What is the problem with the code?

[dullboy]

Here is my code,

class MyString
{
public:
	MyString(const char* str) 
	{
		m_str = new char[strlen(str)+1];
		strcpy_s(m_str,strlen(str)+1, str);
	}

	MyString()
	{
		m_str = new char[1024];
	}

	~MyString()
	{
		delete[] m_str;
	}

	friend MyString operator+(MyString& a, const MyString& b);
    friend ostream& operator<<(ostream& os, MyString& str);
	
private:
	char* m_str;
};

MyString operator+(MyString& a, const MyString& b)
{
	strcat_s(a.m_str, strlen(a.m_str) + strlen(b.m_str) + 1, b.m_str);
	return a.m_str;
}

ostream& operator<<(ostream& os, MyString& str)
{
	os<<str.m_str;
	return os;
}

int _tmain(int argc, _TCHAR* argv[])
{
	MyString s1("Hi");
	MyString s2; 

	s2 = s1 + " there";

	cout<<s2<<endl;
	return 0;
}

Could you point out the problems? Thanks for your inputs.