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);

Answer 2

If I remember correctly, proposals have been made for vector<auto> syntax. They where not accepted by the C++ standard committee.

C++17 will introduce something like std::vector bob = {a,a,a,a}; that just works. Note the lack of <auto>. This may just be the language feature, with actual use in std following afterward.

auto is also added to templates, but auto is always a value never a type. So using auto to replace a type was considered a bad idea.

Here is a use of auto in a template:

template<auto x>
using constant_t=std::integral_constant<decltype(x),x>;
template<auto x>
constexpr constant_t<x> constant{};

and now constant<7> is a std::integral_constant<int,7>. This is considered useful for many reasons.

The answer to your practical problem using current C++ is:

auto a = -SOME_CONST_MAX;
std::vector<decltype(a)> myVec {a, a, a, a};

where we deduce the type of a and pass it to vector.