Question

Should it be possible to write the following code? What I’d like to do is that the do_vector_action could automatically deduce the correct return type of the function (the code I actually have has the function defined in a cpp file, not in the header as here).

class some_class
{
    public:
        std::vector<int> int_vector;
        auto do_vector_action() -> decltype(int_vector_.size())
        {
            decltype(int_vector.size()) something + 1;
            return something;
        }
}

Moreover, I’d like also know, would it be possible to replace typedefs such as

class some_class
{
    public:
        typedef std::vector<int> int_vector_type;
        int_vector_type int_vector;
        int_vector_type::size_type size;
}

with using decltype or some other construct such as

  class some_class
  {
       public:
           std::vector<int> int_vector;
           decltype(int_vector)::size_type size;
  }

as the last snippet with decltype doesn’t compile with Visual Studio 2012 RC.

Answer 1

decltype(int_vector.size()) something + 1;

This is equivalent to:

std::vector<int>::size_type something + 1;

which is ill-formed (you are declaring a variable named something then… adding one to it?

Your second example, using decltype(int_vector)::size_type is valid. Visual C++ 2010 and 2012 reject it due to a compiler bug^(*). As a workaround, you should be able to declare size as:

identity<decltype(int_vector)>::type::size_type size;

assuming the presence of a standard identity template declared as:

template <typename T>
struct identity { typedef T type; };

---

(*) The ability to use decltype in a nested name specifier was added very near the end of the C++11 standardization process (see N3031 [PDF]). This was after Visual C++ 2010 was completed, and support for this addition was not added in Visual C++ 2012.