C++ coerce std::initializer_list to container in templated function

Question

I am not able to pass an initializer list to a templated function, to then call a non templated function which would coerce the initializer list to a container. However, when skipping the intermediate templated function, it compiles.

#include <iostream>
#include <vector>

struct MyLoader {
    static auto load(const std::vector<int> &v) {
        return v;
    }
};

template<typename Loader, typename... Args>
auto makeVector(Args&&... args) {
    return Loader::load(std::forward<Args>(args)...);
}

int main() {
    auto v = makeVector<MyLoader>({ 8, 8, 8 });

    for (auto x : v) {
        std::cout << x << "\n";
    }
}

Renders the error:

main.cpp: In instantiation of 'auto makeVector(Args&& ...) [with Loader = MyLoader; Args = {}]':
main.cpp:18:46:   required from here
main.cpp:13:24: error: no matching function for call to 'MyLoader::load()'
     return Loader::load(std::forward<Args>(args)...);
            ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
main.cpp:6:17: note: candidate: 'static auto MyLoader::load(const std::vector<int>&)'
     static auto load(const std::vector<int> &v) {
                 ^~~~
main.cpp:6:17: note:   candidate expects 1 argument, 0 provided
main.cpp: In function 'int main()':
main.cpp:18:46: error: too many arguments to function 'auto makeVector(Args&& ...) [with Loader = MyLoader; Args = {}]'
     auto v = makeVector<MyLoader>({ 8, 8, 8 });
                                              ^
main.cpp:12:6: note: declared here
 auto makeVector(Args&&... args) {
      ^~~~~~~~~~
main.cpp:18:10: error: 'void v' has incomplete type
     auto v = makeVector<MyLoader>({ 8, 8, 8 });
          ^
main.cpp:20:19: error: unable to deduce 'auto&&' from 'v'
     for (auto x : v) {

However the following compiles perfectly fine:

#include <iostream>
#include <vector>

struct MyLoader {
    static auto load(const std::vector<int> &v) {
        return v;
    }
};

int main() {
    auto v = MyLoader::load({ 8, 8, 8 });

    for (auto x : v) {
        std::cout << x << "\n";
    }
}

Why is the initializer list not coerced to the std::vector argument of MyLoader::load() in the first example?

Answer 1

The problem is that {8, 8, 8} isn't deducible as type.

If you pass it to a method that receive a const std::vector<int>, all goes well because the compiler know that {8, 8, 8} is used to initialize a std::vector<int>.

But if you pass it to a function that receive a variadic list (Args&&... args) of unknown types arguments, this doesn't works because the compiler doesn't know witch type is {8, 8, 8}.

Suggestion: pass the values in a variadic way

auto v = makeVector<MyLoader>(8, 8, 8);

so the compiler can detect Args... as int, int, int, and pass to load() adding graphs

// ------------------V-----------------------------V   
return Loader::load( { std::forward<Args>(args)... } );

If you really (really!) want to pass th values to makeVector() as initialization list, you can intercept they as an array

template <typename Ldr, std::size_t Dim, typename Arg>
auto makeVector (Arg(&&arg)[Dim])

So the compiler can deduce the type (Arg, as int) and the size (Dim, 3 in case of {8, 8, 8}.

But to unpack the array you need the indexes, so a function helper.

Something as

template <typename Ldr, std::size_t ... Is, std::size_t Dim, typename Arg>
auto makeVectorH (std::index_sequence<Is...> const &, Arg(&&arg)[Dim])
 { return Ldr::load({std::forward<Arg>(arg[Is])...}); }

template <typename Ldr, std::size_t Dim, typename Arg>
auto makeVector (Arg(&&arg)[Dim])
 { return makeVectorH<Ldr>(std::make_index_sequence<Dim>{},
                           std::forward<Arg[Dim]>(arg)); }

The following is a full compiling C++14 example for array case (but I suggest the simpler use of variadic arguments)

#include <vector>
#include <utility>
#include <iostream>

struct MyLoader
 { static auto load(const std::vector<int> &v) { return v; } };

template <typename Ldr, std::size_t ... Is, std::size_t Dim, typename Arg>
auto makeVectorH (std::index_sequence<Is...> const &, Arg(&&arg)[Dim])
 { return Ldr::load({std::forward<Arg>(arg[Is])...}); }

template <typename Ldr, std::size_t Dim, typename Arg>
auto makeVector (Arg(&&arg)[Dim])
 { return makeVectorH<Ldr>(std::make_index_sequence<Dim>{},
                           std::forward<Arg[Dim]>(arg)); }

int main ()
 {
   auto v = makeVector<MyLoader>({8, 8, 8});

   for (auto x : v)
        std::cout << x << "\n";
 }