When attempting to prototype a class, for example, with this code:

#include <iostream>

class Foobar;

int main(){
   return 0;
}

class Foobar {
   std::string Bar = "Bar";
   public:
      void Foo(){
         std::cout << Bar;
      }
};

The code compiles fine, no errors there. However, when trying to instance the class Foobar,

/tmp/JZXb2XeBXl.cpp: In function 'int main()':
/tmp/JZXb2XeBXl.cpp:6:11: error: aggregate 'Foobar InstanceOfFoobar' has incomplete type and cannot be defined
    6 |    Foobar InstanceOfFoobar;
      |           ^~~~~~~~~~~~~~~~

Which is the same result as just removing the end definition of the class Foobar. So clearly, the end definition of Foobar is just being ignored, and the compiler is only doing anything with the declaration.

How could I prevent this, and instead make the definition of Foobar not be ignored?