c++ - 不可复制类数据成员的统一初始化导致gcc错误

Question

假设我们有以下代码:

class A {
public:
    A() = default;    
    A(const A&) = delete;
    ~A() = default;
};

class B {
public:    
    B() : a{} { }    
    A a[1];
};

int main() 
{
    B b;
}

该代码在最新的GCC 9.2，Clang 9.2和MSVC 19.22上编译。

但是，当我将默认析构函数更改为~A() { }时，GCC返回错误use of deleted function 'A::A(const A&)'。 Clang和MSVC仍然可以编译。

当我编写A的副本构造函数时，GCC会编译，但是在运行时从未调用过该构造函数。 GCC需要复制构造函数做什么？
是GCC错误吗？ (我已经在GodBolt.org上尝试了所有GCC版本，出现相同的错误。)

Answer 1

这是一个GCC错误。
B的默认构造函数使用聚合初始化来初始化无初始化程序的a。 [dcl.init.aggr]/8:

If there are fewer initializer-clauses in the list than there are elements in a non-union aggregate, then each element not explicitly initialized is initialized as follows:

• If the element has a default member initializer ([class.mem]), the element is initialized from that initializer.

• Otherwise, if the element is not a reference, the element is copy-initialized from an empty initializer list ([dcl.init.list]).

• Otherwise, the program is ill-formed.

[...]

因此，a[0]是从{}复制副本初始化的，它是copy-list-initialization。这可能是GCC开始感到困惑的地方-复制初始化不一定涉及复制构造函数。

[dcl.init.list]/3.4:

List-initialization of an object or reference of type T is defined as follows:

• [...]

• Otherwise, if the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.

• [...]

因此，直接使用A的默认构造函数。没有涉及复制构造函数。也不琐碎。

如果您担心C++ 11和C++ 17之间的区别，请N3337 [dcl.init.aggr]/7:

If there are fewer initializer-clauses in the list than there are members in the aggregate, then each member not explicitly initialized shall be initialized from an empty initializer list ([dcl.init.list]). [...]

复制初始化在这里甚至都不涉及。和N3337 [dcl.init.list]/3.1:

List-initialization of an object or reference of type T is defined as follows:

• If the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.

• [...]

没变。