C++11奇怪的大括号初始化行为

Question

我不明白 C++11 大括号初始化规则在这里是如何工作的。拥有此代码:

struct Position_pod {
    int x,y,z;
};

class Position {
public:
    Position(int x=0, int y=0, int z=0):x(x),y(y),z(z){}
    int x,y,z;
};

struct text_descriptor {
    int             id;
    Position_pod    pos;
    const int       &constNum;
};

struct text_descriptor td[3] = {
     {0, {465,223}, 123},
     {1, {465,262}, 123},
};

int main() 
{
    return 0;
}

注意，数组被声明为有 3 个元素，但只提供了 2 个初始化器。

但是它编译没有错误，这听起来很奇怪，因为最后一个数组元素的引用成员将未初始化。确实，它有 NULL 值:

(gdb) p td[2].constNum 
$2 = (const int &) @0x0: <error reading variable>

现在是“魔法”:我将 Position_pod 更改为 Position

struct text_descriptor {
    int             id;
    Position_pod    pos;
    const int       &constNum;
};

变成这样:

struct text_descriptor {
    int             id;
    Position        pos;
    const int       &constNum;
};

现在它给出了预期的错误:

error: uninitialized const member ‘text_descriptor::constNum'

我的问题:为什么它在第一种情况下编译，什么时候应该给出错误(如在第二种情况下)。不同的是，Position_pod 使用 C 风格的大括号初始化，而 Position 使用 C++11 风格的初始化，调用 Position 的构造函数。但这会如何影响不初始化引用成员的可能性呢？

(更新)编译器:gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2

Answer 1

很明显

struct text_descriptor td[3] = {
     {0, {465,223}, 123},
     {1, {465,262}, 123},
};

是列表初始化，并且初始化列表不为空。

C++11 说 ([dcl.init.list]p3):

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.
• Otherwise, if T is an aggregate, aggregate initialization is performed (8.5.1).
• ...

[dcl.init.aggr]p1:

An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no brace-or-equal-initializers for non-static data members (9.2), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).

td是数组，所以是聚合，所以进行聚合初始化。

[dcl.init.aggr]p7:

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 (8.5.4).

这里就是这种情况，所以 td[2] 是从一个空的初始化列表初始化的，这(再次是 [dcl.init.list]p3)意味着它是值初始化的。

反过来，值初始化意味着 ([dcl.init]p7):

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), ...
• 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.
• ...

你的类 text_descriptor 是一个没有用户提供构造函数的类，所以 td[2] 首先是零初始化，然后调用它的构造函数。

零初始化意味着([dcl.init]p5):

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

• if T is a scalar type (3.9), ...
• 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, ...
• if T is an array type, ...
• if T is a reference type, no initialization is performed.

无论 text_descriptor 的默认构造函数如何，这都是明确定义的:它只是将非引用成员和子成员初始化为零。

然后调用默认构造函数，如果它是非平凡的。下面是默认构造函数的定义方式([special]p5):

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). An implicitly-declared default constructor is an inline public member of its class. A defaulted default constructor for class X is defined as deleted if:

• ...
• any non-static data member with no brace-or-equal-initializer is of reference type,
• ...

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.

因此，隐式定义的构造函数被删除，正如预期的那样，但是如果 pos 是 POD 类型(!)，它也是微不足道的。因为构造函数是微不足道的，所以它没有被调用。因为构造函数没有被调用，所以被删除是没有问题的。

这是 C++11 中的一个大漏洞，现已修复。碰巧已经修复处理inaccessible trivial default constructors ，但固定的措辞也涵盖已删除的琐碎默认构造函数。 N4140(大约 C++14)在 [dcl.init.aggr]p7(强调我的)中说:

• if T is a (possibly cv-qualified) class type without a user-provided or deleted default constructor, then the object is zero-initialized and the semantic constraints for default-initialization are checked, and if T has a non-trivial default constructor, the object is default-initialized;

作为 T.C.在评论中指出，another DR也进行了更改，以便 td[2] 仍然从一个空的初始化程序列表进行初始化，但是该空的初始化程序列表现在意味着聚合初始化。反过来，这意味着 td[2] 的每个成员也是从一个空的初始化程序列表初始化的(再次是 [dcl.init.aggr]p7)，所以似乎初始化了引用{} 的成员。

[dcl.init.aggr]p9 然后说(正如 remyabel 在现已删除的答案中指出的那样):

If an incomplete or empty initializer-list leaves a member of reference type uninitialized, the program is ill-formed.

我不清楚这是否适用于从隐式 {} 初始化的引用，但编译器确实会这样解释它，而且它没有太多其他含义。