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. :(

It's meant as an example of exponent as parameter. 2^8 = 256 — jrok, May 14 '12 at 14:39
@triclosan It could be that way, but it's not intuitive and as I'm not in such a hurry I thought I could ask and learn something new. :) — ivarec, May 14 '12 at 14:40
@haole: The example might have been ambiguous, but it exposed a flaw in what you're trying to do... — Oliver Charlesworth, May 14 '12 at 14:42
Did you mean `(N >= 1) & !(N & (N - 1))`? The number 1 is a power of two, after all. — Robert Cooper, May 14 '12 at 15:04
The compile error for argument value 5 is correct. It's not a power of 2 so the static_assert gives a compile-error (where the usual assert gives a run-time error). — TemplateRex, May 14 '12 at 15:12
Write a sane message in the documentation instead of trying to get a sane error message from the compiler. — BЈовић, May 14 '12 at 15:32
@VJovic sure, but you got to admit that static_assert with a custom message is very cool! :) — ivarec, May 14 '12 at 18:09

score 27 · Accepted Answer · answered Oct 16 '13 at 09:16

27

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

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

answered Oct 16 '13 at 09:16

sehe

374,641
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This is the real solution. – plasmacel Jun 01 '17 at 19:44

TemplateRex · Answer 2 · 2013-10-16T09:18:28.037

23

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);
}

edited Oct 16 '13 at 09:18

answered May 14 '12 at 14:40

TemplateRex

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1

Beat me to it. Might as well mention BOOST_STATIC_ASSERT for C++03. – jrok May 14 '12 at 14:41
Damnit. Was just about to give [this link](http://www.boost.org/doc/libs/1_49_0/doc/html/boost_staticassert.html). I have to learn to type faster. – Eitan T May 14 '12 at 14:42
1

My embedded compiler won't do C++11 yet. I'll try the boost alternative and add this to the answer if it works. – ivarec May 14 '12 at 14:45
Very nice... Although strictly speaking 1 *is* a power of 2. :-) – Grimm The Opiner May 14 '12 at 15:01
1

BOOST_STATIC_ASSERT accepts only one parameter. Also, I'm getting a compile error that I'm trying to figure it out. Gonna update the question. – ivarec May 14 '12 at 15:05
@haole Updated the answer with Ideone output for BOOST_STATIC_ASSERT for C++03 compilers. The BOOST_STATIC_ASSERT_MSG variant takes a string message as well, but this only works for C++11. Note that the compile-time erorr is correct! (This is what static asserts do...) – TemplateRex May 14 '12 at 15:09
1

What you can always do with `BOOST_STATIC_ASSERT` to include a message is this: `BOOST_STATIC_ASSERT(val && "This is SO wrong!")`. (For those who wonder: a string literal in a boolean context evaluates the pointer to the literal which is always non-zero, i.e. true) – Arne Mertz Dec 13 '13 at 08:29
@ArneMertz thanks yes, that is the same as with regular `assert(cond && "message")`. I guess I should have phrased it slightly differently, since my main point was that `static_assert` requires a message (can be "", but still annoying to type). – TemplateRex Dec 13 '13 at 09:24

score 11 · Answer 3 · answered May 14 '12 at 14:41

11

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.");
};

answered May 14 '12 at 14:41

Seth Carnegie

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+1 More bit twiddling hacks here: http://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2. Posted my own version based on `constexpr` :/ – sehe Oct 16 '13 at 09:15
N > 1 should be N > 0, since 1 is 2^0. – boblicious Jan 04 '19 at 08:03

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

3 Answers3

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