String Class + Operator Overloading

#include <iostream>
#include <cstdlib>
#include <cstring>
#include <cstdio>

using namespace std;

class String{
    char *str;
    int size;
    public:
        String(const char* = "");
        ~String();
        int length();
        bool operator >(String&);
        bool operator <(String&);
        bool operator ==(String&);
        String operator +(const String&);
        friend istream& operator >>(istream&, String&);
        friend ostream& operator <<(ostream&, String&);
};

inline String String::operator+(const String &obj){
    int len_1 = size;
    int len_2 = obj.size;
    int concat_len = len_1 + len_2 + 1;
    char *temp = new char[concat_len];
    strcpy(temp, str);
    strcat(temp,obj.str);
    String concat(temp);
    delete[] temp;
    return concat;
}

inline String::String(const char *str){
    String::str = new char[strlen(str) + 1];
    strcpy(String::str, str);
    size = strlen(str);
}

inline String::~String(){
    delete[] str;
    str = NULL;
}

inline int String::length(){
    return size;
}

inline bool String::operator >(String &obj){
    return strcmp(str, obj.str) > 0;
}

inline bool String::operator <(String &obj){
    return strcmp(str, obj.str) < 0;
}

inline bool String::operator ==(String &obj){
    return strcmp(str, obj.str) == 0;
}

istream& operator >>(istream &in, String &obj){
    /* Try to do this in the constructor */
    char temp[1024];
    in.getline(temp, 1024);
    delete[] obj.str;
    obj.str = new char[strlen(temp) + 1];
    strcpy(obj.str, temp);
    obj.size = strlen(obj.str);
    return in;
}

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

int main() {
    system("cls");
    cout << "Enter the first string: ";
    String str1;
    cin >> str1;
    cout << "Enter the second string: ";
    String str2;
    cin >> str2;
    String str3 = str1 + String(" ") + str2;
    cout << str3;
    cin.get();
    return 0;
}