mutable

• Gemini
• Virgo
• Sagittarius
• Pisces

Dictionary of Sexology Project: Main Index

PhoneNumber lookupNumber(const Person& person) const;

This method is declared const because, semantically speaking, looking up a phone number shouldn't change the database. However, if you wanted to speed up your database by caching the last few numbers looked up, you might think to add a private member

std::vector<PhoneNumber> cached;

As written, this will not work, because lookupNumber, being a const method, cannot write to the cache. The preferred way to solve this problem is to declare cached to be mutable.

(The other preferred way is to write in a language such as Scheme or Objective Caml which relegates assignment to the ghetto where it belongs.)

Gorgonzola points out that, like any construct permitting assignment without mutual exclusion, mutable throws all guarantees of thread safety out the window.

Mu"ta*ble (?), a. [L. mutabilis, fr. mutare to change. See Move.]

1.

Capable of alteration; subject to change; changeable in form, qualities, or nature.

Things of the most accidental and mutable nature. South.

2.

Changeable; inconstant; unsettled; unstable; fickle.

Byron.

Syn. -- Changeable; alterable; unstable; unsteady; unsettled; wavering; inconstant; variable; fickle.

© Webster 1913.