jan_14_include_order.md

#include "this.h"
#include "mine.h"
#include "ours.h"

#include <theirs.h>
#include <stdheaders>

#include "this.h" - Foo.cpp should include Foo.h first.
#include "mine.h" - include closely related headers next
#include "ours.h" - working outwards through our codebase

Note those all used "", not <>

Then include 3rd party headers, like Boost, Qt, etc, and the std headers go last

#include <theirs.h>
#include <stdheaders>

But Why

Imagine class Foo uses std::vector ... BUT Foo.h forgot to include <vector> :O

So Foo.h looks like this:

class Foo
{
   std::vector<int> stuff;
};

And Foo.cpp looks like this:

#include <vector>
#include "Foo.h"

Foo foo;

insert everything is fine gif here

What happens when Bar.cpp tries to include and use Foo?

#include "Bar.h"
#include "Foo.h"  // WHAT!?!?! THIS DOESN'T COMPILE?!?!?

It doesn't compile, because the compiler doesn't (yet) know what a std::vector is when it sees std::vector<int> stuff; in the definition of class Foo.

If Foo.cpp instead did:

#include "Foo.h" // VERY FIRST INCLUDE

Foo foo;

Then the author of Foo would notice right away that anyone trying #include "Foo.h" would fail.

Then the author would immediately go and fix Foo.h to include <vector> and whatever other headers it needs.

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NOTE: precompiled headers defeat this :-( - which is one more reason I dislike PCHs.