【求助】类的state成员未定义！

#include <iostream>

using   namespace::std;

class foo {
    public:
        static void print() { cout << sum << endl; }
    private:
        static double sum;
};

int main()
{
    foo x;
    
    x.print();
    
    return 0;
}

[解决办法]

#include <iostream>

class foo {
 public:
  static void print() { std::cout << sum << std::endl; }
 private:
  static double sum;
};

double foo::sum;

int main(int argc, char* argv[]) {
  foo x;

  x.print();

  return 0;
}

[解决办法]
double foo::sum;
类静态成员需要在类外进行定义，类里面只是声明