# include <iostream.h> # include <std.h> int main() { cout<<"hello world"; return 0; }

Question

please tell me what's my mistake

Answer 1

C++ header inclusion doesn't need filename and extension, except for custom headers, you can just use #include <iostream>, and as @Bogdan said, if you want to write it that way you need to declare either:

using namespace std;

Or,

using std::cout;
using std::cin;

Or specifically call std::cout / std::cin each time, proportionally.

Hth, cmiiw

Answer 2

you either write using namespace std;(using std::cout for efficiency) before main or replace cout with std::cout in main function