Decltype of a container from an argument of a function

Question

I'm trying to deduct the type of the stl container inside a function from the function's argument and it doesn't compile:

auto battery_capacity( const vector<double>& v) -> decltype(v)::value_type {
  decltype(v)::value_type b=0;
  return b;
}

error:

main.cpp:10:52: error: 'decltype(v)' (aka 'const vector<double> &') is not a class, namespace, or scoped
      enumeration
auto battery_capacity( const vector<double>& v) -> decltype(v)::value_type {
                                                   ^
main.cpp:11:3: error: 'decltype(v)' (aka 'const vector<double> &') is not a class, namespace, or scoped enumeration
  decltype(v)::value_type b=0;

however this compiles:

double battery_capacity( const vector<double>& v) {
  vector<double> s=v;
  decltype(s)::value_type b=0;
  return b;
}

Update (1)

Addition for when using templates based on Kerrek's answer:

template <typename T>
auto battery_capacity( const T& v) -> typename std::remove_reference<decltype(v)>::type::value_type {
  typename std::remove_reference<decltype(v)>::type::value_type b=0;
  return b;
}

Answer 1

If you really really must force the use of decltype into this very simple code (and I'd call that misguided), then it's like this:

auto battery_capacity(const vector<double> & v) -> std::remove_reference<decltype(v)>::type::value_type
{
    std::remove_reference<decltype(v)>::type::value_type b = 0;
    return b;
}

If you're unsure why you need to write this, you really don't want to be using decltype at all. It's a very specialized tool for a very specialized kind of generic library writing, not for common household use.

Of course the simpler answer is just to write double battery_capacity(...), since you know the type already! (If you don't actually know the type, then you didn't ask the right question, and your question doesn't reflect your actual problem.)

In C++14, you can be a little shorter:

auto battery_capacity(const vector<double> & v)
{
    return std::remove_reference_t<decltype(v)>::value_type { 0 };
}