c++ IntelliSense：表达式必须是可修改的左值

Question

我收到这两个错误，

1>c:\users\owner\documents\visual studio 2010\projects\monopoly\monopoly\xfileentity.cpp(376): error C3490: 'pDrawMesh' cannot be modified because it is being accessed through a const object
IntelliSense: expression must be a modifiable lvalue

我在类中声明了 pDrawMesh，而不是在一个函数中使用它。
这是我的课程

class CXFileEntity
{
        ......
 LPD3DXMESH pDrawMesh;
        .....
};

，这是我使用变量的地方

void CXFileEntity::DrawMeshContainer(LPD3DXMESHCONTAINER meshContainerBase, LPD3DXFRAME frameBase) const
{
 // Cast to our extended frame type
 D3DXFRAME_EXTENDED *frame = (D3DXFRAME_EXTENDED*)frameBase;  

 // Cast to our extended mesh container
 D3DXMESHCONTAINER_EXTENDED *meshContainer = (D3DXMESHCONTAINER_EXTENDED*)meshContainerBase;

 // Set the world transform But only if it is not a skinned mesh. 
 // The skinned mesh has the transform built in (the vertices are already transformed into world space) so we set identity
 // Added 24/08/10
 if (meshContainer->pSkinInfo)
 {
  D3DXMATRIX mat;
  D3DXMatrixIdentity(&mat);
  m_d3dDevice->SetTransform(D3DTS_WORLD, &mat);
 }
 else
  m_d3dDevice->SetTransform(D3DTS_WORLD, &frame->exCombinedTransformationMatrix);


 // Loop through all the materials in the mesh rendering each subset
 for (unsigned int iMaterial = 0; iMaterial < meshContainer->NumMaterials; iMaterial++)
 {
  // use the material in our extended data rather than the one in meshContainer->pMaterials[iMaterial].MatD3D
  m_d3dDevice->SetMaterial( &meshContainer->exMaterials[iMaterial] );
  m_d3dDevice->SetTexture( 0, meshContainer->exTextures[iMaterial] );

  // Select the mesh to draw, if there is skin then use the skinned mesh else the normal one
  pDrawMesh = (meshContainer->pSkinInfo) ? meshContainer->exSkinMesh: meshContainer->MeshData.pMesh;

  // Finally Call the mesh draw function
  pDrawMesh->DrawSubset(iMaterial);
 }
}

Answer 1

您的成员函数是 const 限定的。您不能在 const 限定成员函数中修改任何成员变量，除非它们被声明为可变的。

您需要使 pDrawMesh 可变，从 DrawMeshContainer 中删除 const 限定，或者找到其他方法来完成您想要完成的任务。

Answer 2

pDrawMesh 实际上是this->pDrawMesh。但由于当前方法是一个 const 方法，因此 this 是一个 const CXFileEntity*。所以你不能设置成员pDrawMesh。

如果 DrawMeshContainer 确实应该更改 CXFileEntity，请从方法类型中删除该 const。如果 DrawMeshContainer “有效地”保持 CXFileEntity 常量，并且 pDrawMesh 成员“并不真正计入”您的 const< 的含义/code> 对于对象，您可以将成员更改为可变。