What is the difference between \def and \newcommand (Part II)?

Question

A previous question What is the difference between \def and \newcommand? has discussed the difference between \def and \newcommand at some length. However, I cannot find in that discussion an explanation of the following phenomenon.

This code prints Empty:

\documentclass{article}
\begin{document}
\def\foo{}
\ifx\foo\empty Empty\else Not empty\fi
\end{document}

This code prints Not empty:

\documentclass{article}
\begin{document}
\newcommand\foo{}
\ifx\foo\empty Empty\else Not empty\fi
\end{document}

The only difference is that one has \def and the other has \newcommand. What's going on?

\empty and \foo are different. \foo is defined as \long\def\foo{} which is different than \def\empty{}. Try \newcommand*\foo{} instead. — Qrrbrbirlbel, May 24 '13 at 12:27
See http://tex.stackexchange.com/questions/1050/whats-the-difference-between-newcommand-and-newcommand — Andrew Stacey, May 24 '13 at 12:30
@Qrrbrbirlbel may as well make that an answer, as it is the answer:-) — David Carlisle, May 24 '13 at 12:30
@Qrrbrbirlbel -- might be worth mentioning that \newcommand*\foo{} gives the same result as \def. should be obvious, but ... — barbara beeton, May 24 '13 at 12:43
@Qrrbrbirlbel Goodness me, (La)TeX sure is subtle! I posted a new answer to the old "newcommand vs. def" question, as this point hadn't actually been made there. — John Wickerson, May 24 '13 at 12:47
As you've posted the answer at the other question, maybe this one should now be closed as a duplicate of it. — Andrew Stacey, May 24 '13 at 13:09
@AndrewStacey Happy to. Or I could just delete this question altogether? — John Wickerson, May 24 '13 at 13:10
@JohnWickerson Duplicates do have their place as they make it easier to find the main question. I would only go for deletion if the question shouldn't have been asked here at all (meaning, if the duplicate hadn't already been here). — Andrew Stacey, May 24 '13 at 13:18

What is the difference between \def and \newcommand (Part II)?

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