clang vs gcc CRTP：constexpr变量不能有非文字类型 [英] clang vs gcc CRTP: constexpr variable cannot have non-literal type

问题描述

  template< typename S> 
 class Base 
 {
 public：
 constexpr static S NOT_SET {0}; 
}; 
 
结构派生：public Base< Derived> 
 {
}; 
  

铿锵（5.0.0）不接受：

  5：< source>：5：24：error：constexpr变量不能有非文字类型'const Derived'
 constexpr static S NOT_SET {0} ; 
 
 8：< source>：8：25：注意：在这里请求的模板类'Base< Derived>'的实例中
 struct Derived：public Base< Derived> 
 
 5：< source>：5：24：注意：不完整类型'const Derived'不是字面类型
 constexpr static S NOT_SET {0}; 
 $ 
 8：< source>：8：8：注意：'Derived'的定义直到关闭'}'
 struct Derived：public Base< Derived> 
 ^ 
 1产生的错误。 
编译器退出，结果代码为1 
  

但是gcc（在4.9.2和6.2上测试）接受它就好。

在clang中如何做这件事？

解决方案

当您尝试在类模板 Base 中使用它时，派生的不是完整的类型，所以你不能以这种方式实际使用它。这是因为一个类型必须完整才能声明该类型的变量。没有办法解决它。

综上所述，类型在结尾} 处是完整的（以及其他与您的情况无关的异常，就像在它的成员函数中一样）。
这就是标准所说的（ working draft ）：

类在关闭时被认为是完全定义的对象类型（或完整类型） }在类成员规范中，该类在函数体，默认参数，noexcept-说明符和默认成员初始化符（包括嵌套类中的这些东西）中被认为是完整的， 。
另外，它在自己的类成员规范中被认为是不完整的。

因此，clang是正确的，这个错误说明或多或少是相同的。

---

正如评论中提到的那样，解决这个问题的方法是存在的。只要派生类型是（让我说）可构造，您可以在基类中定义一个constexpr函数，它返回它的 not set 版本（不管它是什么意思）。

I have a CRTP template class here:

template <typename S>
class Base
{
    public:
    constexpr static S NOT_SET{0};
};

struct Derived : public Base<Derived>
{
};

Clang (5.0.0) does not accept this:

5 : <source>:5:24: error: constexpr variable cannot have non-literal type 'const Derived'
    constexpr static S NOT_SET{0};
                       ^
8 : <source>:8:25: note: in instantiation of template class 'Base<Derived>' requested here
struct Derived : public Base<Derived>
                        ^
5 : <source>:5:24: note: incomplete type 'const Derived' is not a literal type
    constexpr static S NOT_SET{0};
                       ^
8 : <source>:8:8: note: definition of 'Derived' is not complete until the closing '}'
struct Derived : public Base<Derived>
       ^
1 error generated.
Compiler exited with result code 1

But gcc (tested on 4.9.2 and 6.2) accepts it just fine.

How does one go about doing this in clang?

解决方案

Derived isn't a complete type when you try to use it in the class template Base, so you cannot actually use it that way. That's because a type must be complete to be able to declare a variable of that type. No way to work around it.
To sum up, a type is complete at the closing } (and other exceptions that are irrelevant for your case, like within its member functions).
This is what the standard says (working draft):

A class is considered a completely-defined object type (or complete type) at the closing } of the class-specifier.
Within the class member-specification, the class is regarded as complete within function bodies, default arguments, noexcept-specifiers, and default member initializers (including such things in nested classes).
Otherwise it is regarded as incomplete within its own class member-specification.

Therefore clang is right and the error says more or less the same.

---

As mentioned in the comments, a way to work around it exists. As long as the derived type is (let me say) constexpr constructible, you can define a constexpr function in the base class that returns you its not set version (whatever it means).

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