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CppInclude

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(C++) #include

A preprocessor statement to add a header file (.h or .hpp) or other files to your program.

There are three different header file #includes:

include <iostream> // 1: STL

include <assert.hpp> // 2: The standard header file directory

include "Unit1.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, i.e. 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 C style include on  ?CppStl STL headers

For backwards compatibility with C one can [[CppInclude

1. include]] C++ header files with the .h extension.

include <stdio.h> //C-stle #include !!!

This header file 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

Implementation (.c or .cpp) files

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 linking error.

'#include' links

• C

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'.

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