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）在頭文件中定義了對這些類型的檢測：

1 template  2 struct std::is_pod; 3 template  4 struct std::is_trivial; 5 template  6 struct std::is_trivially_copyable; 7 template  8 struct std::is_standard_layout;

原文：

阅读(386) | 评论(0) | 转发(1) |

0

上一篇：编译错误：stray ‘\357’ in program的解决方法

下一篇：求职简历撰写要点和模板分享

相关热门文章

• test123
• 编写安全代码——小心有符号数...
• 使用openssl api进行加密解密...
• 一段自己打印自己的c程序...
• sql relay的c++接口

• 我的ChinaUnix博客被锁定了，...
• 虚拟机中ubuntu无线连接问题...
• IBM DS3400 盘阵怎么查看是单...
• 启动auditd时，报错如下，怎么...
• CGSL系统中root密码正确，但无...

给主人留下些什么吧！~~

评论热议