How does include bits stdc h work in C

Question

I have read from a codeforces blog that if we add  include  lt bits stdc   h gt  in a C   program then there is no need to include any other header files  How does  include  lt bits stdc   h gt  work and is it ok to use it instead of including individual header files

User · Answer

include  lt bits stdc   h gt  is an implementation file for a precompiled header   From  software engineering perspective  it is a good idea to minimize the include  If you use  it actually includes a lot of files  which your program may not need  thus increase both compile-time and program size unnecessarily   edit  as pointed out by  Swordfish in the comments that the output program size remains unaffected  But still  it s good practice to include only the libraries you actually need  unless it s some competitive competition   But in contests  using this file is a good idea  when you want to reduce the time wasted in doing chores  especially when your rank is time-sensitive   It works in most online judges  programming contest environments  including ACM-ICPC  Sub-Regionals  Regionals  and World Finals  and many online judges   The disadvantages of it are that it    increases the compilation time  uses an internal non-standard header file of the GNU C   library  and so will not compile in MSVC  XCode  and many other compilers

User · Answer

That header file is not part of the C   standard  is therefore non-portable  and should be avoided   Moreover  even if there were some catch-all header in the standard  you would want to avoid it in lieu of specific headers  since the compiler has to actually read in and parse every included header  including recursively included headers  every single time that translation unit is compiled

User · Answer

Unfortunately that approach is not portable C    so far    All standard names are in namespace std and moreover you cannot know which names are NOT defined by including and header  in other words it s perfectly legal for an implementation to declare the name std  string directly or indirectly when using  include  lt vector gt     Despite this however you are required by the language to know and tell the compiler which standard header includes which part of the standard library  This is a source of portability bugs because if you forget for example  include  lt map gt  but use std  map it s possible that the program compiles anyway silently and without warnings on a specific version of a specific compiler  and you may get errors only later when porting to another compiler or version   In my opinion there are no valid technical excuses because this is necessary for the general user  the compiler binary could have all standard namespace built in and this could actually increase the performance even more than precompiled headers  e g  using perfect hashing for lookups  removing standard headers parsing or loading demarshalling and so on    The use of standard headers simplifies the life of who builds compilers or standard libraries and that s all  It s not something to help users   However this is the way the language is defined and you need to know which header defines which names so plan for some extra neurons to be burnt in pointless configurations to remember that  or try to find and IDE that automatically adds the standard headers you use and removes the ones you don t    a reasonable alternative

User · Answer

It is basically a header file that also includes every standard library and STL include file  The only purpose I can see for it would be for testing and education   Se e g  GCC 4 8 0  bits stdc   h source   Using it would include a lot of unnecessary stuff and increases compilation time   Edit  As Neil says  it s an implementation for precompiled headers  If you set it up for precompilation correctly it could  in fact  speed up compilation time depending on your project   https   gcc gnu org onlinedocs gcc Precompiled-Headers html   I would  however  suggest that you take time to learn about each of the sl stl headers and include them separately instead  and not use  super headers  except for precompilation purposes

[c++] How does #include <bits/stdc++.h> work in C++?

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