Include header files in c++

Question

The C++ Premier I have doesn't say much about what I am about to ask, and this is what I got googling LINK:

When the compiler compiles the #include "example.h" line, it copies the contents of example.h into the current file.

So, if that's true, in the following example why B.h doesn't know about A.h ? Is it how the files are compiled ? Do I have to include A.h in every file that use it and then include every file that use A.h in the program.h that uses these files ?

In program.h
#include "A.h"
#include "B.h"

It does! But it is good practice to include a header directly whenever you need it. — BoBTFish
– BoBTFish, Commented Jun 13, 2013 at 14:51

BoBTFish · Accepted Answer · 2013-06-13 15:14:00Z

1

WARNING: VERY BAD CODE:

a.h

#ifndef A_H
#define A_H

#define SOME_LIT "string lit in A.h"

#endif

b.h

#ifndef B_H
#define B_H

#include <iostream>

void foo() { std::cout << SOME_LIT << '\n'; }

#endif

main.cpp

#include "a.h"
#include "b.h"

int main()
{
    foo();
}

Prints:

$ ./a.out 
string lit in A.h

So you can see b.h knows about the define in a.h. If you forgot the #include "a.h", or put it below #include "b.h", this would break.

As a general rule however, you should explicitly #include a header in any file you need it. That way you know that you only care about foo in main, so you just #include the foo header, which is b.h:

#include "b.h"
int main()
{
    foo();
}

edited Jun 13, 2013 at 15:14

answered Jun 13, 2013 at 15:02

BoBTFish

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9 Comments

StudentX Over a year ago

That's strange, it didn't work for me, for a mvc project I am working, I have numerous subclasses in their separate files, and the base class in it's separate file, then in the controller class file, I am doing something like : #include "basefile.h" #include "child1.h" #include "child2.h" and so on.. but I get compile time errors stating that baseclass doesn't doesn't name a type in child1.h(or something similar)

purew Over a year ago

Well, b.h only knows about the define in a.h in the context of main.cpp in your example. If only b.h is included, you'll get errors.

BoBTFish Over a year ago

2 possibilities: 1) You don't have your compilation configured correctly so it can't pick up one of the headers. 2) You have made a mistake in the order you include things. I think 2 is the more common problem. This is why it is recommended to always include everything you need in the file in which you use it.

BoBTFish Over a year ago

@PureW Yes, that is exactly the question. Was that not clear? Also why I included the warning and the recommendation at the end.

BoBTFish Over a year ago

I don't know how to do it in msvc, but if you can use g++ or clang++, use the -E command line option to just apply the preprocessing. Then you can see that #includes are literally just pasted in.

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