Go back to Richel Bilderbeek's homepage.

Go back to Richel Bilderbeek's C++ page.

 

 

 

 

 

(C++) #include

 

#include is a preprocessor directive to add a header file (.h or .hpp) or other files to your program.

 

There are multiple forms of a header file #includes:

 

#include <iostream> // 1: STL
#include <myassert.hpp> // 2: The standard header file directory
#include "MyUnit.h" // 3: Local

 

 

The first, without the .h extension, means that this header file is from the Standard Template Library. The second #include means that the header is not from the STL but in the standard header file directory. The third #include means that the file is local, that is in the same directory as the program. If the local #include fails, the standard header file directory is checked for this header file.

 

Never #include unnecessary header files [1]. Prefer to #include <iosfwd> when a forward declaration of a stream will suffice [2]. Never #include a header when a forward declaration will suffice [3]. The implementation (.cpp) file of every component should #include its own header (.h) file as the first substantive line of code [7].

 

 

 

 

 

The C style include on STL headers

 

For backwards compatibility with C one can #include C++ header files with the .h extension. Do not do this: call the correct C++ header file. For a list of all C++ standard header files, go to the header file page.

 

#include <stdio.h> //C-stle #include, avoid to do this

 

The header file 'stdio.h' is a wrapper: all it does is call the C++ header file cstdio and then adds a 'using namespace std', as C does not have namespaces.

 

This has the unfortunate side-effect that after calling such a header file all functions and classes in namespace std will be in the global namespace.

 

As it pollutes the global namespace, avoid using namespace std [4-5].

 

An example from [6]:

 

#include <vector> //Carefully avoids polluting the global namespace

vector v1; //Error: no 'vector' in global namespace

#include <stdio.h> //Contains a 'using namespace std'.

vector v2; //Oops: this now works

 

 

 

 

 

#including implementation (.c or .cpp) files

 

Conside not #including these [8], but add these to your project instead. For example, in C++ Builder, select 'Project | Add to Project'.

 

Forgetting to add an implementation file to you project results in a link error.

 

 

 

 

 

References

 

  1. Herb Sutter. Exceptional C++. 2000. ISBN: 0-201-61562-2. Item 26: 'Never #include unnecessary header files'.
  2. Herb Sutter. Exceptional C++. 2000. ISBN: 0-201-61562-2. Item 26: 'Prefer to #include <iosfwd> when a forward declaration of a stream will suffice'.
  3. Herb Sutter. Exceptional C++. 2000. ISBN: 0-201-61562-2. Item 26: 'Never #include a header when a forward declaration will suffice'.
  4. C++ FAQ Lite: http://www.parashift.com/c++-faq-lite/coding-standards.html#faq-27.5. Item 27.5: 'Should I use using namespace std in my code? Probably not.'
  5. Bjarne Stroustrup. The C++ Programming Language (3rd edition). 1997. ISBN: 0-201-88954-4. Chapter C.14.15: 'Don't pollute the global namespace'
  6. Bjarne Stroustrup . The C++ Programming Language (3rd edition). 1997. ISBN: 0-201-88954-4. Chapter 8.2.9.1: 'Namespaces and C'.
  7. John Lakos. Large-Scale C++ Software Design. 1996. ISBN: 0-201-63362-0. Section 3.2, page 110: 'The .c file of every component should include its own .h file as the first substantive line of code'
  8. Joint Strike Fighter Air Vehicle C++ Coding Standards for the System Development and Demonstration Program. Document Number 2RDU00001 Rev C. December 2005. AV Rule 32: 'The #include pre-processor directive will only be used to include header (*.h) files.'

 

 

 

 

 

Go back to Richel Bilderbeek's C++ page.

Go back to Richel Bilderbeek's homepage.

 

Valid XHTML 1.0 Strict

This page has been created by the tool CodeToHtml