Defining a static constexpr member by call to constexpr function

Question

My Problem is the following. I want to sort a list of types based on a list of constexpr values. The problem can be boiled down to this function:

template <typename U, typename V>
auto min(U,V) -> std::conditional_t<U::value < V::value, U, V>
{ return {}; }

whereas value must be some static constexpr member of each type, respecively. The following snippet demonstrates the usage:

// (I)

// This must even be declared outside of a function body due to the statics :(
struct X { static constexpr double value = 2.; };
struct Y { static constexpr double value = 1.; };

int main()
{
    X x;
    Y y;
    auto z = min(x,y);
    std::cout << typeid(z).name() << " : " << z.value << std::endl;
}

My goal is to provide the value as I call the function. The closest thing I got to this goal is the following

template <double (*F)()>
struct Value { static constexpr double value = F(); };

which can be called like this using lambdas:

// (II)
auto w = min(Value<[]{ return 3.14; }>{}, Value<[]{ return 2.71; }>{});
std::cout << typeid(w).name() << " : " << w.value << std::endl;

The actual type to be sorted can be an additional parameter.

The problem is that the above is not valid C++ according to the standard. However, the latest clang does compile this gracefully.

Now, my question is: Is there another standard compliant way to achieve the above (listing (II)), that is, defining a function that computes a type based on constexor objects provided inplace (in some way) as the function argument?

---

P.S.: I'm aware of the solution using std::integral_constant. This, however, is limited to integral types only. I'm interested in a solution that works for all constexpr objects, in particular floating point types, and strings.

Answer 1

Edit:

To deal with floating point values as well as integral types scenarios you could make use of user defined literal template e.g.:

#include <type_traits>
#include <utility>
#include <typeinfo>
#include <iostream>

template <class FloatingPointType, class... Cs>
constexpr FloatingPointType char_list_to_(Cs... cs) {
    char arr[] = {cs...};
    FloatingPointType lhs = 0;
    bool contains_dot = false;
    for (std::size_t i = 0; i < sizeof...(Cs) && !(contains_dot |= (arr[i] == '.')); i++) { 
        lhs *= 10;
        lhs += arr[i] - '0';
    }
    FloatingPointType rhs = 0;
    for (int i = sizeof...(Cs) - 1; i > 0 && arr[i] != '.'; i--) {
       rhs /= 10;
       rhs += arr[i] - '0';
    }
    rhs /= 10;
    return (contains_dot)?lhs+rhs:lhs;
}

template <class FloatingPointType, char... Cs>
struct FloatingPointValue {

    static constexpr FloatingPointType value = char_list_to_<FloatingPointType>(Cs...);

    constexpr operator FloatingPointType() {
        return value;
    }
};

template <class FloatingPointType, char... Cs>
constexpr FloatingPointType FloatingPointValue<FloatingPointType, Cs...>::value;

template <char... Cs>
FloatingPointValue<double, Cs...> operator""_fv() {
    return {};
}


template <typename U, typename V>
auto min(U,V) -> std::conditional_t<(U{}<V{}), U, V>
{ return {}; }

int main() {
   auto w = min(3.14_fv, 2.71_fv);
   std::cout << typeid(w).name() << " : " << w.value << std::endl;
}

Output:

18FloatingPointValueIdJLc50ELc46ELc55ELc49EEE : 2.71

Output of c++filt -t 18FloatingPointValueIdJLc50ELc46ELc55ELc49EEE:

FloatingPointValue<double, (char)50, (char)46, (char)55, (char)49>

[live demo]

---

But if you wish to apply the same to string literal there is currently a lack of support of the feature caused by a c++ standard. There is however a gnu extension supported by clang and gcc if you are capable to accept less portable option:

#include <type_traits>
#include <utility>
#include <typeinfo>
#include <iostream>

template <class CharT, CharT... Cs>
struct Value {

    static constexpr std::size_t size = sizeof...(Cs);
    static constexpr CharT const value[sizeof...(Cs) + 1] = {Cs..., '\0'};

    template <class RHS>
    constexpr bool operator<(RHS) {
        for (std::size_t i = 0; i < size && i < RHS::size; i++) {
            if (value[i] != RHS::value[i]) {
                return value[i] < RHS::value[i];
            }
        }
        return size < RHS::size;
    }
};

template <class CharT, CharT... Cs>
constexpr CharT const Value<CharT, Cs...>::value[sizeof...(Cs) + 1];

template <class CharT, CharT... Cs>
Value<CharT, Cs...> operator""_v() {
    return {};
}


template <typename U, typename V>
auto min(U,V) -> std::conditional_t<(U{}<V{}), U, V>
{ return {}; }

int main() {
   auto w = min("cde"_v, "abc"_v);
   std::cout << typeid(w).name() << " : " << w.value << std::endl;
}

Output:

5ValueIcJLc97ELc98ELc99EEE : abc

Output of c++filt -t 5ValueIcJLc97ELc98ELc99EEE:

Value<char, (char)97, (char)98, (char)99>

[live demo]