一般情况下，QString toDouble是满足使用需求的。

不过也存在QString toDouble后，因为精度问题，会造成加减乘除、比较时，有时候因精度丢失，导致不准确。

现在把需要处理的double 放大10的N次方后，变成整数后，再进行相加相减处理，然后再缩小10的N次方倍。

代码如下：

//获取小数位
int getDecimalNum(QString str)
{
    int n =  str.indexOf(".");
    if(n < 0)
    {
        n=0;
    }
    else
    {
        n = str.length()-n-1;
    }
    return  n;
}
//获取放大后的字符串
QString getZoomIn(QString str,int n)
{
    QString zoomStr;
    int index =  str.indexOf(".");
    if(index < 0)
    {
        index=0;
        zoomStr = str;
    }
    else
    {
        zoomStr+= str.left(index);
        index = str.length()-index-1;
        zoomStr += str.right(index);
    }
    for(int i=0; i < (n - index); i++)
        zoomStr+="0";
    return  zoomStr;
};
//获取缩小后的字符串
QString getZoomOut(QString str,int n)
{
    if(str.isEmpty())
        return "";
    QString zoomStr = str;
    if(str.left(1) == "-")
    {
        zoomStr.remove(0,1);
    }
    if(n >0)
    {
        if(zoomStr.length() <= n)
        {
            int len = (n +1 - zoomStr.length() );
            for(int i =0 ; i < len;i++)
                zoomStr = "0"+zoomStr;
        }
        if(zoomStr.length() > n)
        {
            int index = zoomStr.length()-n;
            zoomStr.insert(index,".");
            for(int i =0 ;i < zoomStr.length(); i++)
            {
                index = zoomStr.length()-1- i;
                if(index < 0)
                    break;
                if(zoomStr.mid(index,1) == "0")
                {
                    zoomStr.remove(index,1);
                    i--;
                }
                else if(zoomStr.mid(index,1) == ".")
                {
                    zoomStr.remove(index,1);
                    break;
                }
                else
                {
                    break;
                }
            }
        }
    }
    if(str.left(1) == "-")
    {
        zoomStr.insert(0,"-");
    }
    return  zoomStr;
};
//两个double字符串相加
QString addDoubleStr(QString t1,QString t2)
{
    int n = qMax(getDecimalNum(t1),getDecimalNum(t2));
    QString zoomStr = QString::number((getZoomIn(t1,n).toInt()+ getZoomIn(t2,n).toInt()));//放大到变成整数后相加
    zoomStr = getZoomOut(zoomStr,n);//再缩小回去
    return  zoomStr ;
}
//两个double字符串相减
QString subtractDoubleStr(QString t1,QString t2)
{
    int n = qMax(getDecimalNum(t1),getDecimalNum(t2));
    int d1 = getZoomIn(t1,n).toInt() ;//放大到变成整数后
    int d2 = getZoomIn(t2,n).toInt() ;
    QString zoomStr;
    if(d1 == d2)
        return  "0";
    else        
    {
        zoomStr = QString::number((d1-d2));
    }
    zoomStr = getZoomOut(zoomStr,n);//再缩小回去
    return  zoomStr;
}

例子：

//转换
void MainWindow::on_pushButton_clicked()
{
    ui->textEdit->clear();
    QString t1 = ui->lineEdit->text();
    QString t2 =ui->lineEdit_2->text();
    ui->lineEdit_3->setText(subtractDoubleStr(t1,t2));
    ui->lineEdit_4->setText(addDoubleStr(t1,t2));
    int n1 =  getDecimalNum(t1);
    int n2 =  getDecimalNum(t2);
    int n = qMax(n1,n2);
    int zoom = qPow(10,n);
    ui->textEdit->append(QString("t1=%1,小数位=%2").arg(t1).arg(n1));
    ui->textEdit->append(QString("t2=%1,小数位=%2").arg(t2).arg(n2));
    ui->textEdit->append(QString("最大小数位=%1,倍数=%2").arg(n).arg(zoom));
    int d1 = getZoomIn(t1,n).toInt();
    int d2 = getZoomIn(t2,n).toInt();
    ui->textEdit->append(QString("放大后 t1=%1,t2=%2").arg(d1).arg(d2));

    QString t3 =ui->lineEdit_5->text();
    QString t4 = subtractDoubleStr(addDoubleStr(t1,t2),t3);
    double d4 = t4.toDouble();
    ui->textEdit->append(QString("(t1+t2) -t3 = %1").arg(d4));
    ui->textEdit->append(QString("%1 + %2 %3 %4").arg(t1).arg(t2).arg((d4==0?"=":(d4>0?">":"<"))).arg(t3));
    qDebug() << t1 << t2 << t3 << t4 ;

}