QList的at与[]10亿次运行速度比较

发表于

看看谁的速度快？

环境

  • windows10系统
  • Qt4.8.7(gcc 4.9.2)
  • Qt5.12.3(gcc 7.3.0)
  • Qt Debug构建
  • 10亿次操作比较

对比

  • 单位ms
  • 源码最后附录
Qt版本 参考 at const at [] const []
4.8.7 14 278 279 639 629
5.12.3 14 325 322 418 411

分析

  • Qt5.12.3整体运行速度快于Qt4.8.7；
  • 无论是Qt4.8.7或是Qt5.12.3的运行速度at都优于[]；
  • 在Qt4.8.7版本下at和[]运行速度相差不大；
  • 在Qt5.12.3版本下at和[]运行速度相差不大。

附录

  • Qt4.8.7与Qt5.12.3的相同QList源码

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    template <typename T>
    inline const T &QList<T>::at(int i) const
    { Q_ASSERT_X(i >= 0 && i < p.size(), "QList<T>::at", "index out of range");
     return reinterpret_cast<Node *>(p.at(i))->t(); }
     
    template <typename T>
    inline const T &QList<T>::operator[](int i) const
    { Q_ASSERT_X(i >= 0 && i < p.size(), "QList<T>::operator[]", "index out of range");
     return reinterpret_cast<Node *>(p.at(i))->t(); }
     
    template <typename T>
    inline T &QList<T>::operator[](int i)
    { Q_ASSERT_X(i >= 0 && i < p.size(), "QList<T>::operator[]", "index out of range");
      detach(); return reinterpret_cast<Node *>(p.at(i))->t(); }

  • 测试源码

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    #include <QList>
    #include <QElapsedTimer>
    #include <QUuid>
    #include <QDebug>

    #define ForLoop for (unsigned int i = 0; i < 10*1000*1000; i++)
    static QList<int> list;
    static QElapsedTimer timer;

    qint64 runningTime()
    {
        timer.start();

        ForLoop {
            int count = 1 + 2 + 3;
        }

        return timer.elapsed();
    }

    qint64 atRunningTime()
    {
        timer.start();

        ForLoop {
            int count = list.at(0) + list.at(1) + list.at(2);
        }

        return timer.elapsed();
    }

    qint64 constAtRunningTime()
    {
        timer.start();

        ForLoop {
            const int count = list.at(0) + list.at(1) + list.at(2);
        }

        return timer.elapsed();
    }

    qint64 squareOperationRunningTime()
    {
        timer.start();

        ForLoop {
            int count = list[0] + list[1] + list[2];
        }

        return timer.elapsed();
    }

    qint64 constSquareOperationRunningTime()
    {
        timer.start();

        ForLoop {
            const int count = list[0] + list[1] + list[2];
        }

        return timer.elapsed();
    }

    int main(int argc, char *argv[])
    {
        list<<0<<1<<2;

        qDebug()<<"SourceRunningTime: "<<runningTime()<<"milliseconds.";
        qDebug()<<"atRunningTime: "<<atRunningTime()<<"milliseconds.";
        qDebug()<<"constAtRunningTime: "<<constAtRunningTime()<<"milliseconds.";
        qDebug()<<"squareOperationRunningTime: "<<squareOperationRunningTime()<<"milliseconds.";
        qDebug()<<"constSquareOperationRunningTime: "<<constSquareOperationRunningTime()<<"milliseconds.";

        return 0;
    }