What are the disadvantages of using templates?

问题:

Some of the disadvantages would be

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

回答1:

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.

回答2:

Hmm, how about...

3: They can be slow to compile

4: They force things to be calculated at compile time rather than run time (this can also be an advantage, if you prefer fast execution speed over runtime flexibility)

5: Older C++ compilers don't handle them, or don't handle them correctly

6: The error messages that they generate when you don't get the code right can be nearly incomprehensible

回答3:

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

回答4:

The only real disadvantage is that if you make any tiny syntax error in a template (especially one used by other templates) the error messages are not gonna be helpful... expect a couple pages of almost-unusable error msgs;-). Compilers' defect are very compiler-specific, and the syntax, while ugly, is not really "complex". All in all, though -- despite the huge issue with proper error diagnostics -- templates are still the single best thing about C++, the one thing that might well tempt you to use C++ over other languages with inferior implementations of generics, such as Java...

回答5:

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).

回答6:

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.

回答7:

Less people understand them, epsecially at the level of meta programming, therefore less people can maintain them.

回答8:

When you use templates, your compiler only generates what you actually use. I don't think there is any disadvantages in using C++ template meta-programming except the compiling time which can be quite long if you used very complex structures as boost or loki libraries do.

回答9:

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.

回答10:

The absolute worst: The compiler error messages you get from bad template code.

回答11:

I have used templates sometimes over the years. They can be handy but from a professional perspective, I am leaning away from them. Two of the reasons are:

1.

The need to either a.) expose the function definitions (not only declarations) "source" code to the "where used" code or b.) create a dummy instantiation in the source file. This is needed for compilation. Option a.) can be done by defining functions in the header or actually including the cpp.

One of the reasons that we tolerate headers in C++ (compared to C# for example) is because of the separation of "interface" from "implementation". Well, templates seem to be inconsistent with this philosophy.

2.

Functions called by a template type parameter instantiation may not be enforced at compile time resulting in link errors. E.g. T example; example.CompilerDoesntKnowIfThisFunctionExistsOnT(); This is "loose" IMHO.

Solutions:

Rather then templates, I lean towards using a base class whereby the derived/container classes know what is available at compile time. The base classes can provide the generic methods and "types" that templates are often used for. This is why source code availability can be helpful if existing code needs to be modified to insert a generic base class in the inheritance hierarchy where needed. Otherwise if, code is closed source, rewrite it better using generic base classes instead of using a template as a work around.

If type is unimportant e.g. vector< T > then how about just using"object". C++ has not provided an "object" keyword and I have proposed to Dr. Bjarne Stroustrup that this would be helpful especially to tell compiler and people reading code that type is not important (for cases when it isn't). I don't that think C++11 has this, perhaps C++14 will?