使用非静态数据成员和嵌套类构造函数的类初始化时出错

问题描述：

以下代码非常简单，我预计它应该编译好。

The following code is quite trivial and I expected that it should compile fine.

struct A
{
    struct B
    {
        int i = 0;
    };

    B b;

    A(const B& _b = B())
        : b(_b)
    {}
};

我用g ++版本4.7.2，4.8.1，clang ++ 3.2和3.3测试了这个代码。除了此代码上的g ++ 4.7.2 segfaults的事实（ http://gcc.gnu.org/bugzilla） /show_bug.cgi?id=57770 ），其他测试的编译器给出的错误消息解释不多。

I've tested this code with g++ version 4.7.2, 4.8.1, clang++ 3.2 and 3.3. Apart from fact that g++ 4.7.2 segfaults on this code (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57770), the other tested compilers give error messages that don't explain much.

g ++ 4.8.1：

g++ 4.8.1:

test.cpp: In constructor ‘constexpr A::B::B()’:
test.cpp:3:12: error: constructor required before non-static data member for ‘A::B::i’ has been parsed
     struct B
            ^
test.cpp: At global scope:
test.cpp:11:23: note: synthesized method ‘constexpr A::B::B()’ first required here 
     A(const B& _b = B())
                       ^

clang ++ 3.2和3.3：

clang++ 3.2 and 3.3:

test.cpp:11:21: error: defaulted default constructor of 'B' cannot be used by non-static data member initializer which appears before end of class definition
    A(const B& _b = B())
                    ^

使这个代码可编译是可能的，它应该没有什么区别。有两个选项：

Making this code compilable is possible and seems like it should make no difference. There are two options:

struct B
{
    int i = 0;
    B(){} // using B()=default; works only for clang++
};

或

struct B
{
    int i;
    B() : i(0) {} // classic c++98 initialization
};

这个代码真的不正确或者编译器错了吗？

Is this code really incorrect or are the compilers wrong?

答

这个代码真的不正确或者编译器错了吗？

Is this code really incorrect or are the compilers wrong?

好吧，没有。该标准有一个缺陷 - 它说， A 被认为是完成，而解析初始化为 B :: i ，并且 B :: B（）（它使用 B :: i 的初始化器） A 的定义。这显然是循环的。考虑这个：

Well, neither. The standard has a defect -- it says both that A is considered complete while parsing the initializer for B::i, and that B::B() (which uses the initializer for B::i) can be used within the definition of A. That's clearly cyclic. Consider this:

struct A {
  struct B {
    int i = (A(), 0);
  };
  A() noexcept(!noexcept(B()));
};

这有一个矛盾： B :: B（）隐含 noexcept iff A（）不抛出， A 不会抛出iff B :: B（）是不是 noexcept 。这一领域还有许多其他循环和矛盾。

This has a contradiction: B::B() is implicitly noexcept iff A() does not throw, and A() does not throw iff B::B() is not noexcept. There are a number of other cycles and contradictions in this area.

这是由核心问题跟踪1360 和 1397 。请特别注意核心问题1397中的此注释：

This is tracked by core issues 1360 and 1397. Note in particular this note in core issue 1397:

也许解决这个问题的最好方法是，静态数据成员初始化器使用其类的默认构造函数。

Perhaps the best way of addressing this would be to make it ill-formed for a non-static data member initializer to use a defaulted constructor of its class.

这是我在Clang中实现的规则的一种特殊情况解决此问题。 Clang的规则是，在解析该类的非静态数据成员初始化器之前，不能使用类的默认默认构造函数。因此，Clang在这里发布诊断：

That's a special case of the rule that I implemented in Clang to resolve this issue. Clang's rule is that a defaulted default constructor for a class cannot be used before the non-static data member initializers for that class are parsed. Hence Clang issues a diagnostic here:

    A(const B& _b = B())
                    ^

...因为Clang在解析默认初始化器之前解析默认参数，并且此默认参数需要 B 的默认初始化器已被解析（为了隐式定义 B :: B（））。

... because Clang parses default arguments before it parses default initializers, and this default argument would require B's default initializers to have already been parsed (in order to implicitly define B::B()).

使用非静态数据成员和嵌套类构造函数的类初始化时出错

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