Re: About instantiation of an abstract class

David Wilkinson <>
Sat, 18 Apr 2009 07:00:55 -0400
Jack wrote:

When I called GetMtl() in the statement above,
pkMtl became NULL.

pNode should point to an instance of some class derived from INode, that
implements the GetMtl() method (which is declared as pure virtual in

What really confused me was the following code block originated from


Wm3::Spatial* WSceneBuilder::BuildMesh (INode* pkMaxNode,
    Wm3::Spatial* pkWm3Node)
    // Convert a Max trimesh to one or more equivalent Magic trimeshes.
    // pkMaxNode:
    // Mesh node in the Max hierarchy.
    // pkWm3Node:
    // Parent node in Magic scene to which newly created.
    // Returns pointer to new child node in the Magic scene. This will
    // directly to a trimesh object, if there is only one Magic mesh, or to
    // a "link" node, whose children are the mulitple trimeshes needed to
    // represent the Max mesh.

    bool bNeedDel = false;
    TriObject* pkTriObj = GetTriObject(pkMaxNode,&bNeedDel);
    if ( !pkTriObj )
        return NULL;
    Mesh* pkMaxMesh = &pkTriObj->GetMesh();

    Mtl* pkMtl = pkMaxNode->GetMtl();
    assert( pkMtl );
    int iMtlID = m_kMtlList.GetID(pkMtl);
    assert( iMtlID >= 0 );

The line
Mtl* pkMtl = pkMaxNode->GetMrl();
returns NULL and assigned to pkMtl.

I left this code snippet intact after applying the application (plugin)
wizard which came with the package.
So it assume the original source was correct...
Finally it came up with 0x00000000 on that line. And I couldn't get out of


I do not see

Mtl* pkMtl = pkMaxNode->GetMrl();

in this code. I see

Mtl* pkMtl = pkMaxNode->GetMtl();

I assume this is a typo.

Whether or not pkMt1 is NULL has nothing to do with the fact that Mt1 is an
abstract class, nor to do with the code you showed here, but rather with what
has previously been placed in the object pointed to by pMaxNode. Clearly the
author of this method expected pkMt1 to be non-NULL. If you are seeing NULL, I
would say you are not using the GeometricalTools library (or whatever it is)

As noted by Giovanni, if Mt1 is abstract and pkMt1 is non-NULL, then it should
(must) be pointing to an instance of a concrete class derived from Mt1.

David Wilkinson
Visual C++ MVP

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