erroe when write the member function“mergeList” in a linear list

[boyhailong]

#include<iostream>
using namespace std;
const int DefaultSize = 100;
class List
{
public:
	List();
	List(size_t size);
	List(List &L);
	~List();
	bool insert(const int);
	void ClearList();
	bool IsEmpty();
	size_t ListLen();
	int GetElem(size_t);
	void append(size_t,int);//在第i个位置插入数值
	void Display();
	void remove(size_t);
	List& merge(List &L);
	int FindElem(int elm);
private:
	size_t MaxSize;
	size_t list_len;
	int *element;
};
List::List()
{
	MaxSize = DefaultSize;
	list_len = 0;
	element = new int[MaxSize];
}
List::List(size_t size)
{
	MaxSize = size;
	list_len = 0;
	element = new int[MaxSize];
}
List::List(List &L)
{
	list_len = 0;
	size_t L_size = L.ListLen();
	MaxSize = 2*(list_len+L_size);
	for (size_t i = 0; i != L_size; ++i)
	{
		insert(L.GetElem(i));
	}
}
List::~List()
{
	delete []element;
}
bool List::insert(const int a)
{
	if (list_len == MaxSize)
		return false;
	element[list_len++] = a;
	return true;
}
bool List::IsEmpty()
{
	return list_len == 0;
}
int List::GetElem(size_t size)
{
	if(size > list_len)
		throw runtime_error("error");
	return element[size];
}
void List::Display()
{
	if(IsEmpty())
		cout << "List is empty" << endl;
	else
		for (size_t i = 0; i < list_len; ++i)
			cout << element[i] << " ";
}
size_t List::ListLen()
{
	return list_len;
}
void List::append(size_t i, int num)
{
	if(i < 0 || i > MaxSize)
	{
		cout << "append invalid" << endl;
	}
	if (i == list_len)
	{
		this->insert(num);
	} 
	else
	{
		for(size_t j = list_len-1; j != i-2; --j)
		{
			if(j == list_len-1)
				this->insert(element[j]);
			else
				element[j+1] = element[j];
		}
		element[i-1] = num;
	}
}
void List::remove(size_t i)
{
	if(i < 0 || i > list_len)
	{
		cout << "remove invalid" << endl;
	}
	else
	{
		for(size_t j = i; j != list_len+1; ++j)
		{
			element[j-1] = element[j];
		}
		--list_len;
	}
}
List& List::merge(List &L)
{
	if(L.IsEmpty())
		return *this;
	size_t size = list_len + L.ListLen();
	List merge_L(MaxSize+L.ListLen());
	for (size_t i = 0; i != size; ++i)
	{
		if (i < list_len)
		{
			merge_L.insert(element[i]);
		} 
		else
		{
			merge_L.insert(L.GetElem(i-list_len));
		}
	}
	return merge_L;
}
int List::FindElem(int elm)
{
	for (size_t i = 0; i != list_len; ++i)
	{
		if (element[i] == elm)
			return i+1;
	}
	return -1;
}
int main()
{
	List l,m,n;
	l.insert(11);
	l.insert(12);
	m.insert(13);
	m.insert(0);
	List Q(l.merge(m));
	cout << endl;
}

when i run it in VS2008，it reads “can't read the memory ‘’0XCCCCC”，i think there is some error in the fuction “merge”，but how the deal with it ？

[Elysia]

Copy constructor is wrong. You never allocate elements.
merge is also wrong. You must never return a reference to a local variable. Return by-value.

[boyhailong]

Copy constructor is wrong. You never allocate elements.
merge is also wrong. You must never return a reference to a local variable. Return by-value.

thank you for your answer. ihave correct it and test correctly.

[iMalc]

You shouldn't have started a second thread for this.

It looks to me like the swap step is missing from that copy and swap I sugested for the Merge, or are you changing the semantics now to return the result and hence be a const function?