Can't assign array values, gives std::out_of_range

See more:

C++

I have this class named String and i have made it so it has a variable called data which is a std::string. I have also done the appropriate stuff so you can do String hey = "hello"; and it assigns "hello" to data. I know this part of it works. But when i try to make an array of Strings like this:

String a = "hello"; //works perfectly
cout << a.data; //prints "hello"
String arr[3];
arr[0] = a; //Crashes, tells me std::out_of_range

This is the only thing showing up in the command prompt:

terminate called after throwing an instance of 'std::out_of_range'
  what(): basic_string::erase __pos (Which is 4294967295) > this->size()

I have overloaded the operator[] but this should not be a problem, they even did it with the std::string class.

As per request here is the first couple of lines of code:

#define cstr std::string
class String {
public:
    cstr data;
    String() {}
    String(char *str) : data(str) {}
    String(const char *str) {
      data = const_cast<char*>(str);
    }

    #pragma region operators
    String& operator=(char* str) {
        if (RemoveLastChar().data != str) {
            data = str + '\0';
        }
        return *this;
    }

    String& operator=(const char* str) {
        if (RemoveLastChar().data != str) {
            data = (char*)str + '\0';
        }
        return *this;
    }

    String& operator=(String& str) {
        if (RemoveLastChar().data != str.data) {
            data = str.data + '\0';
        }
        return *this;
    }

    String& operator=(const String& str) {
        if (RemoveLastChar().data != str.data) {
            data = str.data + '\0';
        }
        return *this;
    }

    char operator[](int index) { //I also tried commenting this out, did not work
        return data[index];
    }
    #pragma endregion

I have continued trying and have found out where the problem is by commenting out sections and running through it.

String hey = "Hello World!"; //This works
String* h = hey.Split(' '); //This works
String p = h[0]; //This causes the CMD to go blank and crash within a second or two. This is how you access array elements no? I have removed the [] operator and still no difference, this specific array seemingly doesn't work like every single array in the history of C++. Why? I don't understand i have been coding for years. What have i missed? Having a full on identity crisis here. xD

What I have tried:

I have tried assigning directly like arr[0] = "hello"; same problem. I have no idea why this is a problem at all.