Some operators in Class String

[Begins]

Please tell me what's wrong with that code.
I have problems with "operator+=" and because of it with "operator+" also.
Besides, please take a look at "operator<<" and "operator>>" and correct me if i have wrote in wrong.
I also have a problem with "operator[]" when i use that operator in the left side. Why is that?
Thank you for your very usefull help.

Header file:

#ifndef STRING_H
#define STRING_H

#include <iostream>

using namespace std; 


class String
{
	char* value;
private:
	virtual void show(ostream&)const;
	void CopyValue(const String& copied);
public:
	String(void);
	String(const char*);
	String(const String&);
	~String(void);
	String& operator= (const String&);
	size_t length() const;
	//String& trim (void);
	char& operator[] (int);
	char operator[] (int) const;
	//String& operator+= (const String&);
	//friend String operator+ (const String&) const;
	friend istream& operator>> (istream&, const String&);
	friend ostream& operator<< (ostream&, const String&);
	friend bool operator== (const String&, const String&);
	friend bool operator!= (const String&, const String&);
};

#endif

Cpp file:

#include "String.h"
#include <iostream>
#include <cstring>

using namespace std;

String::String(void)
{
	value = new char [1];
	value[0] = '\0';
}

String::String(const char* data): value(NULL)
{
	if (data)
	{
		value = new char [strlen(data) + 1];
		strcpy(value,data);
	}
	else
	{
		value = new char[1];
		value[0] = '\0';
	}
}

String::~String(void)
{
	delete [] value;
}

String::String(const String& copied): value(NULL)
{
	//value = new char [strlen(copied.value) + 1];
	//strcpy(value, copied.value);
	this -> CopyValue(copied);
}

String& String:: operator= (const String& copied)
{
	//delete [] value;
	//value = new char [strlen(copied.value) + 1];
	//strcpy(value, copied.value);
	if (&copied != this)
		this -> CopyValue(copied);
	return *this;
}

void String:: CopyValue(const String& copied)
{
	delete [] value;
	value = NULL;
	if (copied.value)
	{
		value = new char [strlen(copied.value) + 1];
		strcpy(value, copied.value);
	}
}

void String:: show(ostream& out)const
{
	if (value)
		out << value << endl;
}

bool operator== (const String& first, const String& second )
{
	if (strcmp(first.value, second.value) == 0)
		return true;
	return false;
}

bool operator!= (const String& first, const String& second)
{
	if (first == second)
		return false;
	return true;
}

ostream& operator<< (ostream& out, const String& obj)
{
	obj.show(out);
	return out;
}

istream& operator>> (istream& in, const String& obj)
{
	in >> obj.value;
	return in;
}

size_t String:: length() const
{
	return strlen(value) + (size_t)1;
}

/*String& String:: operator+= (const String& rhs)
{
	if (this != &rhs)
		strcat(value, rhs.value);
	return *this;
}

String operator+ (const String& rhs) const
{
	return String(*this) += rhs;
}*/

char& String:: operator[] (int place)
{
	return value [place];
}

char String:: operator[] (int place) const
{
	return value [place];
}

main:

#include "String.h"
#include <iostream>

using namespace std;

int main (void)
{
	String a;
	const String b("LINDA IS MY DOG");
	String c(b);
	cout << b;
	//b.show();  BEFORE I HAVE WROTE COUT
	cout << c;
	//c.show();  BEFORE I HAVE WROTE COUT
	cout << a;
	//a.show();  BEFORE I HAVE WROTE COUT
	if (a == b)
		cout << "A and B are the same" << endl;
	if (a != b)
		cout << "A and B are absolutly not the same" << endl;
	a = b;
	cout << a;
	//a.show();  BEFORE I HAVE WROTE COUT
	if (a == b)
		cout << "A and B are the same" << endl;
	if (a != b)
		cout << "A and B are absolutly not the same" << endl;
	cout << b.length() << endl;
	//b [6] = 'k';  ERROR!!
	//b += c;  ERROR!!
	cout << b;
	cout << b [6] << endl;
	String d("Meyers is my friend");
	cin >> d;
	cout << d;

	return 0;
}