A simple constexpr power function (C++)

Since C++11, programmers have been given the power of compile-time calculation without the hassles of template metaprogramming. (And they are hassles, believe me.) The canonical example is a factorial function completely evaluated by the compiler, but here’s another, different use, one that has much more utility.

The Power Function

It’s a sad fact that C++ has no power operator. Sure, we have the std::pow function inherited from C, but that works at runtime, which means we can’t use it in template arguments. Most notably, we can’t say something like array<int, pow(4,4)>. In other words, we aren’t allowed to use a power function when declaring the size of a C++11 array. Most users, of course, will never need it, but you never know when it might come in handy, so here it is:

template <typename T>
constexpr T ipow(T num, unsigned int pow)
    return (pow >= sizeof(unsigned int)*8) ? 0 :
        pow == 0 ? 1 : num * ipow(num, pow-1);

There are a few caveats. First, it’s (obviously) a template, so you can use it on integers, doubles, or anything else that has a binary operator* (i.e., the arithmetic one, not the pointer one). All it does is the classic “raise to a power” algorithm: repeatedly multiply a number by itself. Like the typical factorial function, it’s really written in a function mode. It even has a proper tail call, if you’re into that sort of thing. Incidentally, that’s why the second argument (the power to raise to) must be an unsigned int: you can’t use the elementary-school method with fractional powers, and raising an integer to a negative power gives you a fraction, which means that there could be type issues.

The second thing to notice is that it could use some better error handling. I put a check in to keep somebody from calling it with some huge power like 1,000,000, but all it does is return 0. It should probably throw an exception, instead. Maybe I’ll change it in version 2.

Last, this clearly won’t be useful for a class where “raise to a power” isn’t the same thing as “multiply by itself over and over”. But if I had to guess, I’d say that there aren’t too many places where you’d want to raise that kind of object to a power at compile-time. I’d love to see a counterexample, though.

Anyway, that’s all for this time. I intend to expand on this idea (complex arithmetic at compile-time) later, so I’ll see you then.

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