multilist;

But Eclipse underlines the above declaration and gives this error:

required from here

Partial List_vector implementation:

template<class T>
class List_vector: public Linear_list<T, int> {
public:
  typedef typename Linear_list<T, int>::value_type value_type;
  typedef typename Linear_list<T, int>::position position;

  List_vector();
  List_vector(int);
  List_vector(const List_vector<T>&);
  ~List_vector();
private:
    void change_dimension_(T*&, int, int);
    value_type* elements_;
    int length_; // the length of the list
    int array_dimension_; // array's dimension
};

Compiler output:

g++ -O3 -Wall -c -fmessage-length=0 -o multilista.o "..\\multilista.cpp" 
In file included from ..\multilista.cpp:1:0:
..\list_vector.h: In instantiation of 'List_vector<T>::~List_vector() [with T = List_vector<int>]':
..\multilista.cpp:16:32: required from here
..\list_vector.h:78:5: warning: deleting object of polymorphic class type List_vector<int>' which has non-virtual destructor might cause undefined behaviour [-Wdelete-non-virtual-dtor]
..\list_vector.h: In member function 'List_vector<T>::value_type List_vector<T>::read(List_vector<T>::position) const [with T = int; List_vector<T>::value_type = int; List_vector<T>::position = int]':
..\list_vector.h:136:1: warning: control reaches end of non-void function [-Wreturn-type]
..\list_vector.h: In member function 'List_vector<T>::value_type List_vector<T>::read(List_vector<T>::position) const [with T = List_vector<int>; List_vector<T>::value_type = List_vector<int>; List_vector<T>::position = int]':
..\list_vector.h:136:1: warning: control reaches end of non-void function [-Wreturn-type]
g++ -O3 -Wall -c -fmessage-length=0 -o tester.o "..\\tester.cpp" 
g++ -o Lista.exe tester.o multilista.o 

Answer 1

In file included from ..\multilista.cpp:1:0:
..\list_vector.h: In instantiation of 'List_vector<T>::~List_vector() [with T = List_vector<int>]':
..\multilista.cpp:16:32:   required from here
..\list_vector.h:78:5: warning: deleting object of polymorphic class type 'List_vector<int>' which has non-virtual destructor might cause undefined behaviour [-Wdelete-non-virtual-dtor]

If you're going to derive from Linear_List, you should consider making the destructor virtual, like it says. This is only a warning though, and only you know whether it is really required (there isn't enough code pasted to judge).

..\list_vector.h: In member function 'List_vector<T>::value_type List_vector<T>::read(List_vector<T>::position) const [with T = int; List_vector<T>::value_type = int; List_vector<T>::position = int]':
..\list_vector.h:136:1: warning: control reaches end of non-void function [-Wreturn-type]

You haven't pasted the code for List_vector::read, but it seems to be doing something wrong: every path out of the function should return a List_vector::value_type (unless it throws an exception), but you're letting control reach the end without doing either.

..\list_vector.h: In member function 'List_vector<T>::value_type List_vector<T>::read(List_vector<T>::position) const [with T = List_vector<int>; List_vector<T>::value_type = List_vector<int>; List_vector<T>::position = int]':
..\list_vector.h:136:1: warning: control reaches end of non-void function [-Wreturn-type]