Recursion using template meta programming

Question

Can somebody explain to me, why the first call using the template function is falling into an infinite loop, while the second compile-time function runs correctly?

#include <iostream> using namespace std;  template<int N, int M> struct commondivs {                                                 static const int val = (N<M) ? commondivs<N,(M-N)>::val : commondivs<(N-M),M>::val; };  template<int N> struct commondivs<N,N> {   static const int val = N; };   int commondiv(int N, int M){     if(N==M){         return N;     }        return (N<M)?commondiv(N,(M-N)):commondiv((N-M),M);      }  int main() {      cout << commondivs<9,6>::val << endl;     cout << commondiv(9,6) << endl;     return 0; } 

Answer 1

(N<M) ? commondivs<N,(M-N)>::val : commondivs<(N-M),M>::val 

This line causes instantiation of both commondivs<N,(M-N)>::val and commondivs<(N-M),M>::val, even if the condition is known at compile time and one of the branches will never be taken.

Replace ? : with std::conditional_t, which doesn't have this limitation:

static const int val = std::conditional_t<N < M, commondivs<N,(M-N)>, commondivs<(N-M),M>>::val; 

Answer 2

The problem is all the operands of conditional operator will be evaluated, so both commondivs<N,(M-N)> and commondivs<(N-M),M> get instantiated and their val get evaluated and then leads to recursive template instantiation.

You can apply constexpr if and put it in a constexpr static member function.

If the value is true, then statement-false is discarded (if present), otherwise, statement-true is discarded.

template<int N, int M> struct commondivs {                                                 constexpr static int get_val() {     if constexpr (N<M) return commondivs<N,(M-N)>::val; // if true, the else part won't be evaluated     else return commondivs<(N-M),M>::val;               // vice versa   }   static const int val = get_val(); }; 

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