c++ - g++ 编译器无法编译完全定义的类型

Question

我试图在同一结构中使用该结构的静态实例，但 g++ 编译器告诉我:

C:\Temp\cs2cpp_tries\test\test.cpp: In instantiation of 'struct Static<Test<int> >':
C:\Temp\cs2cpp_tries\test\test.cpp:16:19:   required from 'struct Test<int>'
C:\Temp\cs2cpp_tries\test\test.cpp:20:12:   required from here
C:\Temp\cs2cpp_tries\test\test.cpp:6:4: error: 'Static<T>::t' has incomplete type
  T t;
    ^
C:\Temp\cs2cpp_tries\test\test.cpp:10:8: note: declaration of 'struct Test<int>'
 struct Test
        ^~~~

这是代码示例。您可以看到类型已定义，但 g++ 仍然不喜欢它。

#include <iostream>

template < typename T >
struct Static
{
    T t;
};

template < typename T >
struct Test
{
    static Static<Test<T>> t;
};

template < typename T >
Static< Test<T> > Test<T>::t;

int main (int argc, char **argv)
{
    Test<int> t;
    return 0;
}

但是如果你删除 template < typename T >从测试类和代码变得完全可编译

#include <iostream>

template < typename T >
struct Static
{
    T t;
};

struct Test
{
    static Static<Test> t;
};

Static< Test > Test::t;

int main (int argc, char **argv)
{
    Test t;
    return 0;
}

Answer 1

似乎是 gcc 错误，来自 [class.static]

The declaration of a non-inline static data member in its class definition is not a definition and may be of an incomplete type other than cv void.

所以不管Test<T>或 Static<Test<T>>具有完整的类型，应该允许声明。

来自 [temp.inst]

the class template specialization is implicitly instantiated when the specialization is referenced in a context that requires a completely-defined object type

---

The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions, [...] static data members [...]

表示我们第一次使用 Test<int>声明一个变量，Test<int>必须是完全定义的类型，所以 Test<int>被隐式实例化，但是 Static<Test<int>>不需要是完全定义的对象类型，因为它仍然只是声明的。

有趣的是，gcc 编译得很好

template<typename>
struct Static;

template<typename T>
struct Test
{
    static Static<Test<T>> t;
};

template<typename T>
Static<Test<T>> Test<T>::t;

Test<int> t;
//auto u = t.t;  // error, now requires a completely-defined type

Live

尽管Static甚至没有定义，Test::t永远不需要是完全定义的类型。