类的inline static成员在g++编译正常但clang报错，如何修正？

我有一个简单的C++项目，文件如下：

foo.h #

class foo{  
    inline static const int array[3] = { 1, 2, 3 };  
};

foo.cc #

#include "foo.h"

foo a_foo;

main.cc #

#include "foo.h"
int main()
{
    foo b_foo;
}

使用g++编译：

g++ -c *.cc
g++ *.o -o a.out

仅出现警告：

In file included from foo.cc:2:0:
foo.h:2:25: warning: inline variables are only available with -std=c1z or -std=gnu1z
inline static const int array[3] = { 1, 2, 3 };
^~~~~
In file included from main.cc:3:0:
foo.h:2:25: warning: inline variables are only available with -std=c1z or -std=gnu1z
inline static const int array[3] = { 1, 2, 3 };
^~~~~

但使用clang编译，即使加上-std=c++1z参数：

clang -std=c++1z -c *.cc
clang -std=c++1z *.o -o a.out

会报错：

In file included from foo.cc:2:
./foo.h:2:1: error: 'inline' can only appear on functions
inline static const int array[3] = { 1, 2, 3 };
^
./foo.h:2:25: error: in-class initializer for static data member of type 'const int [3]' requires 'constexpr' specifier
inline static const int array[3] = { 1, 2, 3 };
^ ~~~~~~~~~~~
constexpr
2 errors generated.
In file included from main.cc:3:
./foo.h:2:1: error: 'inline' can only appear on functions
inline static const int array[3] = { 1, 2, 3 };
^
./foo.h:2:25: error: in-class initializer for static data member of type 'const int [3]' requires 'constexpr' specifier
inline static const int array[3] = { 1, 2, 3 };
^ ~~~~~~~~~~~
constexpr
2 errors generated.

请问如何修正代码使其能在clang中正常编译？

根据clang的报错提示，针对C++17标准（对应编译参数-std=c++1z），只需做一处关键修改：给数组类型的静态成员变量加上constexpr修饰符，结合inline即可满足编译器要求。

修正后的foo.h代码如下：

class foo{  
    inline static constexpr int array[3] = { 1, 2, 3 };  
};

如果需要兼容更早的C标准（比如C11/C++14），可以采用分离声明和定义的方式：

兼容C11/C14的修改方案 #

foo.h #

class foo{  
    static const int array[3];  
};

foo.cc #

#include "foo.h"

const int foo::array[3] = {1, 2, 3};
foo a_foo;

两种修改方案都能让g和clang正常编译，第一种更适合C17及以上标准的场景，第二种则兼顾旧版本兼容性。

内容的提问来源于stack exchange，提问作者inprocess