How compile time recursion works?

Question

I found a code here Printing 1 to 1000 without loop or conditionals

Can someone please explain how compile time recursion works, couldn\'t find it in google

Answer 1

It repeatedly instantiates the f1 template with decreasing values for N (f1() calls f1 and so on). The explicit specialization for N==1 ends the recursion: as soon as N becomes 1, the compiler will pick the specialized function rather than the templated one.

f1<1000>() causes the compiler to instantiate f1 999 times (not counting in the final call to f1<1>). This is the reason why it can take a while to compile code that makes heavy use of template meta-programming techniques.

The whole thing relies heavily on the compiler's optimization skills - ideally, it should remove the recursion (which only serves as hack to emulate a for loop using templates) completely.