Assignment Operator Bug

[Mactavish]

I have compiled this code in MS Visual C++ Express 2008 and it works as 
it should be but when i compile this code in Mingw as a part of GCC ver 
4.4.1-2 the input() function should return a temporary object to 'ob' 
object and invoke the assignment operator '=' but it doesn't and it 
shows me error :"no match for 'operator=' in 'ob=input()()'   "

code:
#include<iostream>
#include<conio.h>
#include<cstdio>
#include<cstring>

using namespace std;

class sample{

    char *s;
    public:
        sample(){s=new char('\0');}
        sample(const sample &ob);
        ~sample(){if(s) delete []s; cout<<"Freeing\n";}
        void show(){cout<<s<<endl;}
        void set(char *str);
        sample operator=(sample &ob);

};

sample::sample(const sample &ob)
{
    s=new char[strlen(ob.s)+1];
    strcpy(s,ob.s);
}

void sample::set(char *str)
{
    s=new char[strlen(str)+1];
    strcpy(s,str);
}

sample input()
{
    char instr[20];
    sample str;

    cout<<"Enter a string :";
    cin>>instr;

    str.set(instr);
    return str;

}

sample sample::operator=(sample &ob)
{
    /* If the target memory is not large enough
    then allocate new memory. */
if(strlen(ob.s) > strlen(s))
    {
        delete []s;
        s = new char[strlen(ob.s)+1];
    }
    strcpy(s, ob.s);
    return *this;
}


int main()
{
    sample ob;

    ob=input(); //showing errors
    ob.show();

    _getch();
    return 0;
}