c++ power of integer, template meta programming

Question

I want to make a function which returns a power of integer. Please read the fmuecke's solution in power of an integer in c++ .

However, I want to generalize his solution to the arbitrary type T. Since c++11 has constexpr, I guess this is possible.

Naively, I tried something like,

template<class T, int N>
inline constexpr T pow(const T x){
    return pow<N-1>(x) * x;
}
template<class T>
inline constexpr T pow<T, 1>(const T x){
    return x;
}
template<class T>
inline constexpr T pow<T, 0>(const T x){
    return 1;
}

Actually this approach failed since the partial specialization for function template is not allowed.

And one more question. I heard that it is up to the compiler whether the constexpr function is evaluated in compile time or not. How do I force it to compute for general type. I read from somewhere that one of the simplest hack for integral consts is to wrap it in std::integral_const::value.

Answer 1

Solution using recursion:

#include <iostream>

template<class T>
inline constexpr T pow(const T base, unsigned const exponent)
{
    // (parentheses not required in next line)
    return (exponent == 0) ? 1 : (base * pow(base, exponent-1));
}

int main()
{
    std::cout << "pow(2, 4): " << pow(2, 4) << std::endl;
    std::cout << "pow(5, 0): " << pow(5, 0) << std::endl;
}

Jeremy W. Murphy suggested/requested a version using exponentiation by squaring:

template<class T>
inline constexpr T pow(const T base, unsigned const exponent)
{
    // (parentheses not required in next line)
    return (exponent == 0)     ? 1 :
           (exponent % 2 == 0) ? pow(base, exponent/2)*pow(base, exponent/2) :
           base * pow(base, (exponent-1)/2) * pow(base, (exponent-1)/2);
}

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"I heard that it is up to the compiler whether the constexpr function is evaluated in compile time or not."

True, AFAIK. The compiler isn't required to do constant-initialization at compile-time, but if you use the result of a constexpr function as a non-type template argument, it has to compute the result at compile-time.

std::cout << std::integral_constant<int, pow(2, 4)>::value << std::endl;

Also see the approach using integral_constant as parameter of pow in Andy Prowl's answer.

Here's how you can enforce compile-time evaluation:

#include <iostream>
#include <type_traits>

// insert a constexpr `pow` implementation, e.g. the one from above

template < typename T, T base, unsigned exponent >
using pow_ = std::integral_constant < T, pow(base, exponent) >;

// macro == error prone, you have been warned
#define POW(BASE, EXPONENT) (pow_ < decltype(BASE), BASE, EXPONENT > :: value)

int main()
{
    std::cout << "pow(2, 4): " << pow_<int, 2, 4>::value << std::endl;
    std::cout << "pow(2, 4): " << POW(2, 4) << std::endl;
}

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Please leave a comment if you downvote so I can improve my answer.