static class member not recognised but only for new versions of C compile-CodePudding

The static class member static_member not recognised in the following code.

However, it works on older versions of the compiler. The compiler I use is based on clang.

class my_class {
  public:
    static int static_member;
};

int main() {
  my_class::static_member = 0;
}

To reproduce the error, save above file as c1.cpp and run:

VER_TAG=latest # or VER_TAG=3.1.8

docker run --rm -v $(pwd):/src emscripten/emsdk::$VER_TAG emcc /src/c1.cpp

Leads to error:

wasm-ld: error: /tmp/emscripten_temp_o3wmmq8k/c1_0.o: undefined symbol: my_class::static_member

However, if I use VER_TAG=2.0.22 (earlier release of the compiler), it works fine.

Is there anything wrong with my code? Or is it related to compiler implementation?

CodePudding user response：

From static data member's definition's documentation:

The declaration inside the class body is not a definition and may declare the member to be of incomplete type (other than void), including the type in which the member is declared.

So we have to first provide an out-of class definition for the static data member as shown below:

class my_class {
  public:
    static int static_member;//this is a declaration
};
int my_class::static_member = 0;//this is a definition
int main() {
 std::cout<<my_class::static_member<<std::endl;
}

From C 17 onwards we can use inline keyword so that we'll not need an out-of-class definition for the static data member anymore:

class my_class {
  public:
    inline static int static_member = 0;  // this is a definition
};

//nothing needed here

int main() {
  std::cout<<my_class::static_member; // use the static_member here
}

CodePudding user response：

Is there anything wrong with my code?

Yes, your static_member doesn't have a definition.

class my_class {
  public:
    static int static_member;  // declaration
};

int my_class::static_member{}; // missing definition

int main() {
  my_class::static_member = 0; // now you can use it here
}