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A preprocessor statement to add a header file (.h or .hpp) or other files to your program.
There are three different header file #includes:
1) #include <iostream> // 1: STL
2) #include <myassert.hpp> // 2: The standard header file directory
3) #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 [0] * Prefer to #include <iosfwd> when a forward declaration of a stream will suffice [1] * Never #include a header when a forward declaration will suffice [2]
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 [5] :
#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
These normally are not #included. Instead, these are added to your project. For example, in C++ Builder, select 'Project | Add to Project'.
Forgetting to add an implementation file to you project results in a link error.
[0] Herb Sutter . Exceptional C++. 2000. ISBN: 0-201-61562-2. Item 26: 'Never #include unnecessary header files'.
[1] 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'.
[2] Herb Sutter . Exceptional C++. 2000. ISBN: 0-201-61562-2. Item 26: 'Never #include a header when a forward declaration will suffice'.
[3] 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.'
[4] Bjarne Stroustrup . The C++ Programming Language (3rd edition). 1997. ISBN: 0-201-88954-4. Chapter C.14.15: 'Don't pollute the global namespace'
[5] Bjarne Stroustrup . The C++ Programming Language (3rd edition). 1997. ISBN: 0-201-88954-4. Chapter 8.2.9.1: 'Namespaces and C'.
Go back to Richel Bilderbeek's C++ page.
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