Is it possible to write code to write code?

Question

I've heard that there are some things one cannot do as a computer programmer, but I don't know what they are. One thing that occurred to me recently was: wouldn't it be nice to have a class that could make a copy of the source of the program it runs, modify that program and add a method to the class that it is, and then run the copy of the program and terminate itself. Is it possible for code to write code?

Answer 1

If you want to learn about the limits of computability, read about the halting problem

In computability theory, the halting problem is a decision problem which can be stated as follows: given a description of a program and a finite input, decide whether the program finishes running or will run forever, given that input.

Alan Turing proved in 1936 that a general algorithm to solve the halting problem for all possible program-input pairs cannot exist

Answer 2

Start by looking at quines, then at Macro-Assemblers and then lex & yacc, and flex & bison. Then consider self-modifying code.

Here's a quine (formatted, use the output as the new input):

#include<stdio.h>

main()
{
  char *a = "main(){char *a = %c%s%c; int b = '%c'; printf(a,b,a,b,b);}";
  int b = '"';
  printf(a,b,a,b,b);
}

Now if you're just looking for things programmers can't do look for the opposite of np-complete.

Answer 3

Sure it is. That's how a lot of viruses work!