QList的at与[]10亿次运行速度比较

版权声明： https://blog.csdn.net/nicai_xiaoqinxi/article/details/91050289

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

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

#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;
}