gistfile1.txt

#pragma once
#include <type_traits>

/* template-arg compatable absolute function */
constexpr inline double abs(double x) {
    return (x > 0.0) ? x : -x;
}

/**
 * C++ doesn't support floating point non-type template arguments so
 * this is a little hack to let us do static asserts on floats
 */
template<int N, int D = 1>
struct FakeFloat {
    static constexpr int numerator = N;
    static constexpr int denomenator = D;
    static constexpr double value = static_cast<double>(N) / static_cast<double>(D);
};

/**
 * Here's a struct that we'd like to constrain such that a^2 < abs(b)
 */
template<typename A, typename B>
struct ConstrainedStruct {
  static constexpr double a = A::value;
  static constexpr double b = B::value;

  ConstrainedStruct(typename std::enable_if<A::value * A::value < abs(B::value)>::type* dummy = 0) {
  }
};

int main(void) {
    auto x = ConstrainedStruct<FakeFloat<1>, FakeFloat<5, 3>>();
    auto y = ConstrainedStruct<FakeFloat<10>, FakeFloat<5, 3>>();
    return 0;
}