Inherit from Base class with named constructors problems

class MeshGeneration{
  public:
        static MeshGeneration CreateUnstrMesh() {
          cout<<"Unstr call开发者_Python百科ed"<<endl;
          return MeshGeneration(0);}
        static MeshGeneration CreateStrMesh() {
          cout<<"Str called!"<<endl;
          return MeshGeneration(1);}
        virtual void CreateHybridMesh(){}
  protected:
        MeshGeneration(int mesh_type = -1){
          string mstring;
          if(mesh_type == 0)
            mstring = "unstructured";
          else if(mesh_type == 1)
            mstring = "structured";
          else;
          cout <<"mesh_type = "<<mstring<<endl;
        }
};
class DerivedMeshGeneration:public MeshGeneration{
  public:
    void CreateHybridMesh(){
      cout<<"mesh_type = hybrid"<<endl;
    }
};

int main(int argc, char * argcv[]){
  MeshGeneration m1 = MeshGeneration::CreateUnstrMesh();
  MeshGeneration m2 = MeshGeneration::CreateStrMesh();
  MeshGeneration m3 = DerivedMeshGeneration::CreateUnstrMesh();
  m3.CreateHybridMesh(); // not working as expected..
  return 0;
}

The last function is not working as expected--print out "mesh_type = hybrid". And I think something is wrong when I inherit the base class. Any suggestions are appreciated! it.

Two major problems:

In order to use a polymorphic base class like you're attempting, you must use a reference, pointer, or smart pointer. Since the objects m1, m2, and m3 are plain variables of type MeshGeneration, they will never really be a DerivedMeshGeneration, no matter what the function to the right of the = originally created.

DerivedMeshGeneration::CreateUnstrMesh() is the same function as MeshGeneration::CreateUnstrMesh(), so it never creates a derived object in the first place.

Here your code prints:

Unstr called
mesh_type = unstructured
Str called!
mesh_type = structured
Unstr called
mesh_type = unstructured

and it is what it should happen.

m1, m2 and m3 are objects of type MeshGeneration, and MeshGeneration::CreateHybridMesh does not print anything.

In order to print mesh_type = hybrid you should have an object of type DerivedMeshGeneration (or a pointer/reference to a DerivedMeshGeneration or a MeshGeneration pointing/referencing to an instance of DerivedMeshGeneration).

In this line:

MeshGeneration m3 = DerivedMeshGeneration::CreateUnstrMesh();

You are making a copy of the return value of DerivedMeshGeneration::CreateUnstrMesh(), and that copy is of type MeshGeneration. Thus the function that gets called is the one in MeshGeneration.

You should be using pointers or references instead.

The problem is

DerivedMeshGeneration::CreateUnstrMesh()

doesn't create an instance of DerivedMeshGeneration, rather it creates an instance of MeshGeneration.

Thanks guys. Now this works as I expected:

DerivedMeshGeneration * m3 = new DerivedMeshGeneration;

m3->CreateHybridMesh();