char*参数构造函数报错

#include <iostream>
#include <string>
using namespace std;
class String
{
public:
    String() { p = NULL; }
    String(char* str);
    friend bool operator  >(String& string1, String& string2);
    friend bool operator  <(String& string1, String& string2);
    friend bool operator==(String& string1, String& string2);
    void display();
private:
    char* p;
};
String::String(char* str)
{
    p = str;
}

void String::display()
{
    cout << p;
}

bool operator>(String& string1, String& string2)
{
    if (strcmp(string1.p, string2.p) > 0)
        return  true;
    else
        return  false;
}
bool operator<(String& string1, String& string2)
{
    if (strcmp(string1.p, string2.p) < 0)
        return true;
    else
        return false;
}

bool operator==(String& string1, String& string2)
{
    if (strcmp(string1.p, string2.p) == 0)
        return true;
    else
        return false;
}

int main()
{
    String string1("Hello"), string2("Book"), string3("Computer");//这一行构造函数传入参数出错
    cout << (string1 > string2) << endl;
    cout << (string1 < string3) << endl;
    cout << (string1 == string2) << endl;
    return 0;
}

什么错？