What one doesn't understand, one doesn't possess.
– Johann Wolfgang von Goethe

Page 9 of: C++ auto and decltype Explained, by Thomas Becker   about me  

Miscellaneous Properties of decltype

An important property of decltype is that its operand never gets evaluated. For example, you can use an out-of-bounds element access to a vector as the operand of decltype with impunity:
std::vector<int> vect;
typedef decltype(vect[0]) integer;
Another property of decltype that is worth pointing out is that when decltype(expr) is the name of a plain user defined type (not a reference or pointer, not a basic or function type), then decltype(expr) is also a class name. This means that you can access nested types directly:
template<typename R>
class SomeFunctor {
  typedef R result_type;
  result_type operator()() {
    return R();

SomeFunctor<int> func;
typedef decltype(func)::result_type integer; // access nested type
You can even use decltype(expr) to specify a base class:
auto foo = [](){return 42;};

class DerivedFunctor : public decltype(foo)
    MyFunctor(): decltype(foo)(foo) {}

  // ...