由于存在默认成员初始值设定项，class 类型的非平凡性
Non-triviality of class type due to presence of default member initializer(s)
以下所有标准参考均指N4659: March 2017 post-Kona working draft/C++17 DIS。
正如人们所料，对非静态数据成员使用默认成员初始值设定项使得 class 非常重要：
// Well-formed according to both GCC and Clang (-std=c++17).
#include <type_traits>
struct Trivial { int a; int b; };
struct NotTrivial { int a; int b{0}; };
static_assert(std::is_trivial_v<Trivial>, "");
static_assert(!std::is_trivial_v<NotTrivial>, "");
int main() {}
(C++17) 标准的哪个段落规定 NotTrivial 是一个非平凡的 class？
以下所有标准参考均指N4659: March 2017 post-Kona working draft/C++17 DIS。
[class]/6 支配什么是 琐碎的 class [摘录]:
[...] A trivial class is a class that is trivially copyable and has one or more default constructors, all of which are either trivial or deleted and at least one of which is not deleted. [...]
让我们将 琐碎的 class 要求表示如下：
- (A) class 可简单复制，
- (B) class 的所有默认构造函数都是微不足道的或已删除，并且
- (C) class 至少有一个 non-deleted 默认构造函数。
如下所示，NonTrivial 满足要求 (A) 和 (C)，但未满足要求 (B)，因此不是微不足道的。
要求 (A)：平凡可复制[已实现]
要求 (A) 受 [class]/6:
约束
A trivially copyable class is a class:
- (6.1) where each copy constructor, move constructor, copy assignment operator, and move assignment operator ([class.copy],
[over.ass]) is either deleted or trivial,
- (6.2) that has at least one non-deleted copy constructor, move constructor, copy assignment operator, or move assignment operator,
and
- (6.3) that has a trivial, non-deleted destructor.
其中第一个 sub-requirement (6.1) 涵盖构造函数和赋值运算符的 琐碎性 由 [class.copy.ctor]/11 and [class.copy.assign]/9，分别是：
[class.copy.ctor]/11
A copy/move constructor for class X is trivial if it is not
user-provided and if:
- (11.1) class
X has no virtual functions and no virtual base classes, and
- (11.2) the constructor selected to copy/move each direct base class subobject is trivial, and
- (11.3) for each non-static data member of
X that is of class type (or array thereof), the constructor selected to copy/move that member
is trivial;
otherwise the copy/move constructor is non-trivial.
[class.copy.assign]/9
A copy/move assignment operator for class X is trivial if it is not
user-provided and if:
- (9.1) class
X has no virtual functions and no virtual base classes, and
- (9.2) the assignment operator selected to copy/move each direct base class subobject is trivial, and
- (9.3) for each non-static data member of
X that is of class type (or array thereof), the assignment operator selected to copy/move that
member is trivial;
otherwise the copy/move assignment operator is non-trivial.
NonTrivial.
全部满足
第二个 sub-requirement (6.2) 涵盖 构造函数和赋值运算符的存在 ， 受限时隐式声明的特殊函数（如本例所示）分别由[class.copy.ctor]/6 and [class.copy.assign]/2控制：
[class.copy.ctor]/6
If the class definition does not explicitly declare a copy
constructor, a non-explicit one is declared implicitly. If the class
definition declares a move constructor or move assignment operator,
the implicitly declared copy constructor is defined as deleted;
otherwise, it is defined as defaulted. [...]
[class.copy.assign]/2
If the class definition does not explicitly declare a copy assignment
operator, one is declared implicitly. If the class definition
declares a move constructor or move assignment operator, the
implicitly declared copy assignment operator is defined as deleted;
otherwise, it is defined as defaulted. [...]
因此，将为 NonTrivial 隐式声明一个复制构造函数和复制赋值运算符。从 [class.copy.ctor]/8 and [class.copy.assign]/4 4 可以看出，这同样适用于移动构造函数和移动赋值运算符。因此，NonTrivial 满足 sub-requirement (6.2).
第三个 sub-requirement (6.3) 由 [class.dtor]/6:
管辖
A destructor is trivial if it is not user-provided and if:
- (6.1) the destructor is not virtual,
- (6.2) all of the direct base classes of its class have trivial destructors, and
- (6.3) for all of the non-static data members of its class that are of class type (or array thereof), each such class has a trivial
destructor.
Otherwise, the destructor is non-trivial.
由 NonTrivial 满足，因此 (A) 对 NonTrivial.
成立
要求 (B)：简单或已删除的默认构造函数[未满足]
根据 [class.ctor]/4 [摘录]:
[...] If there is no user-declared constructor for class X, a non-explicit constructor having no parameters is implicitly declared as defaulted ([dcl.fct.def]).
将为 NonTrivial class 隐式声明默认构造函数。但是，根据 [class.ctor]/6, particularly [class.ctor]/6.2，这个隐式声明的默认构造函数 并不简单 :
A default constructor is trivial if it is not user-provided and if:
- [...]
- (6.2) no non-static data member of its class has a default member initializer ([class.mem]), and [...]
因此 NonTrivial class 不满足 (B) 的要求，因此不是微不足道的。
我们可能注意到子句 [class.ctor]/6.2 源自 non-static 数据成员初始值设定项 N2628 的原始提案，该提案建议添加
no non-static data member of its class has an assignment-initializer, and
作为 [class.ctor] 的额外琐碎要求。
需求 (C)：存在默认构造函数[已实现]
为了完整起见，我们可能会注意到 NonTrivial class 满足要求 (C)，根据 [class.ctor]/4（引用于上面的部分）。
以下所有标准参考均指N4659: March 2017 post-Kona working draft/C++17 DIS。
正如人们所料，对非静态数据成员使用默认成员初始值设定项使得 class 非常重要：
// Well-formed according to both GCC and Clang (-std=c++17).
#include <type_traits>
struct Trivial { int a; int b; };
struct NotTrivial { int a; int b{0}; };
static_assert(std::is_trivial_v<Trivial>, "");
static_assert(!std::is_trivial_v<NotTrivial>, "");
int main() {}
(C++17) 标准的哪个段落规定 NotTrivial 是一个非平凡的 class？
以下所有标准参考均指N4659: March 2017 post-Kona working draft/C++17 DIS。
[class]/6 支配什么是 琐碎的 class [摘录]:
[...] A trivial class is a class that is trivially copyable and has one or more default constructors, all of which are either trivial or deleted and at least one of which is not deleted. [...]
让我们将 琐碎的 class 要求表示如下：
- (A) class 可简单复制，
- (B) class 的所有默认构造函数都是微不足道的或已删除，并且
- (C) class 至少有一个 non-deleted 默认构造函数。
如下所示，NonTrivial 满足要求 (A) 和 (C)，但未满足要求 (B)，因此不是微不足道的。
要求 (A)：平凡可复制[已实现]
要求 (A) 受 [class]/6:
约束A trivially copyable class is a class:
- (6.1) where each copy constructor, move constructor, copy assignment operator, and move assignment operator ([class.copy], [over.ass]) is either deleted or trivial,
- (6.2) that has at least one non-deleted copy constructor, move constructor, copy assignment operator, or move assignment operator, and
- (6.3) that has a trivial, non-deleted destructor.
其中第一个 sub-requirement (6.1) 涵盖构造函数和赋值运算符的 琐碎性 由 [class.copy.ctor]/11 and [class.copy.assign]/9，分别是：
[class.copy.ctor]/11
A copy/move constructor for class
Xis trivial if it is not user-provided and if:
- (11.1) class
Xhas no virtual functions and no virtual base classes, and- (11.2) the constructor selected to copy/move each direct base class subobject is trivial, and
- (11.3) for each non-static data member of
Xthat is of class type (or array thereof), the constructor selected to copy/move that member is trivial;otherwise the copy/move constructor is non-trivial.
[class.copy.assign]/9
A copy/move assignment operator for class
Xis trivial if it is not user-provided and if:
- (9.1) class
Xhas no virtual functions and no virtual base classes, and- (9.2) the assignment operator selected to copy/move each direct base class subobject is trivial, and
- (9.3) for each non-static data member of
Xthat is of class type (or array thereof), the assignment operator selected to copy/move that member is trivial;otherwise the copy/move assignment operator is non-trivial.
NonTrivial.
第二个 sub-requirement (6.2) 涵盖 构造函数和赋值运算符的存在 ， 受限时隐式声明的特殊函数（如本例所示）分别由[class.copy.ctor]/6 and [class.copy.assign]/2控制：
[class.copy.ctor]/6
If the class definition does not explicitly declare a copy constructor, a non-explicit one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted. [...]
[class.copy.assign]/2
If the class definition does not explicitly declare a copy assignment operator, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy assignment operator is defined as deleted; otherwise, it is defined as defaulted. [...]
因此，将为 NonTrivial 隐式声明一个复制构造函数和复制赋值运算符。从 [class.copy.ctor]/8 and [class.copy.assign]/4 4 可以看出，这同样适用于移动构造函数和移动赋值运算符。因此，NonTrivial 满足 sub-requirement (6.2).
第三个 sub-requirement (6.3) 由 [class.dtor]/6:
管辖A destructor is trivial if it is not user-provided and if:
- (6.1) the destructor is not virtual,
- (6.2) all of the direct base classes of its class have trivial destructors, and
- (6.3) for all of the non-static data members of its class that are of class type (or array thereof), each such class has a trivial destructor.
Otherwise, the destructor is non-trivial.
由 NonTrivial 满足，因此 (A) 对 NonTrivial.
要求 (B)：简单或已删除的默认构造函数[未满足]
根据 [class.ctor]/4 [摘录]:
[...] If there is no user-declared constructor for class
X, a non-explicit constructor having no parameters is implicitly declared as defaulted ([dcl.fct.def]).
将为 NonTrivial class 隐式声明默认构造函数。但是，根据 [class.ctor]/6, particularly [class.ctor]/6.2，这个隐式声明的默认构造函数 并不简单 :
A default constructor is trivial if it is not user-provided and if:
- [...]
- (6.2) no non-static data member of its class has a default member initializer ([class.mem]), and [...]
因此 NonTrivial class 不满足 (B) 的要求，因此不是微不足道的。
我们可能注意到子句 [class.ctor]/6.2 源自 non-static 数据成员初始值设定项 N2628 的原始提案，该提案建议添加
no non-static data member of its class has an assignment-initializer, and
作为 [class.ctor] 的额外琐碎要求。
需求 (C)：存在默认构造函数[已实现]
为了完整起见，我们可能会注意到 NonTrivial class 满足要求 (C)，根据 [class.ctor]/4（引用于上面的部分）。