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OpenCascade的拓扑对象的方向，存在继承关系。通常来说，下层拓扑对象的最终方向，依赖上层对象的方向。例如：一个Edge的方向，依赖所在Face方向最终确定。

在OCCT中，当一个拓扑对象反向时，仅仅需要将方向属性反向，如下面代码所示：

inline void TopoDS_Shape::Reverse()
{
myOrient = TopAbs::Reverse(myOrient);
}

子对象的方向将根据父对象的方向属性而进行调整。

    例如：如果一个Face反向了，通常裁剪环Wire也需要反向，Wire中的Edge也需要反向，这样才能保证拓扑结构的正确性。而实际上，仅仅需要调用Face的Reverse()方法，反向Face的方向即可。

OCCT中拓扑子对象遍历，有两个工具类：TopoDS_Iterator和TopExp_Explorer，其中TopExp_Explorer是基于TopoDS_Iterator类实现的。我们看看TopoDS_Iterator类：

//! Creates an Iterator on sub-shapes.
//! Note:
//! - If cumOri is true, the function composes all
//! sub-shapes with the orientation of S.
//! - If cumLoc is true, the function multiplies all
//! sub-shapes by the location of S, i.e. it applies to
//! each sub-shape the transformation that is associated with S.

TopoDS\_Iterator(const TopoDS\_Shape& S, const Standard\_Boolean cumOri = Standard\_True, const Standard\_Boolean cumLoc = Standard\_True);