Sometimes for algebraic types it is convenient to have a constructor that takes a literal value 0 to denote the neutral element, or 1 to denote the multiplicative identity element, even if the underlying type is not an integer.
The problem is that it is not obvious how to convince the compiler only to accept, 0 or 1 without accepting any other integer.
Is there a way to do this in C++14 or beyond, for example combining literals, constexpr or static_assert?
Let me illustrate with a free function (although the idea is to use the technique for a constructor that take a single argument. Contructors cannot take template parameters either).
A function that accepts zero only could be written in this way:
constexpr void f_zero(int zero){assert(zero==0); ...}
The problem is that, this could only fail at runtime. I could write f_zero(2) or even f_zero(2.2) and the program will still compile.
The second case is easy to remove, by using enable_if for example
template<class Int, typename = std::enable_if_t<std::is_same<Int, int>{}> >
constexpr void g_zero(Int zero){assert(zero==0);}
This still has the problem that I can pass any integer (and it only fails in debug mode).
In C++ pre 11 one had the ability to do this trick to only accept a literal zero.
struct zero_tag_{};
using zero_t = zero_tag_***;
constexpr void h_zero(zero_t zero){assert(zero==nullptr);}
This actually allowed one to be 99% there, except for very ugly error messages.
Because, basically (modulo Maquevelian use), the only argument accepted would be h_zero(0).
This is situation of affairs is illustrated here https://godbolt.org/z/wSD9ri . I saw this technique being used in the Boost.Units library.
1) Can one do better now using new features of C++?
The reason I ask is because with the literal 1 the above technique fails completely.
2) Is there an equivalent trick that can be applied to the literal 1 case? (ideally as a separate function).
I could imagine that one can invent a non-standard long long literal _c that creates an instance of std::integral_constant<int, 0> or std::integral_constant<int, 1> and then make the function take these types. However the resulting syntax will be worst for the 0 case. Perhaps there is something simpler.
f(0_c);
f(1_c);
EDIT: I should have mentioned that since f(0) and f(1) are potentially completely separate functions then ideally they should call different functions (or overloads).