c++ - 编译时错误: Union default constructor is deleted

Question

以下C++代码无法编译。就我研究此问​​题而言，我知道该问题是因为联合的默认构造函数已被编译器删除。在线注释中指出:

If a union contains a non-static data member with a non-trivial default constructor, the default constructor of the union is deleted by default unless a variant member of the union has a default member initializer.

struct A {
   int val;
   A() : val(0) {}
};

union B
{
   A a;
};

B b;

为什么将struct A的默认构造函数视为不平凡的？如何解决此问题以使此代码成功编译？

Answer 1

Why is the default constructor of struct A considered non-trivial?

因为它是用户声明的。

确实具有简单构造函数的类的示例:

struct Trivial {
    int val;
};

struct Trivial2 {
    int val;
    Trivial2() = default;
};

作为奖励，这是不平凡的:

struct NonTrivial {
    int val;
    NonTrivial();
};
NonTrivial::NonTrivial = default;

但是，如果希望 A::val初始化为零，则需要向联合添加默认成员初始化器:

union B {
   A a = {};
};