the relationship among the QwtLinearScaleEngine,QwtLogScaleEngine,QwtTransfor m

[cp]

hi,
I have some questions about the QwtLinearScaleEngine,QwtLogScaleEngine,QwtTransfor m.I don't the relationship among those class clearly.I show some code to describe my question.
Fist code :

QwtLinearScaleEngine *logSE=new QwtLinearScaleEngine();
    logSE->setTransformation(new QwtLogTransform());
    plot->setAxisScaleEngine(QwtPlot::xTop,logSE);

QwtLinearScaleEngine *logSE=new QwtLinearScaleEngine();
    logSE->setTransformation(new QwtLogTransform());
    plot->setAxisScaleEngine(QwtPlot::xTop,logSE);

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second code:

QwtLogScaleEngine *logSE=new QwtLogScaleEngine();
    plot->setAxisScaleEngine(QwtPlot::xTop,logSE);

QwtLogScaleEngine *logSE=new QwtLogScaleEngine();
    plot->setAxisScaleEngine(QwtPlot::xTop,logSE);

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why are the results different?what is the difference between the two methods?I don't understand.
Thanks in advance!