(C++) const

const (an abbreviation of 'constant') is a keyword to ensure read-only operations.

int main()
{
  const int dozen = 12;
}

  There are five types of const:

1. const variable
2. const argument
3. const return type
4. const member function
5. const member

Using the const keyword, design flaws can be spotted at compile time.

Const is not deep as one can delete a pointer-to-const.

Active use of const is termed const correctness.

In function design consider using:

• const arguments, whenever these need to be read-only
• const variables, whenever these need not change after initialization
• In C++98 only: const return types

In class design consider using:

• const members: for member variables that must not change after construction
• const member functions: for member functions that must not change the class's state  

Advice

• Prefer const data to mutable data [15]
• Use const whenever possible [1-6,14].
• Be const correct [7-10, 12]
• Prefer const over #defines [5]
• Avoid magic constants [13]
• Avoid constants at file scope in header files [11]
• Consider casting away const only as the very last resort [15]

External links

• C++ Weekly - Ep 104 - Learning "Modern" C++ - 4 : const and constexpr

References

• [1] Bjarne Stroustrup. The C++ Programming Language (3rd edition). ISBN: 0-201-88954-4 7.9.3: 'Use const extensively and consistently'.
• [2] Scott Meyers. Effective C++ (3rd edition).ISBN: 0-321-33487-6. Item 3: 'Use const whenever possible'.
• [3] Jarrod Hollingworth, Bob Swart, Mark Cashman, Paul Gustavson. Sams C++ Builder 6 Developer's Guide. ISBN: 0-672-32480-6. Chapter 3: 'Understand and use const in your code'.
• [4] Jesse Liberty. Sams teach yourself C++ in 24 hours. ISBN: 0-672-32224-2. Hour 8, chapter 'Const member functions': 'Use const whenever possible.'.
• [5] Scott Meyers. Effective C++ (3rd edition). ISBN: 0-321-33487-6. Item 2: 'Prefer consts, enums and inlines to #defines'.
• [6] Herb Sutter, Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Item 15: 'Use const proactively'.
• [7] Herb Sutter. Exceptional C++ style. 2005. ISBN: 0-201-76042-8. Item 1 guideline: 'Be const correct'.
• [8] Marshall Cline, Greg Lomow and Mike Girou. C++ FAQs. ISBN: 0-201-3098301. FAQ 14.05: 'Is const correctness tedious?' (Answer: no).
• [9] The C++ FAQ Lite. Item 18.1: What is 'const correctness' (Answer: 'A good thing')?
• [10] Bruce Eckel. Thinking in C++, second edition, volume 1. 2000. ISBN: 0-13-979809-9. Item 8: 'Constants', paragraph 'Summary': 'const-correctness can be a lifesaver for projects'.
• [11] John Lakos. Large-Scale C++ Software Design. 1996. ISBN: 0-201-63362-0. Chapter 2.3.3
• [12] John Lakos. Large-Scale C++ Software Design. 1996. ISBN: 0-201-63362-0. Chapter 9.1.6: 'Every object in a system should be const-correct'
• [13] Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 6.6. Advice. page 169: '[6] Avoid "magic constants"'
• [14] Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 12.7. Advice. page 341: '[13] Use const extensively and consistently'
• [15] Gottschling, Peter. Discovering Modern C++: An Intensive Course for Scientists, Engineers, and Programmers. Addison-Wesley Professional, 2015. Chapter 5.2.3.2: 'Consider casting away const only as the very last resort'
• [16] C++ Core Guidelines: P.10: Prefer immutable data to mutable data