No matching constructor for initialization of variadic template in C++

Question

Trying to learn Variadic templates and have no clue why this won't compile, the error is: no matching constructor for initialization of 'test<>'.

While we are at it, I like to ask a second question by clarifying my comments within the code.

template <typename ...Args>     // this is just how Variadic template are defined
class test {
 public:
 int arr[sizeof...(Args)];       // this is packing or unpacking? why are 
                                 // dots outside of bracket?

 test(Args... arg)               // this is called packing?
               : arr{arg...}{}   // this is called un-packing?
};

int main(){
 test<> t(1,2,3);
 return 0;
}

Edit: seems like i need to do test <int, int, int>, but why do I have to as this other example works as is:

template <typename ...Args> 
int func(Args... arg)
{
   int a[] = {arg...};
   return sizeof...(arg);
}

int main(void)
{
    std::cout << func(1,2,3,4,5,6) << std::endl;
    return 0;
}

func doesn't need the <int, int ,int.. part.

`t` is of type `test<>` - an instantiation of `test` template with empty `Args` pack. Therefore, its constructor takes no arguments. — Igor Tandetnik, Feb 25 '20 at 14:58
`sizeof...` is a single operator, the dots are not something additional or separate. There's documentation for this stuff around. >_> — CodeClown42, Feb 25 '20 at 15:01
With sufficiently modern compiler, you can write `test t(1,2,3);` This automatically deduces `Args` to be `int, int, int` — Igor Tandetnik, Feb 25 '20 at 15:02
related/maybe dupe: https://stackoverflow.com/questions/984394/why-not-infer-template-parameter-from-constructor — NathanOliver, Feb 25 '20 at 15:10

NutCracker · Accepted Answer · 2020-02-25T15:10:31.627

Your first example does not work because you need to specify types like:

int main() {
    test<int, int, int> t(1,2,3);
    return 0;
}

If you are able to use C++17, you can take advantage of class template argument deduction and have something like:

int main() {
   test t(1,2,3);
   return 0;
}

But prior to C++17, class template argument deduction did not exist. However, function template argument deduction existed. That's why your second example works without explicitly specifying template types.

No matching constructor for initialization of variadic template in C++

1 Answers1