What are the advantages of auto in template parameters that will (possibly) be introduced with C++17?
Is it just a natural extension of auto when I want to instantiate template code?
auto v1 = constant<5>; // v1 == 5, decltype(v1) is int
auto v2 = constant<true>; // v2 == true, decltype(v2) is bool
auto v3 = constant<'a'>; // v3 == 'a', decltype(v3) is char
What else do I gain from this language feature?
The template <auto> feature (P0127R1) was accepted into C++ in the ISO C++ 2016 meeting in Oulu, Finland.
An auto keyword in a template parameter can be used to indicate a non-type parameter the type of which is deduced at the point of instantiation. It helps to think of this as a more convenient way of writing:
template <typename Type, Type value>
For example,
template <typename Type, Type value> constexpr Type constant = value;
constexpr auto const IntConstant42 = constant<int, 42>;
can now be written as
template <auto value> constexpr auto constant = value;
constexpr auto const IntConstant42 = constant<42>;
where you don't need to explicitly spell out the type any more. P0127R1 also includes some simple but good examples where using template <auto> with variadic template parameters is very handy, for example for implementations of compile-time lists constant values:
template <auto ... vs> struct HeterogenousValueList {};
using MyList1 = HeterogenousValueList<42, 'X', 13u>;
template <auto v0, decltype(v0) ... vs> struct HomogenousValueList {};
using MyList2 = HomogenousValueList<1, 2, 3>;
In pre-C++1z, while HomogenousValueList could be simply written as
template <typename T, T ... vs> struct Cxx14HomogenousValueList {};
using MyList3 = Cxx14HomogenousValueList<int, 1, 2, 3>;
writing an equivalent of HeterogenousValueList would not be possible without wrapping the values in some other templates, for example:
template <typename ... ValueTypes> struct Cxx14HeterogenousValueList {};
using MyList4 = Cxx14HeterogenousValueList<constant<int, 42>,
constant<char, 'X'> >;