c++ - 什么是微不足道的功能？

Question

[basic.def.odr]/3 引用了术语“非平凡函数”，我在标准 (N4140) 中找不到它的定义。

[basic.def.odr]/3

A variable x whose name appears as a potentially-evaluated expression ex is odr-used by ex unless applying the lvalue-to-rvalue conversion (4.1) to x yields a constant expression (5.19) that does not invoke any nontrivial functions and, if x is an object, ex is an element of the set of potential results of an expression e, where either the lvalue-to-rvalue conversion (4.1) is applied to e, or e is a discarded-value expression (Clause 5).

Answer 1

“非平凡函数”是“平凡特殊成员函数”的补充。对于平凡和不平凡的默认/复制/移动构造函数、复制/移动赋值运算符或析构函数有什么定义 - 仅属于特殊成员函数的特征，并决定是否这些需要在某些情况下调用。

这些的定义可以在第 12 章中找到。

默认构造函数，§12.1/4:

A default constructor is trivial if it is not user-provided and if:

• its class has no virtual functions (10.3) and no virtual base classes (10.1), and
• no non-static data member of its class has a brace-or-equal-initializer, and
• all the direct base classes of its class have trivial default constructors, and
• for all the non-static data members of its class that are of class type (or array thereof), each such class has a trivial default constructor.

Otherwise, the default constructor is non-trivial.

复制/移动构造函数，第 12.8/12 节:

A copy/move constructor for class X is trivial if it is not user-provided, its parameter-type-list is equivalent to the parameter-type-list of an implicit declaration, and if

• class X has no virtual functions (10.3) and no virtual base classes (10.1), and
• class X has no non-static data members of volatile-qualified type, and
• the constructor selected to copy/move each direct base class subobject is trivial, and
• 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.

复制/移动赋值运算符，§12.8/26:

A copy/move assignment operator for class X is trivial if it is not user-provided, its parameter-type-list is equivalent to the parameter-type-list of an implicit declaration, and if

• class X has no virtual functions (10.3) and no virtual base classes (10.1), and
• class X has no non-static data members of volatile-qualified type, and
• the assignment operator selected to copy/move each direct base class
• 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.

析构函数，§12.4/5:

A destructor is trivial if it is not user-provided and if:

• the destructor is not virtual,
• all of the direct base classes of its class have trivial destructors, and
• 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