Can't create an instance of a class (no default constructor) - c++

Question

Edit:
I've found the solution by myself. I'm sorry for this question, that wasn't fully understanble for You.
Okey, I have so much code in my program. I'm writing a tree with using a list,that I'm going to write. I can't use STL library.
30_Contaner.h:
#pragma once
#include "Container_5_30.h"
#include "Tree.h"
#include "List.h"
using std::cout;
using std::cin;
class Tree : public AbstractTree
{//Cathy
protected:
struct ListStruct{
void* key;
size_t size;
ListStruct* next;
};
//Iterator *globalIterator;
class List : public AbstractList{
public:
/*some code*/
List(MemoryManager& mem) :AbstractList(mem){//constructor
}
virtual ~List(){
}
//some functions
};
struct TreeStruct{
TreeStruct *parent;
//OneLinkedList* Children = new OneLinkedList;
List *Children;//error
void *elem;
size_t size;
int childIndex;
};
int numberOfElements;
bool globalBoolForRecursion;
//TreeStruct *TreeStructInstance = new TreeStruct;
List *ListInstance = new List;//error
public:
Tree(MemoryManager& mem) :AbstractTree(mem) {
//root = new TreeStruct;//root - корень дерева (элемент структуры Дерева)
numberOfElements = 0;//кол-во элементов в дереве пока что 0
}
//some functions
};
main.cpp:
#include "30_Container.h"
#include "30_Mem.h"
#include "Tree.h"
//********************************
void main(){
Mem mem(100);
Tree tree(mem);
//some code
}
30_Mem.h
#pragma once
#include "MemoryManager.h"
// Простейший менеджер памяти, использует ::new и ::delete
class Mem : public MemoryManager
{
public:
Mem(size_t sz) :MemoryManager(sz) {}
void* allocMem(size_t sz) { return new char[sz]; }
void freeMem(void* ptr) { delete[] ptr; }
};
All methods in AbstractTree,MemoryManager and AbstractList classes are virtual. I should get instance of List class inside Tree class. But, I have a problem: I don't have default constructor for List and, of course, when I write List *list = new List; I have an error. My teacher told me that I should use pointer to memory manager of tree, or address, may be. Do you have any idea?
P.S. I need 30_Mem.h in the future. I should be writing my own new

Answer 1

Instead of default initialization for ListInstance add initialization of ListInstance to constructor
Tree(MemoryManager& mem) :AbstractTree(mem), ListInstance(mem) {
//root = new TreeStruct;//root - корень дерева (элемент структуры Дерева)
numberOfElements = 0;//кол-во элементов в дереве пока что 0
}