What are the disadvantages of using templates?

Question

Some of the disadvantages would be

1. its syntax is complex
2. compiler generates extra code

Answer 1

So far no-one seems to have mentioned the main disadvantage I find with templates: code readability plummets!

I'm not referring to syntax issues -- yes the syntax is ugly, but I can forgive that. What I mean is this: I find that with never-seen-before non-templated code, however large the application is, if I start at main() I can usually decode the broad strokes of what a program is doing without problems. And code that merely uses vector or similar doesn't bother me in the slightest. But once code starts to define and use its own templates for purposes beyond simple container types, understandability rapidly goes out the window. And that has very negative implications for code maintenance.

Part of that is unavoidable: templates afford greater expressiveness via the complicated partial-order overload resolution rules (for function templates) and, to a lesser degree, partial specialisation (for class templates). But the rules are so damn complicated that even compiler writers (who I'm happy to acknowledge as being an order of magnitude smarter than I am) are still getting them wrong in corner cases.

The interaction of namespaces, friends, inheritance, overloading, automatic conversions and argument-dependent lookup in C++ is already complicated enough. But when you add templates into the mix, as well as the slight changes to rules for name lookup and automatic conversions that they come with, the complexity can reach proportions that, I would argue, no human can deal with. I just don't trust myself to read and understand code that makes use of all these constructs.

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An unrelated difficulty with templates is that debuggers still have difficulty showing the contents of STL containers naturally (as compared to, say, C-style arrays).