inline static member variable

问题

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

The above code generates an error:

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;};
                     ^~~~~~

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

回答1:

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.

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.

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) );

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().

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

As a workaround you can explicitly provide the troublesome constructor:

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

来源：https://stackoverflow.com/questions/51959135/inline-static-member-variable