Introduction into metaproramming

Starting with C, many programming languages have support for multistage or metaprogramming. This means that there is a second language, usually with destinct syntax, that is interpretered before the the main language. For C this is the preprocessor. It is a simple text replacement mechanism that allows to automatically generate code. A simple example is

#define MSG "Hello World!"
#include <stdio.h>

int main()
{
  printf(MSG"\n");
}

This generates the output

Hello World!