msvc内联静态成员变量重新定义

Question

I'm new to C++17. Considering the following code:

// ---------------
// in MyClass.hpp
// ---------------
#pragma once

class MyClass {
public:
    static const int A;
};

inline const int MyClass::A = 100;

// ---------------
// in test.cpp
// ---------------
#include <stdio.h>
#include "MyClass.hpp"

void test() {
    printf("test: %p\n", &MyClass::A);
}

// ---------------
// in main.cpp
// ---------------
#include <stdio.h>
#include "MyClass.hpp"

extern void test();

int main() {
    printf("main: %p\n", &MyClass::A);
    test();
}

When compiled with MinGW-W64 g++ 8.1.0

g++ -std=c++17 main.cpp test.cpp -o test.exe

The output is

main: 00000000004044B0
test: 00000000004044B0

which works as expected.

However, in MSVC 2017

cl /std:c++17 main.cpp test.cpp

I got a compiler error, saying redefinition of "public: static int const MyClass::A". (Sorry, the compiler output contains Chinese characters. It's not appropriate to post here directly.)

Why the code works under g++, but fails in MSVC? Did I do something wrong?

Answer 1

I can confirm that Clang accepts your code without any warning.

What worries me is that cppreference shows the following note:

The inline specifier cannot re-declare a function or variable (since C++17) that was already defined in the translation unit as non-inline.

I could not really identify the real cause for that note in C++ standard. But as cppreference is generally correct in its warnings, I assume that it is the reason why MSVC chokes on your code. It would probably expect:

// ---------------
// in MyClass.hpp
// ---------------
#pragma once

class MyClass {
public:
    static const int A = 100;
};

in order to avoid a former non inline declaration followed with an inline definition.