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

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.

#include <bits/stdc++.h> is an implementation file for a precompiled header.

From, software engineering perspective, it is a good idea to minimize the include. If you use <bits/stdc++.h> 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