Is declval() the same as (*(T*)nullptr)?

Question

Is declval() just a replacement for the old trick of (*(T*)NULL) to get an instance of T in a decltype without needing to worry about T\'s

Answer 1

declval() has the advantage that if it is used in an evaluated context (i.e., odr-used) then the program is ill-formed (20.2.4p2), and a diagnostic is required to be issued (per 1.4p1). Typically this is enforced through a static_assert in the library:

c++/4.7/type_traits: In instantiation of '[...] std::declval() [...]':
source.cpp:3:22:   required from here
c++/4.7/type_traits:1776:7: error: static assertion failed: declval() must not be used!

declval also works on reference types:

using S = int &;
using T = decltype(std::declval());
using U = decltype(*(S *)nullptr);  // fails

Where the type is not a reference type, declval will give an rvalue type where nullptr gives an lvalue.