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Does using namespace ... increase compilation time or can it somehow affect performance? I heard that this affects the compilation time slightly, but still, why?

Aykhan Hagverdili
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Артём Гаркавый
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    1. Slightly. 2. No, assuming you're talking about runtime performance. – user207421 Feb 09 '20 at 10:56
  • `using namespace std;` in global can lead to unexpected behavior, `using namespace` in global scope in general is a responsibility of programmer – Swift - Friday Pie Feb 09 '20 at 11:00
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    For more reasons why not to do `using namespace std;` read [Why is “using namespace std;” considered bad practice?](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice) – Ted Lyngmo Feb 09 '20 at 11:04

3 Answers3

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It increases the compilation time ever so slightly because more places are considered during a name lookup, but nothing significant. It doesn't affect the runtime performance at all.

Aykhan Hagverdili
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tldr No, it doesn't affect neither compilation time nor run-time

Strictly speaking every change in the source code modifies the compilation time. Using directives, using statements and all forms of aliases don't affect compilation time in any meaningful or note-worthy way.

As for run-time performance the generate code is identical, so absolutely no performance impact whatsoever.

In conclusion don't worry about it, not in the slightest.

I would like to challenge Ayxan statement that the compilation time is is increased because the more places are considered during name lookup. I could argue the opposite: that compilation time is decreased because nested namespaces are looked up just once on the using directive, instead of on every qualified name.

Consider this:

auto x = nsa::nsb::nsc::nsd::class_x{};
auto y = nsa::nsb::nsc::nsd::class_y{};
auto z = nsa::nsb::nsc::nsd::class_z{};

vs

using namespace nsa::nsb::nsc::nsd;

auto x = class_x{};
auto y = class_y{};
auto z = class_z{};

In the first case:

  • nsa must be looked up on the global namespace
  • nsb must be looked up on the nsa namespace
  • nsc must be looked up on the nsb namespace
  • nsd must be looked up on the nsc namespace
  • class_x must be looked up on the nsd namespace
  • repeat all above thee times (for all the qualified items)

In the second case

  • just like before, look up nsa in the global namespace, nsb in the nsa etc. Except just once, during the using directive.
  • add entry in the table lookup for the using directive
  • look up class_x in the global namespace + the entries brought by the using directives. Do this for all the unqualified names.

I am not saying one is faster than the other. I am saying it's not as simple and black and white. Without a proper benchmark (study really as the codebases wildly differ) we don't know. Anyway, my point is that it doesn't matter anyway as any difference practically doesn't matter.

bolov
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About performance, it does not.

Namespaces are used to make logical groupings and to avoid potential naming conflicts within C++ applications. Use of namespaces incurs no overhead to application size or performance.

But...

...bringing the entire contents of such a namespace into current scope in this fashion defeats the purpose of namespaces.

https://faq.cprogramming.com/cgi-bin/smartfaq.cgi?answer=1046398336&id=1043284351

About compile time, there is not much documentation I can find about it, but it seems to be consensual that though it slightly affects it, the differences are negligible, bordering the unmeasurable.

C++ performance: using declaratives Vs using declarations Vs using names directly from the library

For C#

Does using namespaces affect performance or compile time?

anastaciu
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  • True, but this talks about application performance only. It doesn't touch on the compilation impact. – bolov Feb 09 '20 at 11:33