N3797 :: 9.5 / 2 [class.union]说：
If any non-static data member of a union has a non-trivial default
constructor (12.1),copy constructor (12.8),move constructor (12.8),
copy assignment operator (12.8),move assignment operator (12.8),or
destructor (12.4),the corresponding member function of the union must
be user-provided or it will be implicitly deleted (8.4.3) for the
union
我试图通过例子来理解这个说明：
#include <iostream>
#include <limits>
struct A
{
A(const A&){ std::cout << "~A()" << std::endl; } //A has no default constructor
};
union U
{
A a;
};
U u; //error: call to implicitly-deleted default constructor of 'U'
int main()
{
}
DEMO
这种行为对我来说不是很清楚. struct A没有隐式声明的默认构造函数,因为12.1 / 4：[class.ctor]说：
If there is no user-declared constructor for class X,a constructor
having no parameters is implicitly declared as defaulted (8.4).
这意味着struct A没有一个非平凡的默认构造函数(根本没有默认构造函数,特别是非平凡的).这是联合U不必具有删除的默认构造函数.怎么了？
相关措辞在C 11 [class.ctor] p5(强调我的)：
A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X,a constructor having no parameters is implicitly declared as defaulted (8.4). […] A defaulted default constructor for class X is defined as deleted if:
[…]
X is a union-like class that has a variant member with a non-trivial default constructor,
[…]
- any direct or virtual base class,or non-static data member with no brace-or-equal-initializer,has class type
M (or array thereof) and either M has no default constructor or overload resolution (13.3) as applied to M‘s default constructor results in an ambiguity or in a function that is deleted or inaccessible from the defaulted default constructor,or
[…]
您的类A没有默认构造函数,因此默认的默认构造函数(不管是隐式还是显式)对于包含类型A的非静态数据成员的类X(无论是联合还是非联合)都不使用初始化,导致默认构造函数X被删除.它必须：编译器根本没有办法生成任何其他默认构造函数.
至于您在评论中的后续问题：
如果代替A没有默认构造函数,它有一个非平凡的默认构造函数,那么在一个union和一个非union类中使用它们之间有区别,也是[class.ctor] p5的一部分：这是我之前引用的第一个重点,我没有重点介绍.
