Aside: Over-engineered Min() [C++, variadic templates, constexpr, fold left]

Q: Given a function constexpr int Min(int a, int b), construct a function constexpr int Min(Args... args) that returns the minimum of all the provided args. Fail to justify your over-engineering.

A: Rename Min(int, int) as MinImpl(int, int) or stick it in a namespace. Overloading the function is not only unnecessary, it gets in the way of the implementation.

constexpr int MinImpl(int a, int b)
  return a < b ? a : b;

Implement a constexpr fold left function. If we can use it for Min(), we should be able to do the same for Max(), and other similar functions. Should we be able to find any (#prematuregeneralization).

template<typename ArgA, typename ArgB, typename Func>
constexpr auto foldl(Func func, ArgA a, ArgB b)
  return func(a, b);

template<typename ArgA, typename ArgB,
typename Func, typename ...Args>
constexpr auto foldl(Func func, ArgA a, ArgB b, Args... args)
  return foldl(func, func(a, b), args...);

Combine the two.

template<typename ...Args>
constexpr auto Min(Args... args)
  return foldl(MinImpl, args...);

Add the bare minimum amount of testing for a constexpr function: slap a static_assert() on it.

static_assert(Min(6, 4, 5, 3, 9) == 3), "Nope");

I did so with Visual Studio 2015 Update 2. It did not object.

Addendum: Some discussion with @nlguillemot and @DrPizza led to this attempt to do something similar with a C++17/C++1z fold-expression:

#include <limits.h>

constexpr int MinImpl1(int a, int b) { return a < b ? a : b; }

constexpr void MinImpl2(int* m, int a, int b) { *m = a < b ? a : b; }

template<typename ...Args>
constexpr int Min(Args... args)
  int m = INT_MAX;
  // a binary expression in an operand of a fold-expression 
  // is not allowed, so this won't compile:
  //((m = MinImpl1(m, args), ...);

  // But this does:
  (MinImpl2(/*out*/&m, m, args), ...);
  return m;

int main()
  static_assert(Min(3,4,5) == 3, "nope");

This compiles with a gcc-6 pre-release snapshot.

Update: Here’s a further updated version, based on a refinement by @dotstdy.


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