内联静态成员变量 [英] inline static member variable

问题描述

struct sa
{
  struct sb { int a = 123;};
  inline static sb b;
};

上面的代码生成错误：

main.cpp:25:20: error: default member initializer for ‘sa::sb::a’ required before the end of its enclosing class
   inline static sb b;
                    ^
main.cpp:24:21: note: defined here
   struct sb { int a = 123;};
                     ^~~~~~

删除内联关键字或默认成员初始化程序起作用。但是仅仅从输出中，我不理解为什么这种用法是错误的。

Removing the inline keyword or the default member initializer works. But just from the output, I don't understand why this usage is wrong.

推荐答案

我认为这段代码是正确的，应该被接受;为了避免核心问题1397 。

I think this code is correct and should be accepted; gcc and clang are erring on the side of caution in order to avoid the defect of Core Issue 1397.

该问题规定，如果NSDMI（非静态数据成员初始化器）导致程序格式错误，类的默认默认构造函数。

That issue ruled that a program is ill-formed if a NSDMI (non-static data member initializer) causes the class's defaulted default constructor to be generated.

但是您的代码却不这样做。 NSDMI只是整数文字。导致此问题的示例的代码如 int a =（（sa（），123））;

However your code doesn't do that. The NSDMI is just an integer literal. The example that prompted this issue had code like int a = ( (sa(), 123) );

我猜可能正在发生：该标准还说，在处理NSDMI时，应该将类 sa 视为完整类。因此，也许编译器会推迟NSDMI处理，直到达到 sa 的大括号为止。然后标记错误，因为内联静态sb b; 会生成 sb :: sb（）。

What I guess might be happening is: The standard also says that , when processing the NSDMI, the class sa should be treated as complete. So perhaps the compilers are deferring the NSDMI processing until after the closing brace of sa is reached; and then flagging the error because inline static sb b; would generate sb::sb().

可能该标准仍然存在缺陷，直到现在为止还没有人想到您的示例。

Possibly the standard is still defective and nobody thought of your example until now.

作为一种解决方法，您可以明确提供麻烦的构造函数：

As a workaround you can explicitly provide the troublesome constructor:

struct sb { int a = 123; sb() {} };

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