Why should I not #include <bits/stdc++.h>?

Including <bits/stdc++.h> appears to be an increasingly common thing to see on Stack Overflow, perhaps something newly added to a national curriculum in the current academic year.

I imagine the advantages are vaguely given thus:

• You only need write one #include line.
• You do not need to look up which standard header everything is in.

Unfortunately, this is a lazy hack, naming a GCC internal header directly instead of individual standard headers like <string>, <iostream> and <vector>. It ruins portability and fosters terrible habits.

The disadvantages include:

• It will probably only work on that compiler.
• You have no idea what it'll do when you use it, because its contents are not set by a standard.
• Even just upgrading your compiler to its own next version may break your program.
• Every single standard header must be parsed and compiled along with your source code, which is slow and results in a bulky executable under certain compilation settings.