question

#include <iostream>
 using namespace std;
 int main()
 { 
    cout << "Calculator" << endl;
    cin >> "Dec: " >> dec >> 255 >> endl;   
    cout << "Bin: " << bin << 255 << endl;  
    cout << "Hex: " << hex << 255 << endl;
    cout << "Oct: " << oct << 255 << endl;
 }

what is wrong with this code

It's written in C++.

Other than that, using namespace std;, the cin line, the line with bin, the lack of return 0; and usage of iostream altogether. Also, lack of a var if you want it to work as a dec to bin, hex and oct converter.

Get of /prog/ and don't return until you can write non-trivial code in a real language, please!