Hello Std Proposals,

I have been benchmarking the cost of including the standard library headers
alone without actually using anything in them. This is because it is common
for large C++ programs to end up with headers that have massive include
graphs that slow down compilation considerably. Even when the individual
translation units use only a small fraction of what is included in them.

The executive summary is that the compile time costs incurred by using the
standard library have roughly doubled between C++11 and C++23. Simply
changing the standard library in use on the command line could add 0.5-1.0
seconds to the compile time for each file in your project. This is because
of the surprising amount of additional code that gets included when you
change language standards.

Even the cost of loading the standard library as a PCH has been going up
and it could still add up to a considerable amount of time across hundreds
of files.

I would like to strongly discourage adding more code to existing headers in
future standards.

As for floating a proposal on this list, having things like <algorithm-core>
that includes only the contents of <algorithm> as defined by C++11 would be
a way to cut back. I can't say there are any clean and satisfying solutions
that come to mind right now.

Here is the script I used to generate the following document if you want to
check my methods: https://github.com/whatchamacallem/cxx-std-include-bench
The script and the following documents are also attached to this email.

Regards,
Adrian



Benchmarking Including The C++ Standard Library

All timings are the median of 51 runs. Timings collected on an MSI Stealth
A16 AI+ A3XVFG.

This script measures the compilation overhead of including but not using a
large set of the C++ standard library headers with GCC and Clang for C++11,
C++14, C++17, C++20, and C++23. Nothing in the headers is actually used.
Startup time is the time taken to compile an empty file with no includes at
all and is deducted from all subsequent measurements. GCC uses libstdc++
and Clang uses libc++.
Compiler Version Startup (ms)
GCC g++ (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0 6
Clang Ubuntu clang version 18.1.3 (1ubuntu1) 15Headers Under Test

These are C++11 headers that were chosen as representative of normal use in
a large C++ program with a complex include graph. Choosing C++11 headers
allows showing how they change across all language versions being tested.

<algorithm> <array> <atomic> <bitset> <chrono> <condition_variable> <deque>
<forward_list> <fstream> <functional> <future> <iostream> <iterator> <list>
<locale> <map> <memory> <mutex> <numeric> <queue> <random> <regex> <set>
<sstream> <stack> <stdexcept> <string> <thread> <tuple> <typeindex>
<unordered_map> <unordered_set> <utility> <valarray> <vector>
Compilation Time

Time taken to compile the selected standard library headers in an otherwise
empty translation unit. Also lists the lines of code seen by the
preprocessor for that translation unit. Times do not include compiler
startup.
Standard GCC LOC GCC ms (net) Clang LOC Clang ms (net)
C++11 73860 354 60612 500
C++14 75657 370 61644 527
C++17 83880 451 68122 629
C++20 108899 814 83963 938
C++23 112865 868 86602 995Compilation Time Using A Precompiled Header

Time to compile an empty translation unit that uses only a precompiled
header (PCH) containing the selected standard library headers. The PCH is
rebuilt for each version of the standard. Times do not include compiler
startup.
Standard GCC PCH ms (net) Clang PCH ms (net)
C++11 59 71
C++14 64 81
C++17 76 99
C++20 237 94
C++23 238 102Lines Of Code Per-Header (GCC)

Lines of code seen by preprocessor when including each header individually.
Each column shows the absolute LOC and the delta from the prior standard,

if any.
Header C++11 C++14 C++17 C++20 C++23
algorithm 10759 11302 (🔺543) 12383 (🔺1081) 21184 (🔺8801) 23940 (🔺2756)
array 5678 6132 (🔺454) 6655 (🔺523) 9944 (🔺3289) 10473 (🔺529)
atomic 3400 3461 (🔺61) 3950 (🔺489) 7767 (🔺3817) 7920 (🔺153)
bitset 13518 14430 (🔺912) 17834 (🔺3404) 23084 (🔺5250) 24116 (🔺1032)
chrono 3866 4691 (🔺825) 5233 (🔺542) 60024 (🔺54791) 61446 (🔺1422)
condition_variable 13409 14298 (🔺889) 15020 (🔺722) 39998 (🔺24978) 41170
(🔺1172)
deque 10097 10975 (🔺878) 13439 (🔺2464) 18509 (🔺5070) 19428 (🔺919)
forward_list 8014 8892 (🔺878) 11372 (🔺2480) 16517 (🔺5145) 17436 (🔺919)
fstream 21748 22654 (🔺906) 26071 (🔺3417) 31503 (🔺5432) 32620 (🔺1117)
functional 5562 6185 (🔺623) 26147 (🔺19962) 31571 (🔺5424) 33415 (🔺1844)
future 24537 25785 (🔺1248) 28081 (🔺2296) 42231 (🔺14150) 43412 (🔺1181)
iostream 20077 20983 (🔺906) 24365 (🔺3382) 29683 (🔺5318) 30800 (🔺1117)
iterator 16064 16970 (🔺906) 20349 (🔺3379) 25582 (🔺5233) 26607 (🔺1025)
list 7765 8220 (🔺455) 10701 (🔺2481) 15850 (🔺5149) 16769 (🔺919)
locale 21210 22116 (🔺906) 25500 (🔺3384) 30869 (🔺5369) 31894 (🔺1025)
map 10651 11789 (🔺1138) 13556 (🔺1767) 18705 (🔺5149) 19624 (🔺919)
memory 15094 15978 (🔺884) 16785 (🔺807) 36698 (🔺19913) 37736 (🔺1038)
mutex 7855 8794 (🔺939) 9553 (🔺759) 15850 (🔺6297) 16758 (🔺908)
numeric 1940 2733 (🔺793) 4563 (🔺1830) 7113 (🔺2550) 7308 (🔺195)
queue 13832 14717 (🔺885) 17136 (🔺2419) 22246 (🔺5110) 23195 (🔺949)
random 25481 26429 (🔺948) 33162 (🔺6733) 38388 (🔺5226) 40628 (🔺2240)
regex 46191 47475 (🔺1284) 50374 (🔺2899) 56217 (🔺5843) 57376 (🔺1159)
set 9099 10191 (🔺1092) 13268 (🔺3077) 18417 (🔺5149) 19336 (🔺919)
sstream 20689 21595 (🔺906) 24977 (🔺3382) 30514 (🔺5537) 31631 (🔺1117)
stack 10339 11224 (🔺885) 13585 (🔺2361) 18663 (🔺5078) 19603 (🔺940)
stdexcept 12944 13850 (🔺906) 17232 (🔺3382) 22478 (🔺5246) 23502 (🔺1024)
string 12471 13377 (🔺906) 16781 (🔺3404) 22035 (🔺5254) 23066 (🔺1031)
thread 8983 10372 (🔺1389) 11045 (🔺673) 36539 (🔺25494) 37711 (🔺1172)
tuple 3590 3755 (🔺165) 4275 (🔺520) 11078 (🔺6803) 11931 (🔺853)
typeindex 178 178 (🔺0) 178 (🔺0) 3570 (🔺3392) 3625 (🔺55)
unordered_map 12589 13538 (🔺949) 15287 (🔺1749) 20639 (🔺5352) 21558
(🔺919)
unordered_set 12553 13502 (🔺949) 15166 (🔺1664) 20522 (🔺5356) 21441
(🔺919)
utility 2224 2353 (🔺129) 2812 (🔺459) 4609 (🔺1797) 4812 (🔺203)
valarray 15263 15806 (🔺543) 20819 (🔺5013) 29697 (🔺8878) 33653 (🔺3956)
vector 10848 11726 (🔺878) 14206 (🔺2480) 19333 (🔺5127) 20261 (🔺928)