How do I check if a template parameter is a power of two?

Question

I want to create a structure that allocates statically an array of 2^N bytes, but I don't want the users of this structure to specify this size as the exponent. Example:

my_stupid_array<char, 32> a1; // I want this!
my_stupid_array<char, 5> a2; // And not this...

How do I check if this template parameter is a power of two and warn the user with a nice message about this?

I've been able to check for this with a simple template:

template<int N>
struct is_power_of_two {
    enum {val = (N >= 1) & !(N & (N - 1))};
};

However, I'm unable to warn the user about this with a sane message. Any ideas?

EDIT

Fixed the ambiguous example.

EDIT

1 is a power of two indeed. Fixed that! :)

EDIT

Using BOOST_STATIC_ASSERT, I'm getting this compile error for this code with GCC:

template<int N>
struct is_power_of_two {
    enum {val = (N >= 1) & !(N & (N - 1))};
    BOOST_STATIC_ASSERT(val);
};

Error

..\main.cpp:29:1: error: invalid application of 'sizeof' to incomplete type 'boost::STATIC_ASSERTION_FAILURE<false>' 

http://ideone.com/cMfEf

EDIT

Oh, I get it. That was the message that I'm supposed to get when the assert fails. But that fails to give the user some sane message. :(

Answer 1

These days, with constexpr and the bit twiddling hacks you can just

constexpr bool is_powerof2(int v) {
    return v && ((v & (v - 1)) == 0);
}

Answer 2

static_assert to the rescue (C++11 only, uncomment BOOST_STATIC_ASSERT for C++03):

#include<iostream>
// #include <boost/static_assert.hpp>

template<int N>
struct is_power_of_two {
    enum {val = N && !(N & (N - 1))};
    static_assert(val, "should use a power of 2 as template parameter");
    // BOOST_STATIC_ASSERT(val); // without C++11 support, won't take a string message
};

int main()
{
        std::cout << is_power_of_two<2>::val << "\n";
        std::cout << is_power_of_two<3>::val << "\n";
}

Ideone output for C++11

Ideone output for C++03

UPDATE1: other idea (I know you don't want this, but it is a lot easier for large exponents):

template<int N>
make_power_of_two
{
    enum { val = 1 << N };
};

my_stupid_array<char, make_power_of_two<5>::val > a1; // size 2^5 = 32

UPDATE2: based on comments by @sehe in the chat, you can do this for constexpr functions as well

constexpr bool is_power_of_two(int x)
{
    return x && ((x & (x-1)) == 0);
}

Answer 3

You can use static_assert to provide an error message:

template<int N>
struct is_power_of_two {
    static_assert((N > 1) & !(N & (N - 1)), "Template parameter must be a power of two.");
};