CODEWITHSUNDEEP
c++performancetemplates
user293398
user293398

Reputation:

Do c++ templates make programs slow?

I have heard from many people that usage of templates make the code slow. Is it really true. I'm currently building a library. There are places where if templates are not created, it would result in code management problem. As of now I can think two solutions to this problem:

e.g. typedef Graph<int> GraphI32; etc.

Is there anyway, to restrict user from creating various template instances on their own.

Help on above queries would be highly regarded.

Upvotes: 46

Views: 28405

Answers (5)

Jerry Coffin
Jerry Coffin

Reputation: 490058

The short answer is no. For the longer answer please read on.

As others have already noted, templates don't have a direct run-time penalty -- i.e. all their tricks happen at compile time. Indirectly, however, they can slow things down under a few circumstances. In particular, each instantiation of a template (normally) produces code that's separate and unique from other instantiations. Under a few circumstances, this can lead to slow execution, by simply producing enough object code that it no longer fits in the cache well.

With respect to code size: yes, most compilers can and will fold together the code for identical instantiations -- but that's normally the case only when the instantiations are truly identical. The compiler will not insert code to do even the most trivial conversions to get two minutely different instantiations to match each other. For example, a normal function call can and will convert T * to T const * so calls that use either const or non-const arguments will use the same code (unless you've chosen to overload the function on constness, in which case you've probably done so specifically to provide different behavior for the two cases). With a template, that won't happen -- instantiations over T * and T const * will result in two entirely separate pieces of code being generated. It's possible the compiler (or linker) may be able to merge the two after the fact, but not entirely certain (e.g., I've certainly used compilers that didn't).

But in the end, templates have positive effects on speed far more often than negative.

Upvotes: 74

duffymo
duffymo

Reputation: 308743

They do make object code bigger, because C++ generates code for every type you use. But I don't believe this will slow execution speed. I have no numbers to suggest that it would.

It certainly does improve your lot in life during code development, reading and maintenance. I would not let coding urban myths discourage you from using a language feature that's clearly useful.

Upvotes: 6

anon
anon

Reputation:

No they don't. When ever you find that you have "heard" something, and cannot name the source, you can almost certainly guarantee that what you have heard is wrong. In fact, templates tend to speed code up.

Rather than depending on hearing things, it's a good idea to read an authoritative book on the subject - I recommend C++ Templates - The Complete Guide.

Upvotes: 16

sepp2k
sepp2k

Reputation: 370112

Since template instantiation happens at compile time, there is no run-time cost to using templates (as a matter of fact templates are sometimes used to perform certain computations at compile-time to make the program run faster). Heavy use of templates can however lead to long compile times.

Upvotes: 26

fabmilo
fabmilo

Reputation: 48310

Template make the Compilation slow. But most of the time it makes the program faster.

Upvotes: 8

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