Is vector<auto> not allowed ? (error: invalid use of ‘auto’)

Question

I have:

#include <cstdlib>
#include <vector>

using namespace std;

int main()
{
   auto a = -SOME_CONST_MAX;
   vector<auto> myVec {a, a, a, a};
}

I don't know the type of the SOME_CONST_MAX but I want to make a vector of the type of -SOME_CONST_MAX. I assumed vector<auto> would work as it will deduce from type of a.

I'm getting these errors running: g++ -std=c++14 main.cpp

main.cpp:9:9: error: invalid use of ‘auto’
  vector<auto> myVec {a, a, a, a};
         ^
main.cpp:9:13: error: template argument 1 is invalid
  vector<auto> myVec {a, a, a, a};
             ^
main.cpp:9:13: error: template argument 2 is invalid
main.cpp:9:32: error: scalar object ‘myVec’ requires one element in initializer
  vector<auto> myVec {a, a, a, a};
                                ^

Is vector<auto> not allowed? What am I doing wrong?

Answer 1

I find Slava's solution really simple and elegant

vector<decltype(a)> myVec {a, a, a, a};

But just to show another way, you can use the variadic template function

template <typename T, typename ... Ts>
std::vector<T> getVect (T const & t, Ts const & ... ts)
 { return { t, ts... } ; }

You can use auto again

auto myVec = getVect(a, a, a, a, a);