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 a1; // I want this! my_stupid_array 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 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 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_…

Accepted Answer

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

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

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

Solution

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