When should one use Custom Tag in CFML?

What are some common use cases for implementing CFML Custom Tag (not CFX ta开发者_Go百科g)? In 3 yrs of my CF exp I've never written one. Would someone please enlighten me, under which use case / situation would one choose custom tag over cfc / udf?

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Remember that custom tags were, at one time, the only method available to extend CFML (up until version 4) - UDFs came later (CF 5) and CFCs later still (CF MX). They're not as commonly used as they once were for the simple reason that there are more options.

Custom tags are basically procedural in nature in a language that, with CFCs, become more and more OO in practice. This is another reason that they're not very common.

But there's still cases where they come in handy (but are never required) - mostly for interface work. The ability to create both a start and end state can definately come in handy. A simple example could be a "wrapper" for page content the opening tag might add the HTML header and page navigation while the closing tag would add the footer and end the page.

In this way your page content could be nothing more than:

<cfmodule... >
       Page Content!
</cfmodule>

Of course there are other ways to do this as well - but sometimes the classics still have value. ;^)

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Look at the CFUniform project for a great example of custom tag usage. Custom Tags are great when building reusable pieces for the UI portion of an application.

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I think that, for the most part, custom tags have mostly fallen by the wayside since UDFs, CFCs, and integration with Java (and to a lesser degree .NET) allowed easier and more straightforward ways to do similar things.

Looking back to when I started in CF5, I can think of several examples. A good one might be CFX)Zip, which allowed interaction with Zip files before that was available directly through CF.

The only use I can think of offhand in a more modern context would be to provide precompiled code that wasn't written in Java or .NET, such as proprietary doodads written in C. That's a pretty niche use, though.

Honestly, I imagine at this point they exist more or backwards comatibility than anything else.

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Ever since CFCs came out I've stopped using custom tags simply because of the overhead. They take too long to initiate and execute. But like @Jim Davis said, they may be useful where you need to write a tag that wraps around other content.

But in a well defined solution, you can do way with them all together.