什么是平凡的功能？

Question

[basic.def.odr]/3 引用了术语 "nontrivial function"，我在标准 (N4140) 中找不到其定义。

[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

"non-trivial function"是"trivial special member function"的补码。对于什么是平凡的和非平凡的 default/copy/move 构造函数，copy/move 赋值运算符或析构函数有定义 - 仅适用于特殊成员函数的特征，并决定是否例如。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.

Copy/move 构造函数, §12.8/12:

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.

Copy/move 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

Answer 2

可能是这个小例子将帮助您理解 [basic.def.odr]/3

上下文中的非平凡函数

struct C { 
        int l; 
        constexpr C(int _l) : l(_l) { } 
        constexpr C(const C&c) : q(c.l* 2) { } 
      }; 

      int main(void) { 
        constexpr C c(42); 
        constexpr int m= c.l; 
        struct K{ 
         int foo() { return c.l; } 
        } l; 
        return l.foo(); 
      }

如果您查看标准中的以下行

将左值到右值转换 (4.1) 应用于 x 会产生一个不调用任何非平凡函数的常量表达式 (5.19)

这里c满足出现在常量表达式中的要求，但是将左值到右值转换应用于 a 会调用 a 非平凡的功能。

为什么它会调用一个非平凡的函数？

当在未计算的操作数或其子表达式中发生左值到右值的转换时，不会访问引用对象中包含的值。否则，如果 glvalue 具有 class 类型，则转换从 glvalue 复制初始化类型 T 的临时值，并且转换的结果是临时值

的纯右值

因此使用 class C 的复制构造函数创建了一个纯右值，并且由于复制构造函数是用户声明的，它是非平凡的，因此 c 在此处未使用 ODR

A copy/move 赋值运算符对于 class X 是微不足道的，如果它不是用户提供的，它的参数类型列表等同于参数类型列表隐式声明

我希望这个例子能澄清你的疑问

什么是平凡的功能？

What is a trivial function?

c++

language-lawyer

c++14