C / C ++ Preprocessor Concatenation

Question

Take a look at this code:

#include <iostream> using namespace std; #define foo bar #define string(a) #a #define concat(a,b) string(a ## b) #define concat2(a,b) concat(a,b) int main() { cout << concat(foo, baz) << '\n' << concat2(foo, baz) << endl; } 

The output of this program:

 foobaz barbaz 

Why does adding an extra level of indirection change the behavior of this code?