How does #include work in C++?

Question

If a library is included in a class header and then this header is included in another class do I have to include the library again?

For example:

#ifndef A_H
#define A_H

#include<someLibrary.h>

class A{
  ...
}

#endif

Then another class includes the A.h header

#include<A.h>   //include class A
class B{
   ...
}

Do I have to include the "someLibrary.h" in class B?

Answer 1

No, #includes are transitive.

However, if your second file itself uses symbols from someLibrary, it's good style to re-include the header. That way you're not "hoping and praying" that you never remove the intermediate include. Your codebase will be more robust if every source file #includes everything that it directly needs. Header guards prevent this from being a wasteful policy.

Answer 2

The preprocessor #include directive does exactly what the name implies, it actually includes the file at the place of the directive.

Simple example: Lets say we have to files, first a header file named a.h

// Our class
class A
{
    // Stuff for the class
};
// End of the class

Then a source file a.cpp:

// Include header file
#include "a.h"
// Header file included

int main()
{
    // Code
}

The preprocessor generates a single file looking like

// Include header file
// Our class
class A
{
    // Stuff for the class
};
// End of the class
// Header file included

int main()
{
    // Code
}

This inclusion is recursive, so if a header file is including another header file, that other header file will also be included.

The source file generated by the preprocessor is called a translation unit and is what the compiler actually sees.

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The above is a simplification on how a modern preprocessor works, and while it can be run separately to create a preprocessed source file, it's more common that the preprocessor is part of the compiler to streamline the parsing process.

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You should also note that the terminology you use is not correct. A library can (and usually do) have one or more header files, which are used when compiling your source code. A library then often (but not always) contain a special library file that is linked with the object files created by the compiler.

A library that has no linker library is called a header only library.

Answer 3

You don't include classes or libraries, you just include headers, and that is a textual operation (a bit like a copy & paste done by the preprocessor).

Read more about the C/C++ preprocessor, and the GNU cpp.

You can ask your compiler to show you the preprocessed form of your source file foo.cc, e.g. with g++ -Wall -C -E foo.cc (it will spills on stdout the preprocessed form)

For a small project (e.g. less than 200KLOC), having just one single common header file included by all your translation units is sensible (and I believe that having many small header files is bad habit, so I usually put more than one class definition per header file). BTW, that (single header file) approach is friendly for precompiled headers. Some people prefer to have several of their own #include-d subheaders in that single header.

Notice that most C++ standard headers (like <map> or <vector>....) bring a lot of text, so you don't want to have tiny compilation units (on my Linux system, #include <vector> is dragging more than ten thousand lines, so having only a few dozen of your source code lines after is inefficient for the compiler)

Also notice that C++ class definitions usually have lots of inlined member functions (and you practically want to give the body of that inlined function in the same header file). So C++ header code tends to be quite big...

^{(some people prefer to break a single header file in many subheaders, which are always included together)}