Class String + BMSearch (upd)

#include <iostream>
#include <string.h>
#include <algorithm>

using namespace std;

class String{
    char *str;
    int len;
    public:
        String(int l = 0) {
            str = new char[l+1];
            len = 0;
            strcpy(str,"\0");
        };
        String(const char * a) {
            str = new char[strlen(a)+1];
            strcpy(str,a);
            len = strlen(a);
        };
        String(String & a) {
            str = new char[a.len+1];
            len = a.len;
            strcpy(str,a.str);
        };
        ~String() { if(str) delete []str; str = NULL; }
        int Len();
        String & operator = (const String &);
        char &operator [](int);
        bool operator == (const String &);
        bool operator != (const String &);
        String operator + (const String &);
        String & operator += (const String &);
        friend istream & operator >> (istream & in, String & a);
        friend ostream & operator << (ostream & out, String & a);
        int BMSearch(String &);
};

int String::Len() {
    return len;
}
String & String::operator = (const String &a) {
    if (this!=&a) {
        delete [] str;
        str = new char[a.len+1];
        strcpy(str,a.str);
        len = a.len;
    }
    return *this;
}
char & String::operator [](int a) {
    if (a<0 || a>=len) exit(1);
    return str[a];
}
bool String::operator == (const String &a) {
    if(strcmp(str,a.str)) return false;
    return true;
}
bool String::operator != (const String &a) {
    if(strcmp(str,a.str)) return true;
    return false;
}
String String::operator + (const String &a) {
    String z(len+a.len+1);
    z.len = len + a.len;
    strcpy(z.str,str);
    strcat(z.str,a.str);
    return z;
}
String & String::operator += (const String &a) {
    char  *t = new  char[len + 1];
    strcpy(t, str);
    delete  [] str;
    len = len + a.len;
    str = new char[len + 1];
    strcpy( str, t);
    strcat( str, a.str);
    delete  [] t;
    return *this;
}
istream & operator >> (istream & in, String & a) {
    in>>a.str;
    a.len = strlen(a.str);
    return in;
}
ostream & operator << (ostream & out, String & a) {
    cout<<a.str;
    return out;
}
int String::BMSearch(String & a) {
    int m = a.len;
    int n = len;
    int t[256];
    fill_n(t,256,m);
    for (int i = 0; i < m-1;i++) {
        t[a.str[i]] = m-1-i;
    }
    int i = m-1,j = m-1;
    while ( i < n && j>=0) {
        int k = i;
        j = m-1;
        while (j>=0) {
            if(str[k]!=a.str[j]) {
                i += t[str[i]];
                j = m - 1;
                break;
            }
            j--;
            k--;
        }
    }
    if(j>=0) return -1;
    else return i+1-m;
}

int main()
{
    String a, b("qwertyuiop");
    String c("qwerty");

    cout<<"operator[] = "<<b[3]<<endl<<endl;

    cout<<"b = "<<b<<endl<<"c = "<<c<<endl;

    a = b + c;
    cout<<"a = b + c : "<<a<<endl;

    a+=c;
    cout<<"a+= : "<<a<<endl<<endl;

    b = "qwerty";
    cout<<"c = "<<c<<endl<<"b = "<<b<<endl;
    if(c==b) cout<<"c equal b\n\n";

    b = a;
    cout<<"c = "<<c<<endl<<"b = "<<b<<endl;
    if(c!=b) cout<<"c not equal b\n\n";


    a = "abcabccbaaaghabcabcde";
    b = "ghab";

    cout<<"Match at "<<a.BMSearch(b)<<" position\n\n"<<"a = "<<a<<endl<<"b = "<<b<<endl<<endl;

    cout<<"operator[] = "<<b[0]<<endl<<endl;

    String h(100);
    cout<<"Enter NewString: ";
    cin>>h;
    cout<<"NewString: "<<h<<"\n"<<"NewStrLen: "<<h.Len();

    return 0;
}