constexpr function not calculate value in compile time

Question

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;
}

Answer 1

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)};

Answer 2

With gcc, at least, you can get the constexpr value to be computed at compile time by making it a static variable:

static const unsigned fibNUM = fib(NUM);

As I read the standard, it's still allowed to compute the value at startup, but in practice it will be computed at compile time.