为什么显示“无法打开包括文件:“iostream.h”: No such file or directory”

#include<iostream.h>
#include<stdlib.h>
class counter
{
    static int count;
    int objnumber;
    public:
        counter()
        {
            count++;
            objnumber=count;
        }
        void show()
        {
            cout<<"obj"<<objnumber<<" ";
            cout<<"cout="<<count<<"\n";
        }
};

int counter::count=0;


main()
{
    counter obj1;
    obj1.show();
    cout<<"------------------";
    counter obj2;
    obj1.show();
    obj2.show();
    cout<<"----------------------";
    counter obj3;
    obj1.show();
    obj2.show();
    obj3.show();
    return 1;
}