c – 当包装在模板中时,不完整类型的新的编译器

发布时间：2020-12-16 04:59:18 所属栏目：百科来源：网络整理

导读：考虑这个代码,有一个明显的编译错误：(1) struct A;struct B { B() { new A(); } // error: allocation of incomplete type 'A'}; 使用unique_ptr将无助于：(2) struct A;struct B { B() { std::make_uniqueA(); } // error: due to ~unique_ptr()}; 然后(我

考虑这个代码,有一个明显的编译错误：(1)

struct A;
struct B {
  B() { new A(); } // error: allocation of incomplete type 'A'
};

使用unique_ptr将无助于：(2)

struct A;
struct B {
  B() { std::make_unique<A>(); } // error: due to ~unique_ptr()
};

然后(我的惊喜)我发现,这将编译：(3)

struct A;
struct B {
  B() { std::make_unique<A>(); }
};
struct A {}; // OK,when a definition is added **below**

然后我检查,这是否有助于新的 – 不是：(4)

struct A;
struct B {
  B() { new A(); } // error: allocation of incomplete type 'A'
};
struct A {};

我认为它与模板有关,事实上：在模板中封装新的内容会编译：(5)

template <typename T> 
T* my_new() { return new T(); } // OK,when wrapped in template
struct A;
struct B {
  B() { my_new<A>(); }
};
struct A {};

为了完整起见,删除A的定义再次出现错误：(6)

template <typename T> 
T* my_new() { return new T(); } // error: allocation of incomplete type 'A'
struct A;
struct B {
  B() { my_new<A>(); }
}; 
// do note: definition of A removed

这里发生了什么？据我所知,编译器必须知道A的大小/定义来分配它,因此仅仅声明它是不够的.另外我相信这个定义必须在分配之前.

这似乎是正确的,当直接使用新的(1,4).但是,当新的包装,显然我错了(2,3,5,6).

到目前为止我发现的可能的解释是：

>完成类型的检查将被延迟,直到出现模板实例化.我认为这是正确的,但在我的情况下,直接使用新的A()和对my_new< A>()的调用几乎在同一个位置.所以这不可能是原因.对？
>使用不完整的类型作为模板参数可能是未定义的行为.这是真的吗即使启用所有警告,编译器也不会抱怨.比较5和6似乎表明,编译器足够聪明,可以确定下面的定义(从而实际上使得类型完整).

为什么4被认为是不正确的,而5个编译(或者5是只是虚假地编译未定义的行为[但是3必须有缺陷,对吗？

btw：用cl -3.5.0和g -4.9.2测试

解决方法

§14.6.4.1[temp.point] / p1,8,强调我的：

1 For a function template specialization,a member function template
specialization,or a specialization for a member function or static
data member of a class template,if the specialization is implicitly
instantiated because it is referenced from within another template
specialization and the context from which it is referenced depends on
a template parameter,the point of instantiation of the specialization
is the point of instantiation of the enclosing specialization.
Otherwise,the point of instantiation for such a specialization
immediately follows the namespace scope declaration or definition that
refers to the specialization.

8 A specialization for a function template,a member function template,
or of a member function or static data member of a class template may
have multiple points of instantiations within a translation unit,and
in addition to the points of instantiation described above,for any
such specialization that has a point of instantiation within the
translation unit,the end of the translation unit is also considered a
point of instantiation. A specialization for a class template has at
most one point of instantiation within a translation unit. A
specialization for any template may have points of instantiation in
multiple translation units. If two different points of instantiation
give a template specialization different meanings according to the one
definition rule (3.2),the program is ill-formed,no diagnostic
required.

在my_new< A>中有两点实例化,一个在B定义的结尾,一个在翻译单元的末尾.由于这两点会导致不同的含义(代码片段3和5),程序是不完整的NDR(即它有未定义的行为).

（编辑：李大同）

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