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

来源：互联网发布：sql日期相减编辑：程序博客网时间：2024/04/27 17:29

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：

1 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;

原文：

阅读(386) | 评论(0) | 转发(1) |

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