出现了这个错误，因为两个命名重复了。

#include "mainwindow.h"
#include <QApplication>
#include <QDebug>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    double aa = -19.3;
    double b = 9.7;
    double c = qAbs(aa);//绝对值
    double max = qMax(b, c);//最大值
    int bn = qRound(b);//返回一个与浮点数最接近的数值，个人认为相当于四舍五入
    int cn = qRound(c);
    qDebug() << "a = " <<aa;
    qDebug() << "b = " << b;
    qDebug() << "c = " << c;
    qDebug() << "max = " << max;
    qDebug() << "bn = " << bn;
    qDebug() << "cn = " << cn;
    qSwap(bn, cn);
    qDebug() << "bn = " << bn << "cn = " << cn;
    return a.exec();
}