16f6c0aecf
Inline adoc files when they are included exactly once. Also fix language tags because this inlining gives us better information on what language the code is written in.
129 lines
4.0 KiB
Plaintext
129 lines
4.0 KiB
Plaintext
== Why is this an issue?
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Most classes should not directly handle resources, but instead, use members that perform resource handling for them:
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* For memory, it can be ``++std::unique_ptr++``, ``++std::shared_ptr++``, ``++std::vector++``...
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* For files, it can be ``++std::ofstream++``, ``++std::ifstream++``...
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* ...
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Classes that avoid directly handling resources don't need to define any of the special member functions required to properly handle resources: destructor, copy constructor, move constructor, copy-assignment operator, move-assignment operator. That's because the versions of those functions provided by the compiler do the right thing automatically, which is especially useful because writing these functions correctly is typically tricky and error-prone.
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Omitting all of these functions from a class is known as the Rule of Zero because no special function should be defined.
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In some cases, this rule takes a slightly different shape, while respecting the fact that no definition of those functions will be provided:
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* For the base class of a polymorphic hierarchy, the destructor should be declared as ``++public++`` and ``++virtual++``, and defaulted (``++=default++``). The copy-constructor and copy-assignment operator should be deleted. (If you want to copy classes in a polymorphic hierarchy, use the ``++clone++`` idiom.) The move operation will be automatically deleted by the compiler.
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* For other kinds of base classes, the destructor should be ``++protected++`` and non-``++virtual++``, and defaulted (``++=default++``).
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=== Noncompliant code example
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[source,cpp]
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----
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class FooPointer { // Noncompliant. The code is correct (it follows the rule of 5), but unnecessarily complex
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Foo* pFoo;
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public:
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FooPointer(int initValue) {
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pFoo = new Foo(initValue);
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}
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~FooPointer() {
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delete pFoo;
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}
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FooPointer(FooPointer const &fp) = delete;
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FooPointer const & operator=(FooPointer const &fp) = delete;
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FooPointer(FooPointer &&fp) noexcept {
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pFoo = fp.pFoo;
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fp.pFoo = nullptr;
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}
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FooPointer const & operator=(FooPointer &&fp) {
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FooPointer temp(std::move(fp));
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std::swap(temp.pFoo, pFoo);
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return *this;
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}
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};
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----
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=== Compliant solution
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[source,cpp]
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----
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class FooPointer { // Compliant, std::unique_ptr is use to handle memory management
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unique_ptr<Foo> pFoo;
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public:
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FooPointer(int initValue) : pFoo(std::make_unique<Foo>(initValue) {}
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};
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----
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A polymorphic base class can look like this:
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[source,cpp]
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----
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class Base { // Compliant, the virtual destructor is defaulted
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public:
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virtual ~Base() = default;
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Base(Base const &) = delete;
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Base &operator=(Base const &) = delete;
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};
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----
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=== Exceptions
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* Empty destructors are treated as though they were defaulted.
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* There are several cases when this rule should not be followed. For instance, if your class is manually handling a resource, logging when being constructed/copied, maintaining some kind of counter, having non-transient data that should not be copied (like ``++capacity++`` for ``++std::vector++``)... In that case, it should still follow the rule of 5 (S3624). And you should consider if you can isolate this specific behavior in a base class or a dedicated member data, which would allow you to still follow the rule of 0.
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== Resources
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* S3624
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* S1235
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ifdef::env-github,rspecator-view[]
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'''
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== Implementation Specification
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(visible only on this page)
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=== Message
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Remove this "[copy constructor|copy-assignment operator|destructor|move constructor|move-assignment operator]" so that the class follows the rule of Zero.
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=== Highlighting
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class name
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'''
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== Comments And Links
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(visible only on this page)
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=== is related to: S3624
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=== on 6 Nov 2018, 20:15:10 Ann Campbell wrote:
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Double-check my changes [~loic.joly]
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=== on 7 Nov 2018, 09:20:19 Loïc Joly wrote:
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\[~ann.campbell.2] I just replaced
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____
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though they were ``++=default++``-ed.
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____
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By
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____
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though they were defaulted.
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____
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Because a destructor can be defaulted in two ways :
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* It's not mentioned in the code
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* It's present, but with ``++=default++``
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endif::env-github,rspecator-view[]
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