tmp/tmpi1m9rgrd/{from.md → to.md}
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| 1 |
+
## Properties of classes <a id="class.prop">[[class.prop]]</a>
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A *trivially copyable class* is a class:
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- that has at least one eligible copy constructor, move constructor,
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copy assignment operator, or move assignment operator ([[special]],
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[[class.copy.ctor]], [[class.copy.assign]]),
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- where each eligible copy constructor, move constructor, copy
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assignment operator, and move assignment operator is trivial, and
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- that has a trivial, non-deleted destructor [[class.dtor]].
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A *trivial class* is a class that is trivially copyable and has one or
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more eligible default constructors [[class.default.ctor]], all of which
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are trivial.
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[*Note 1*: In particular, a trivially copyable or trivial class does
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not have virtual functions or virtual base classes. — *end note*]
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A class `S` is a *standard-layout class* if it:
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- has no non-static data members of type non-standard-layout class (or
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array of such types) or reference,
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- has no virtual functions [[class.virtual]] and no virtual base classes
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[[class.mi]],
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- has the same access control [[class.access]] for all non-static data
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members,
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- has no non-standard-layout base classes,
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- has at most one base class subobject of any given type,
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- has all non-static data members and bit-fields in the class and its
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base classes first declared in the same class, and
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- has no element of the set M(S) of types as a base class, where for any
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type `X`, M(X) is defined as follows.[^1]
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\[*Note 2*: M(X) is the set of the types of all non-base-class
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subobjects that may be at a zero offset in `X`. — *end note*]
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- If `X` is a non-union class type with no (possibly inherited
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[[class.derived]]) non-static data members, the set M(X) is empty.
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- If `X` is a non-union class type with a non-static data member of
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type X₀ that is either of zero size or is the first non-static data
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member of `X` (where said member may be an anonymous union), the set
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M(X) consists of X₀ and the elements of M(X₀).
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- If `X` is a union type, the set M(X) is the union of all M(Uᵢ) and
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the set containing all Uᵢ, where each Uᵢ is the type of the iᵗʰ
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non-static data member of `X`.
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- If `X` is an array type with element type Xₑ, the set M(X) consists
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of Xₑ and the elements of M(Xₑ).
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- If `X` is a non-class, non-array type, the set M(X) is empty.
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[*Example 1*:
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``` cpp
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struct B { int i; }; // standard-layout class
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struct C : B { }; // standard-layout class
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struct D : C { }; // standard-layout class
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struct E : D { char : 4; }; // not a standard-layout class
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struct Q {};
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struct S : Q { };
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struct T : Q { };
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struct U : S, T { }; // not a standard-layout class
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```
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— *end example*]
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A *standard-layout struct* is a standard-layout class defined with the
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*class-key* `struct` or the *class-key* `class`. A
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*standard-layout union* is a standard-layout class defined with the
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*class-key* `union`.
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[*Note 3*: Standard-layout classes are useful for communicating with
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code written in other programming languages. Their layout is specified
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in [[class.mem]]. — *end note*]
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[*Example 2*:
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``` cpp
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struct N { // neither trivial nor standard-layout
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int i;
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int j;
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virtual ~N();
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};
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struct T { // trivial but not standard-layout
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int i;
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private:
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int j;
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};
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struct SL { // standard-layout but not trivial
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int i;
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int j;
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~SL();
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};
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struct POD { // both trivial and standard-layout
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int i;
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int j;
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};
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```
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— *end example*]
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[*Note 4*: Aggregates of class type are described in
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[[dcl.init.aggr]]. — *end note*]
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A class `S` is an *implicit-lifetime class* if it is an aggregate or has
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at least one trivial eligible constructor and a trivial, non-deleted
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destructor.
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