c++ - Restrict variadic template arguments

Question

Can we restrict variadic template arguments to a certain type? I.e., achieve something like this (not real C++ of course):

struct X {};

auto foo(X... args)

Here my intention is to have a function which accepts a variable number of X parameters.

The closest we have is this:

template <class... Args>
auto foo(Args... args)

but this accepts any type of parameter.

Answer 1

Yes it is possible. First of all you need to decide if you want to accept only the type, or if you want to accept a implicitly convertible type. I use std::is_convertible in the examples because it better mimics the behavior of non-templated parameters, e.g. a long long parameter will accept an int argument. If for whatever reason you need just that type to be accepted, replace std::is_convertible with std:is_same (you might need to add std::remove_reference and std::remove_cv).

Unfortunately, in C++ narrowing conversion e.g. (long long to int and even double to int) are implicit conversions. And while in a classical setup you can get warnings when those occur, you don't get that with std::is_convertible. At least not at the call. You might get the warnings in the body of the function if you make such an assignment. But with a little trick we can get the error at the call site with templates too.

So without further ado here it goes:

---

The testing rig:

struct X {};
struct Derived : X {};
struct Y { operator X() { return {}; }};
struct Z {};

foo_x : function that accepts X arguments

int main ()
{
   int i{};
   X x{};
   Derived d{};
   Y y{};
   Z z{};
   
   foo_x(x, x, y, d); // should work
   foo_y(x, x, y, d, z); // should not work due to unrelated z
};

---

C++20 Concepts

Not here yet, but soon. Available in gcc trunk (March 2020). This is the most simple, clear, elegant and safe solution:

#include <concepts>

auto foo(std::convertible_to<X> auto ... args) {}

foo(x, x, y, d); // OK
foo(x, x, y, d, z); // error:

We get a very nice error. Especially the

constraints not satisfied

is sweet.

Dealing with narrowing:

I didn't find a concept in the library so we need to create one:

template <class From, class To>
concept ConvertibleNoNarrowing = std::convertible_to<From, To>
    && requires(void (*foo)(To), From f) {
        foo({f});
};

auto foo_ni(ConvertibleNoNarrowing<int> auto ... args) {}

foo_ni(24, 12); // OK
foo_ni(24, (short)12); // OK
foo_ni(24, (long)12); // error
foo_ni(24, 12, 15.2); // error

C++17

We make use of the very nice fold expression:

template <class... Args,
         class Enable = std::enable_if_t<(... && std::is_convertible_v<Args, X>)>>
auto foo_x(Args... args) {}

foo_x(x, x, y, d, z);    // OK
foo_x(x, x, y, d, z, d); // error

Unfortunately we get a less clear error:

template argument deduction/substitution failed: [...]

Narrowing

We can avoid narrowing, but we have to cook a trait is_convertible_no_narrowing (maybe name it differently):

template <class From, class To>
struct is_convertible_no_narrowing_impl {
  template <class F, class T,
            class Enable = decltype(std::declval<T &>() = {std::declval<F>()})>
  static auto test(F f, T t) -> std::true_type;
  static auto test(...) -> std::false_type;

  static constexpr bool value =
      decltype(test(std::declval<From>(), std::declval<To>()))::value;
};

template <class From, class To>
struct is_convertible_no_narrowing
    : std::integral_constant<
          bool, is_convertible_no_narrowing_impl<From, To>::value> {};

C++14

We create a conjunction helper:
_{please note that in C++17 there will be a std::conjunction, but it will take std::integral_constant arguments}

template <bool... B>
struct conjunction {};

template <bool Head, bool... Tail>
struct conjunction<Head, Tail...>
    : std::integral_constant<bool, Head && conjunction<Tail...>::value>{};

template <bool B>
struct conjunction<B> : std::integral_constant<bool, B> {};

and now we can have our function:

template <class... Args,
          class Enable = std::enable_if_t<
              conjunction<std::is_convertible<Args, X>::value...>::value>>
auto foo_x(Args... args) {}


foo_x(x, x, y, d); // OK
foo_x(x, x, y, d, z); // Error

C++11

just minor tweaks to the C++14 version:

template <bool... B>
struct conjunction {};

template <bool Head, bool... Tail>
struct conjunction<Head, Tail...>
    : std::integral_constant<bool, Head && conjunction<Tail...>::value>{};

template <bool B>
struct conjunction<B> : std::integral_constant<bool, B> {};

template <class... Args,
          class Enable = typename std::enable_if<
              conjunction<std::is_convertible<Args, X>::value...>::value>::type>
auto foo_x(Args... args) -> void {}

foo_x(x, x, y, d); // OK
foo_x(x, x, y, d, z); // Error