求助啊，这段代买在VC++上可以运行，但是DEV c++就不行了

#include <iostream>
using namespace std;
class sample
{
    char*string;
public:
    sample(){string=new char('\0');}
    sample(const sample &ob);
    ~sample(){if(string) delete string;cout<<"Freeing string\n";}
    void show_string(){cout<<string<<"\n";}
    void set_string(char*s);
    sample operator=(sample &ob);
};
void sample::set_string(char*s)
{
    string=new char[strlen(s)+1];
    if(!string)
    {
        cout<<"Allocation error\n";
        exit(1);
    };
    strcpy(string,s);
}
sample::sample(const sample &ob)
{
    string=new char[strlen(ob.string)+1];
    if(!string)
    {
        cout<<"Allocation error\n";
        exit(1);
    };
    strcpy(string,ob.string);
}
sample sample::operator =(sample &ob)
{
    if(strlen(ob.string)>strlen(string))
    {
        delete string;
        string=new char[strlen(ob.string)+1];
        if(!string)
        {
            cout<<"Allocation error\n";
            exit(1);
        }
    }
    strcpy(string,ob.string);
    return *this;
}
sample input_string()
{
    char instr[80];
    sample str;
    cout<<"Enter a string:";
    cin>>instr;
    str.set_string(instr);
    return str;
}
int main()
{
    sample ob;
    ob=input_string();    //DEV C++中显示这里是错的：no match for 'operator=' in 'ob = input_string()()' ，而VC++中没问题。
    ob.show_string();
    return 0;
}

求大神帮忙改下，能在DEVc++中也运行。