How to tex $:\iff$ correctly?

Question

In the thread Symbol for "defined to be logically equivalent" (:⇔), we discussed how to tex ":⇔" correctly. As we learnt, the solution is to define the following new command:

\newcommand*{\logeq}{\ratio\Leftrightarrow}

Now, this only works with

\Leftrightarrow

I am interested in how to tex the "colon \iff" correctly, that is, with a

\Longleftrightarrow

instead of a

\Leftrightarrow

Answer 1

\iff is similar to \Longleftrightarrow but forces an amount of space on either side - a \; or \thickmuskip (see What commands are there for horizontal spacing?). That,

\def\pshow#1{{\let\protect\show#1}}
\pshow\iff

yields

> \iff =\long macro:
->\;\Longleftrightarrow \;.
\iff ->\protect \iff  

in the .log. In contrast,

\pshow\Longleftrightarrow

yields

> \Longleftrightarrow =\long macro:
->\Leftarrow \joinrel \Rightarrow .

in the .log - a \join or the relations \Leftarrow and \Rightarrow, without adding spaces around the outside.

\documentclass{article}
\usepackage{colonequals}
\newcommand*{\logeq}{\ratio\Leftrightarrow}
\newcommand*{\longeq}{\ratio\Longleftrightarrow}
\begin{document}
\[ A \logeq B \longeq C \]
\end{document}