Advantages of auto in template parameters in C++17

Question

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?

Answer 1

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'> >;

Answer 2

Actually, the case of real values in mceo's (original) answer is explicitly not covered as non-type template parameter.

template <auto ... vs> struct HeterogenousValueList {};
using MyList1 = HeterogenousValueList<42, 'X', 1.3f>;

See the example given in the mentioned proposal: Modify §14.3.2 paragraph 2:

template<auto n> struct B { /* ... */ };
B<5> b1;   // OK: template parameter type is int
B<'a'> b2; // OK: template parameter type is char
B<2.5> b3; // error: template parameter type cannot be double

Just stumbled over the same misconception myself a few days ago.

Answer 3

Here is another example ( originally presented by @Rakete1111 as an answer for template template parameter of unknown type ):

Extracting the value of SIZE without knowing its type:

template<std::size_t SIZE>
class Foo {};

template <template<auto> class T, auto K>
auto extractSize(const T<K>&) {
    return K;
}

int main() {
    Foo<6> f1;
    Foo<13> f2;
    std::cout << extractSize(f1) << std::endl;
    std::cout << extractSize(f2) << std::endl;
}

Answer 4

C++20 seemed to add another usage

... but it doesn't :-(

The concepts technical specification allowed a function parameter with auto template argument placeholder, eg:

void printPair(const std::pair<auto, auto>& p) {
    std::cout << p.first << ", " << p.second << std::endl;
}

But it was removed later in the C++20 specification.

This is a neat usage... I am hoping it will get in again in C++23!

See: 'auto' as a template argument placeholder for a function parameter