Why does omission of “#include ” only sometimes cause compilation failures?

Question

I am a beginner with C++. When I write the code sometimes I write #include and the code works, other times I don\'t write #include

Answer 1

As Branko said:

It is possible that other headers that you do include have #include in them.

Let's take a look at iostream includes:

#include 
#include 
#include 

if you check istream you can see some include so like this, we have:

iostream => istream => ios => iosfwd

and in iosfwd we have string library! but it's not standard, it's for forward declaration. in iosfwd we have:

#include // For string forward declarations.

and in stringfwd.h:

@file bits/stringfwd.h
This is an internal header file, included by other library headers.
Do not attempt to use it directly. @headername{string}

so, You can use string without #include .