I want to compare meta programming and use of constexpr in c++0x. then I write a fib function in both model. when I use meta programming model, answer print out very fast because it calculated in compile time. but when I use constexpr funcion it calculate value in run time, not in compile time. I using g++( gcc ) 4.8 .can any body help me?
#include <iostream>
using namespace std;
#define NUM 42
template <unsigned int N>
struct Fibonacci {
enum { value = Fibonacci<N - 1>::value + Fibonacci<N - 2>::value };
};
template <>
struct Fibonacci<1> {
enum { value = 1 };
};
template <>
struct Fibonacci<0> {
enum { value = 1 };
};
constexpr unsigned int fib(unsigned int n)
{
return (n > 1 ? fib(n-1) + fib(n-2) : 1 );
}
int main()
{
cout << "Meta_fib(NUM) : " << Fibonacci<NUM>::value << endl; // compile time :)
cout << "Constexpr_fib(NUM) : " << fib(NUM) << endl; // run time :-?
return 0;
}
I believe the reason is that constexpr
is not guaranteed to execute at compile-time. To enforce compile-time evaluation, you have to assign it to a compile-time alias. Like,
enum {i = fib(NUM)};