Using swap() without #including its library? [duplicate]

Question

I have a simple program in which I arrange the elements of an int array in ascending or descending order, and I use the swap() function to move the elements around. I compiled the program without any errors, and it ran like a charm. I only noticed afterwards that I had forgotten to #include the library that swap() is defined in (<algorithm>, or <utility> as of C++11) before I compiled.

Why did it still work? The top of my program looked like this:

#include <iostream>
#include <cstdlib>
using namespace std;

I tried taking out <iostream>, just to see what would happen, and it predictably put out a bunch of 'cout/cin/endl' was not declared in this scope errors, but I was surprised to see that it gave some 'swap' was not declared in this scope errors as well. Does that mean swap() is defined in <iostream>? I don't think it should be, should it?

Anyways, this is probably a big long question for a simple answer, but I'm pretty curious. I'm still learning C and C++, so I don't know a lot of things, and I couldn't find an answer to this particular mystery via the "Almighty" Google Machine, so here I am.

Thanks in advance!

Answer 1

Generally, do NOT rely on the header files that includes other header files.

Always include and only include the header files you need.

For example, if you want to use std::swap(), Google it and you'll see if requires <algorithm> in c++98 and <utility> in c++11, so you should include the file to make sure your code compiles.