How to properly implement a function with variadic number of std::string_view arguments?

Question

Desired behavior

What I basically want is to create a function like this:

void func(std::string_view... args)
{
    (std::cout << ... << args);
}

It should be able to work only with classes that are convertible to std::string_view .

Example:

int main()
{
    const char* tmp1 = "Hello ";
    const std::string tmp2 = "World";
    const std::string_view tmp3 = "!";

    func(tmp1, tmp2, tmp3, "\n");

    return 0;
}

should print: Hello World!

---

Accomplished behavior

So far, I got here:

template<typename... types>
using are_strings = std::conjunction<std::is_convertible<types, std::string_view>...>;

template<typename... strings, class = std::enable_if_t<are_strings<strings...>::value, void>>
void func(strings... args)
{
    (std::cout << ... << args);
}

int main()
{
    const char* tmp1 = "Hello ";
    const std::string tmp2 = "World";
    const std::string_view tmp3 = "!";

    func(tmp1, tmp2, tmp3, "\n");

    return 0;
}

This actually works as expected, but there is still one big problem.

---

Problem

Only classes that are convertible to std::string_view can be used in this function and that's great.
However, even though classes are convertible, they are not converted to std::string_view !

This leads to needless copying of data(for example when std::string is passed as argument).

---

Question

Is there a way to force implicit conversion of variadic arguments to std::string_view ?

---

Note

I know about std::initializer_list , but I would like to keep function call simple, without {} .

Answer 1

namespace impl{
  template<class...SVs>
  void func(SVs... svs){
    static_assert( (std::is_same< SVs, std::string_view >{} && ...) );
    // your code here
  }
}
template<class...Ts,
  std::enable_if_t< (std::is_convertible<Ts, std::string_view >{}&&...), bool > =true
>
void func( Ts&&...ts ){
  return impl::func( std::string_view{std::forward<Ts>(ts)}... );
}

or somesuch.

Answer 2

#include <string_view>
#include <utility>

template <typename>
using string_view_t = std::string_view;

template <typename... Ts>
void func_impl(string_view_t<Ts>... args)
{
    (std::cout << ... << args);
}

template <typename... Ts>
auto func(Ts&&... ts)
    -> decltype(func_impl<Ts...>(std::forward<Ts>(ts)...))
{
    return func_impl<Ts...>(std::forward<Ts>(ts)...);
}

DEMO

Answer 3

If you simply want to avoid needless copying of data, use a forward reference and then perform explicit casts (if still required). This way no data is copied but forwarded (in your main.cpp example, all params are passed as const references)

template <typename... strings,
          class = std::enable_if_t<are_strings<strings...>::value, void>>
void func(strings&&... args) {
  (std::cout << ... << std::string_view{args});
}

Answer 4

Not exactly what you asked... but if you can set a superior limit for a the length of args... (9 in following example) I propose the following solution: a foo<N> struct that inherit N func() static function that accepting 0, 1, 2, ..., N std::string_view .

This way, func() function are accepting what is convertible to std::string_view and all argument are converted to std::string_view .

That is exactly

void func(std::string_view... args)
{ (std::cout << ... << args); }

with the difference that func() functions are static methods inside foo<N> , that there is a limit in args... length and that there is a func() method for every supported length.

The full example is the following.

#include <string>
#include <utility>
#include <iostream>
#include <type_traits>

template <std::size_t ... Is>
constexpr auto getIndexSequence (std::index_sequence<Is...> is)
   -> decltype(is);

template <std::size_t N>
using IndSeqFrom = decltype(getIndexSequence(std::make_index_sequence<N>{}));

template <typename T, std::size_t>
struct getType
 { using type = T; };

template <typename, typename>
struct bar;

template <typename T, std::size_t ... Is>
struct bar<T, std::index_sequence<Is...>>
 {
   static void func (typename getType<T, Is>::type ... args)
    { (std::cout << ... << args); }
 };

template <std::size_t N, typename = std::string_view, 
          typename = IndSeqFrom<N>>
struct foo;

template <std::size_t N, typename T, std::size_t ... Is>
struct foo<N, T, std::index_sequence<Is...>> : public bar<T, IndSeqFrom<Is>>...
 { using bar<T, IndSeqFrom<Is>>::func ...; };


int main ()
 {
    const char* tmp1 = "Hello ";
    const std::string tmp2 = "World";
    const std::string_view tmp3 = "!";

    foo<10u>::func(tmp1, tmp2, tmp3, "\n");
 }

Answer 5

Make it a two-stage production:

template <class... Args>
std::enable_if_t<... && std::is_same<Args, std::string_view>()>
func(Args... args)
{
    (std::cout << ... << args);
}

template <class... Args>
auto func(Args&&... args)
-> std::enable_if_t<... || !std::is_same<std::decay_t<Args>, std::string_view>(),
    decltype(func(std::string_view(std::forward<Args>(args))...))>
{
    func(std::string_view(std::forward<Args>(args))...);
}