Why auto is not allowed in template function for creating a built-in array?

Question

This code below does not compile:

template<typename... Ts>
void CreateArr(const Ts&... args)
{
    auto arr[sizeof...(args) + 1]{ args... };
}

int main()
{
    CreateArr(1, 2, 3);
}

due to the following errors:

'arr': in a direct-list-initialization context, the type for 'auto [6]' can only be deduced from a single initializer expression
auto [6]': an array cannot have an element type that contains 'auto'
'initializing': cannot convert from 'const int' to 'std::initializer_list<int>'

My questions are:

Why cannot I use auto to define the type of the array?
How to define it properly to work with the template?

Nothing to do with templates, you can't use `auto` for a built-in array as the error says. — chris, Sep 12 '21 at 20:34
Note that the array size should be `sizeof...(args)` not `sizeof...(args) + 1`, if you intend to have the array of as many parameters you passed to the function. — JeJo, Sep 12 '21 at 20:53

JeJo · Accepted Answer · 2021-09-12T20:51:35.323

Why cannot I use auto to define the type of the array?

For the same reason, following does not work/ allowed!

auto ele[]{ 1, 2, 3 };

More reads: Why can't I create an array of automatic variables?

How to define it properly to work with the template?

Use the std::common_type_t for specifying the type

#include <type_traits> //  std::common_type_t

template<typename... Ts>
void CreateArr(const Ts&... args) 
{
    std::common_type_t<Ts...> arr[sizeof...(args)]{ args... };
   
    static_assert(std::is_array_v<int[sizeof...(args)]>, "is not array!");
}

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Why auto is not allowed in template function for creating a built-in array?

1 Answers1