why do i need to include to use them?

Question

I am new to C++ and trying to figure out how to use vector.

More specifically, I want to know when I need to use #include. I

Answer 1

vector is not actually built into C++, it is only part of its standard library which is guaranteed to be available to you if you use C++. vector (or, by its full name, std::vector) is itself implemented in C++.

By writing #include , you are telling the compiler to not only use your own code, but to also compile a file called vector. This file is actually somewhere on your harddrive (if you use GNU/Linux, it's probably located at /usr/include/c++/[GCC_VERSION]/vector).

You cannot use std::vector without including that file, because the compiler then doesn't know a class called std::vector. The compiler only knows the language C++, not its standard library!

If some programs use std::vector without including its header file, it's because some header file that they have already included, has an #include somewhere. There may be good reasons for that (e.g. some C++ courses ship with a file that includes all necessary headers and that is used in the first few lessons). However there may also be standard library headers that include vector (some implementations of iostream do that). Relying upon that is not a good idea because it differs from implementation to implementation, so your program might work in Visual C++ 2010, but it doesn't compile on GNU or in a newer version of Visual C++.