C/C++ program that prints its own source code as its output

Question

Wikipedia says it\'s called a quine and someone gave the code below:

char*s=\"char*s=%c%s%c;main(){printf(s,34,s,34);}\";main(){printf(s,34,s,34);}
         


        

Answer 1

Here's a version that will be accepted by C++ compilers:

#include
const char*s="#include%cconst char*s=%c%s%c;int main(int,char**){printf(s,10,34,s,34);return 0;}";int main(int,char**){printf(s,10,34,s,34);return 0;}

test run:

$ /usr/bin/g++ -o quine quine.cpp
$ ./quine | diff quine.cpp - && echo 'it is a quine' || echo 'it is not a quine'
it is a quine

The string s contains mostly a copy of the source, except for the content of s itself - instead it has %c%s%c there.

The trick is that in the printf call, the string s is used both as format and as the replacement for the %s. This causes printf to put it also into the definition of s (on the output text, that is)

the additional 10 and 34s correspond to the linefeed and " string delimiter. They are inserted by printf as replacements of the %cs, because they would require an additional \ in the format-string, which would cause the format- and replacement-string to differ, so the trick wouldn't work anymore.