From ad6f8631527ec124437819ed1bd4953894285114 Mon Sep 17 00:00:00 2001 From: Bjarne Stroustrup Date: Sun, 7 May 2017 15:59:55 -0400 Subject: [PATCH] Added not about notation to Con.2 in response to #902 --- CppCoreGuidelines.md | 30 +++++++++++++++++++++++++++++- 1 file changed, 29 insertions(+), 1 deletion(-) diff --git a/CppCoreGuidelines.md b/CppCoreGuidelines.md index b351420..6a2168c 100644 --- a/CppCoreGuidelines.md +++ b/CppCoreGuidelines.md @@ -1,6 +1,6 @@ # C++ Core Guidelines -May 1, 2017 +May 7, 2017 Editors: @@ -14676,6 +14676,34 @@ Example: Note that this wrapper solution is a patch that should be used only when the declaration of `f()` cannot be be modified, e.g. because it is in a library that you cannot modify. +##### Note + +A `const` member function can modify the value of an object that is `mutable` or accessed through a pointer member. +A common use is to maintain a cache rather than repeatedly do a complicated computation. +For example, here is a `Date` that caches (mnemonizes) its string representation to simplify repeated uses: + + class Date { + public: + // ... + const string& string_ref() const + { + if (string_val=="") compute_string_rep(); + return string_val; + } + // ... + private: + void compute_string_rep() const; // compute string representation and place it in string_val + mutable string string_val; + // ... + }; + +Another way of saying this is that `const`ness is not transitive. +It is possible for a `const` member function to change the value of `mutable` members and the value of objects accessed +through non-`const` pointers. +It is the job of the class to ensure such mutation is done only when it makes sense according to the semantics (invariants) +it offers to its users. + +See also [PIMPL](#???). ##### Enforcement