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Update C.67 and C.130 to address #1151

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Herb Sutter
2021-06-17 11:33:14 -07:00
parent 52e6263377
commit 528cb9e832
1 changed files with 10 additions and 6 deletions
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CppCoreGuidelines.md
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@@ -4665,7 +4665,7 @@ Copy and move rules:
* [C.64: A move operation should move and leave its source in a valid state](#Rc-move-semantic)
* [C.65: Make move assignment safe for self-assignment](#Rc-move-self)
* [C.66: Make move operations `noexcept`](#Rc-move-noexcept)
* [C.67: A polymorphic class should suppress copying](#Rc-copy-virtual)
* [C.67: A polymorphic class should suppress public copy/move](#Rc-copy-virtual)
Other default operations rules:
@@ -4774,12 +4774,13 @@ defined as defaulted.
};
To prevent slicing as per [C.67](#Rc-copy-virtual),
`=delete` the copy and move operations and add a `clone`:
make the copy and move operations protected or `=delete`d, and add a `clone`:
class ClonableBase {
public:
virtual unique_ptr<ClonableBase> clone() const;
virtual ~ClonableBase() = default;
CloneableBase() = default;
ClonableBase(const ClonableBase&) = delete;
ClonableBase& operator=(const ClonableBase&) = delete;
ClonableBase(ClonableBase&&) = delete;
@@ -6289,12 +6290,14 @@ This `Vector2` is not just inefficient, but since a vector copy requires allocat
(Simple) A move operation should be marked `noexcept`.
### <a name="Rc-copy-virtual"></a>C.67: A polymorphic class should suppress copying
### <a name="Rc-copy-virtual"></a>C.67: A polymorphic class should suppress public copy/move
##### Reason
A *polymorphic class* is a class that defines or inherits at least one virtual function. It is likely that it will be used as a base class for other derived classes with polymorphic behavior. If it is accidentally passed by value, with the implicitly generated copy constructor and assignment, we risk slicing: only the base portion of a derived object will be copied, and the polymorphic behavior will be corrupted.
If the class has no data, `=delete` the copy/move functions. Otherwise, make them protected.
##### Example, bad
class B { // BAD: polymorphic base class doesn't suppress copying
@@ -6321,6 +6324,7 @@ A *polymorphic class* is a class that defines or inherits at least one virtual f
class B { // GOOD: polymorphic class suppresses copying
public:
B() = default;
B(const B&) = delete;
B& operator=(const B&) = delete;
virtual char m() { return 'B'; }
@@ -6945,7 +6949,7 @@ Designing rules for classes in a hierarchy summary:
* [C.127: A class with a virtual function should have a virtual or protected destructor](#Rh-dtor)
* [C.128: Virtual functions should specify exactly one of `virtual`, `override`, or `final`](#Rh-override)
* [C.129: When designing a class hierarchy, distinguish between implementation inheritance and interface inheritance](#Rh-kind)
* [C.130: For making deep copies of polymorphic classes prefer a virtual `clone` function instead of copy construction/assignment](#Rh-copy)
* [C.130: For making deep copies of polymorphic classes prefer a virtual `clone` function instead of public copy construction/assignment](#Rh-copy)
* [C.131: Avoid trivial getters and setters](#Rh-get)
* [C.132: Don't make a function `virtual` without reason](#Rh-virtual)
* [C.133: Avoid `protected` data](#Rh-protected)
@@ -7469,7 +7473,7 @@ at the cost of the functionality being available only to users of the hierarchy.
* ???
### <a name="Rh-copy"></a>C.130: For making deep copies of polymorphic classes prefer a virtual `clone` function instead of copy construction/assignment
### <a name="Rh-copy"></a>C.130: For making deep copies of polymorphic classes prefer a virtual `clone` function instead of public copy construction/assignment
##### Reason
@@ -7480,8 +7484,8 @@ Copying a polymorphic class is discouraged due to the slicing problem, see [C.67
class B {
public:
virtual owner<B*> clone() = 0;
B() = default;
virtual ~B() = default;
B(const B&) = delete;
B& operator=(const B&) = delete;
};