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

Templates expose your implementation to the clients of your code, which makes maintaining your ABI harder if you pass templated objects at library boundaries.

Answer 2

They're complicated for the compiler to parse which means your compilation time will increase. Also it can be hard to parse compiler error messages if you have advanced template constructions.

Answer 3

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<int> 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).

Answer 4

They are hard to validate. Template code which doesn't get used tends to be seldom compiled at all. Therefore good coverage of test cases is a must. But testing is time-consuming, and then it may turn out the code never needed to be robust in the first place.

Answer 5

A disadvantage: template errors are only detected by the compiler when the template is instantiated. Sometimes, errors in the methods of templates are only detected when the member method is instantiated, regardless if the rest of the template is instantiated.

If I have an error in a method, of a template class, that only one function references, but other code uses the template without that method, the compiler will not generate an error until the erroneous method is instantiated.