C++中的Trivial 、POD、non-POD和Standard Layout概念

POD types
non-POD types
Standard Layout types

A Formal Definition

Informally, a standard layout class is one without direct or indirect virtual member functions, reference data members or virtual base classes. Formally, a standard-layout class is a class that:

• Has no non-static data members of type non-standard-layout class (or array of such types) or reference. In simpler words, a standard layout class shall not have reference variables as data members or member objects of a non-standard layout class.
• Has no virtual functions and no virtual base classes.
• Has the same access control for all non-static data members.
• Has no non-standard-layout base classes.
• Either has no non-static data members in the most derived class and at most one base class with non-static data members, or has no base classes with non-static data members.
• Has no base classes of the same type as the first non-static data member.

standard-layout 类型的类是指：

• 没有 non-standard-layout类型（或这些类型的数组）和引用的非静态数据成员

• 没有虚函数和虚基类

• 非静态数据成员的访问控制必须是相同的
• 没有non-standard-layout的基类

• 在最底层的派生类中没有非静态数据成员，而且在最多有一个基类拥有非静态数据成员，或者没有拥有非静态数据成员

• 相同基类类型的非静态数据成员不能作为第一个成员

C++标准把trivial类型定义如下：

一个拷贝不变（trivially copyable）类型是指：

• 没有non-trivial 的复制构造函数

• 没有non-trivial的转移构造函数

• 没有non-trivial的赋值操作符

• 没有non-trivial的转移赋值操作符

• 有一个trivial的析构函数

更多请参考：

点击(此处)折叠或打开

struct N { // neither trivial nor standard-layout
   int i;
   int j;
   virtual ~N();
};
// 有一个虚拟函数就不是trivial和standard-layout
struct T { // trivial but not standard-layout
    int i;
private:
    int j;
};
// 数据成员访问类型不同就不是standard-layout，但可以是trivial
struct SL { // standard-layout but not trivial
    int i;
    int j;
    ~SL();
};
// 有非虚拟的析构或非默认的构造函数，就不是trivial，但可以是standard-layout
struct POD { // both trivial and standard-layout
    int i;
    int j;
};

這裏有個幾個例子能讓你彻底明白每個trivial類型：

点击(此处)折叠或打开

				1 // empty classes are trivial
				 2 struct Trivial1 {};
				 3
				 4 // all special members are implicit
				 5 struct Trivial2 {
				 6 int x;
				 7 };
				 8
				 9 struct Trivial3 : Trivial2 { // base class is trivial
				10     Trivial3() = default; // not a user-provided ctor
				11 int y;
				12 };
				13
				14 struct Trivial4 {
				15 public:
				16 int a;
				17 private: // no restrictions on access modifiers
				18 int b;
				19 };
				20
				21 struct Trivial5 {
				22     Trivial1 a;
				23     Trivial2 b;
				24     Trivial3 c;
				25     Trivial4 d;
				26 };
				27
				28 struct Trivial6 {
				29     Trivial2 a[23];
				30 };
				31
				32 struct Trivial7 {
				33     Trivial6 c;
				34     void f(); // it's okay to have non-virtual functions
				35 };
				36
				37 struct Trivial8 {
				38 int x;
				39      static NonTrivial1 y; // no restrictions on static members
				40 }
				41
				42 struct Trivial9 {
				43      Trivial9() = default; // not user-provided
				44 // a regular constructor is okay because we still have default ctor
				45      Trivial9(int x) : x(x) {};
				46 int x;
				47 }
				48
				49 struct NonTrivial1 : Trivial 3 {
				50     virtual f(); // virtual members make non-trivial ctors
				51 }
				52
				53 struct NonTrivial2 {
				54     NonTrivial2() : z(42) {} // user-provided ctor
				55 int z;
				56 }
				57
				58 struct NonTrivial3 {
				59     NonTrivial3(); // user-provided ctor
				60 int w;
				61 }
				62 NonTrivial3::NonTrivial3() = default; // defaulted but not on first declaration
				63 // still counts as user-provided
				64 struct NonTrivial5 {
				65     virtual ~NonTrivial5(); // virtual destructors are not trivial
				66 };

這裏有個幾個例子能讓你彻底明白每個standard-layout類型：

点击(此处)折叠或打开

				1 // empty classes have standard-layout
				 2 struct StandardLayout1 {};
				 3
				 4 struct StandardLayout2 {
				 5 int x;
				 6 };
				 7
				 8 struct StandardLayout3 {
				 9 private: // both are private, so it's ok
				10 int x;
				11 int y;
				12 };
				13
				14 struct StandardLayout4 : StandardLayout1 {
				15 int x;
				16 int y;
				17
				18     void f(); // perfectly fine to have non-virtual functions
				19 };
				20
				21 struct StandardLayout5 : StandardLayout1 {
				22 int x;
				23     StandardLayout1 y; // can have members of base type if they're not the first
				24 };
				25
				26 struct StandardLayout6 : StandardLayout1, StandardLayout5 {
				27 // can use multiple inheritance as long only
				28 // one class in the hierarchy has non-static data members
				29 };
				30
				31 struct StandardLayout7 {
				32 int x;
				33 int y;
				34     StandardLayout7(int x, int y) : x(x), y(y) {} // user-provided ctors are ok
				35 };
				36
				37 struct StandardLayout8 {
				38 public:
				39     StandardLayout8(int x) : x(x) {} // user-provided ctors are ok
				40 // ok to have non-static data members and other members with different access
				41 private:
				42 int x;
				43 };
				44
				45 struct StandardLayout9 {
				46 int x;
				47     static NonStandardLayout1 y; // no restrictions on static members
				48 };
				49
				50 struct NonStandardLayout1 {
				51     virtual f(); // cannot have virtual functions
				52 };
				53
				54 struct NonStandardLayout2 {
				55     NonStandardLayout1 X; // has non-standard-layout member
				56 };
				57
				58 struct NonStandardLayout3 : StandardLayout1 {
				59     StandardLayout1 x; // first member cannot be of the same type as base
				60 };
				61
				62 struct NonStandardLayout4 : StandardLayout3 {
				63 int z; // more than one class has non-static data members
				64 };
				65
				66 struct NonStandardLayout5 : NonStandardLayout3 {}; // has a non-standard-layout base class

結論：

在新的標准下，很多新類型成为POD類型，而且，就算一個類型不是POD類型，我們也可以分別利用POD類型的特性（只要這個類型是trivial或者standard-layout）。

標准模板塊（STL）在頭文件<type_traits>中定義了對這些類型的檢測：

template <typename T> 2 struct std::is_pod; 3 template <typename T> 4 struct std::is_trivial; 5 template <typename T> 6 struct std::is_trivially_copyable; 7 template <typename T> 8 struct std::is_standard_layout;

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