成员不归零，一个clang ++错误？

在原来的C ++ 11规范中， B {} 将执行值初始化，导致 ai 为零初始化。对于

...在C ++ 98中作为聚合初始化处理，但作为值初始化C ++ 11 FDIS。

但是，这种情况下的行为已更改核心问题1301 ，它通过强制执行聚合初始化来恢复C ++ 98行为，只要聚合由 braced-init-列表。由于这个问题被认为是一个DR，它被视为事实上应用于C ++标准的早期版本，因此一个合格的C ++ 11编译器应该在这里执行聚合初始化，而不是值 -

最终，依靠值初始化来初始化数据成员是一个坏主意，特别是对于具有用户提供的构造函数的类。

Consider the following code:

Compiled with -std=c++11 in clang++, p->i turns out to be zero, but p->a.i doesn't. Shouldn't the whole object be zeroed as long as its class doesn't have user-provided constructor?

EDIT: Since there are some extensive discussion in the comments, I think it's better to add some excerpt from the standard here:

To value-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type (Clause 9) with a user-provided constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
• if T is a (possibly cv-qualified) non-union class type without a user-provided constructor, then the object is zero-initialized and, if T’s implicitly-declared default constructor is non-trivial, that constructor is called.
• if T is an array type, then each element is value-initialized;
• otherwise, the object is zero-initialized.

To zero-initialize an object or reference of type T means:

• if T is a scalar type (3.9), the object is set to the value 0 (zero), taken as an integral constant expression, converted to T;
• if T is a (possibly cv-qualified) non-union class type, each non-static data member and each base-class subobject is zero-initialized and padding is initialized to zero bits;
• if T is a (possibly cv-qualified) union type, the object’s first non-static named data member is zero-initialized and padding is initialized to zero bits;
• if T is an array type, each element is zero-initialized;
• if T is a reference type, no initialization is performed.

The second bullet of each applies here.

Clang is correct, per the C++11 standard plus relevant DRs

In the original C++11 specification, B{} would perform value-initialization, resulting in a.i being zero-initialized. This was a change in behavior compared to C++98 for cases like

... which were handled as aggregate initialization in C++98 but treated as value-initialization in C++11 FDIS.

However, the behavior in this case was changed by core issue 1301, which restored the C++98 behavior by mandating that aggregate initialization is used whenever an aggregate is initialized by a braced-init-list. Since this issue is considered a DR, it is treated as de facto applying to earlier revisions of the C++ standard, so a conforming C++11 compiler would be expected to perform aggregate initialization here rather than value-initialization.

Ultimately, it's a bad idea to rely on value-initialization to initialize your data members, especially for a class that has user-provided constructors.