C++之error: cannot bind non-const lvalue reference of type ‘myString&’ to an rvalue of type ‘myString’

//header.h
#ifndef _HEADER_H
#define _HEADER_H
#define defaultSize 128
#include<iostream>
#include<string.h>
using namespace std;
class myString
{
	private:
		char *ch;
		int curLength;
		int maxSize;
	public:
		myString(int sz=defaultSize);
		myString(const char *init);
		myString(const myString& ob);
		~myString(){delete []ch;}
		void print();
		int Length()const;
		myString operator()(int pos, int len);
		myString& operator = (myString& ob);
};
myString& myString::operator = (myString& ob)
{
	if(&ob!=this)
	{
		delete[]ch;

		this->ch = new char[ob.maxSize];
		this->maxSize = ob.curLength;
		strcpy(this->ch, ob.ch);
		this->curLength = ob.curLength;
	}
	else
	{
		cerr<<"String copy error\n";
	}
	return *this;
}

myString myString::operator()(int pos, int len)
{
	myString temp;
	if(pos<0 || len<=0 || pos+len-1>=this->maxSize)
	{
		temp.curLength = 0;
		temp.ch[0] = ‘\0‘;
	}
	else
	{
		if(pos+len-1 >= this->curLength)
			len = this->curLength-pos;
		temp.curLength = len;
		for(int i=0,j=pos; i<len; ++i,++j)
			temp.ch[i] = this->ch[j];
		temp.ch[len] = ‘\0‘;
	}
	return temp;
}

int myString::Length()const
{
	return this->curLength;
}

void myString::print()
{
	cout<<this->ch<<endl;
}

myString::myString(int sz)
{
	this->maxSize = sz;
	this->ch = new char[this->maxSize+1];
	if(this->ch == NULL)
	{
		cerr<<"Allocation ERROR\n";
		exit(1);
	}
	this->curLength = 0;
	ch[0] = ‘\0‘;
}

myString::myString(const char *init)
{
	int len = strlen(init);
	this->maxSize = (len > defaultSize) ? len : defaultSize;
	this->ch = new char[this->maxSize+1];
	if(this->ch == NULL)
	{
		cerr<<"Application Memory ERROR\n";
		exit(1);
	}

	this->curLength = len;
	strcpy(this->ch, init);

}

myString::myString(const myString& ob)
{
	this->maxSize = ob.maxSize;
	this->ch = new char[this->maxSize+1];
	if(this->ch == NULL)
	{
		cerr<<"Application Memory ERROR\n";
		exit(1);
	}
	this->curLength = ob.curLength;
	strcpy(this->ch, ob.ch);

}

#endif

//main.cpp
#include"header.h"

int main()
{
	myString st(10), st1("ABCDEFG");
	myString st2(st1);
	st.print(), st1.print(), st2.print();
	st = st1(0, 4);//???
	st.print();
	return 0;
}