C:\Users\tanji\Desktop\QCP\Demo03\mainwindow.cpp:86: error: 'startSelection' is a protected member of 'QCPSelectionRect'

这个错误是因为你尝试在 `QCPSelectionRect` 类外部调用其受保护的成员函数 `startSelection`，而受保护的成员函数只能在该类的派生类或友元类中访问。

如果你想在 `mainwindow.cpp` 中使用 `startSelection` 函数，你需要将 `QCPSelectionRect` 类的定义添加到 `mainwindow.h` 的头文件中，并在 `QCPSelectionRect` 类的定义前加上 `friend class MainWindow;`，以使 `MainWindow` 类成为 `QCPSelectionRect` 类的友元类。这样，在 `MainWindow` 类中就可以访问 `QCPSelectionRect` 的受保护成员函数了。

示例代码：

mainwindow.h：

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include "qcustomplot.h"

class QCPSelectionRect;

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
    QCustomPlot *customPlot;
    QCPSelectionRect *selectionRect;

private slots:
    void selectionRectStarted(QMouseEvent *event);
    void selectionRectChanged(const QRect &rect, QMouseEvent *event);
    void selectionRectEnded(QMouseEvent *event);
};

#endif // MAINWINDOW_H

mainwindow.cpp：

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include "qcustomplot.h"

class QCPSelectionRect;

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    // create graph and assign data to it:
    customPlot = new QCustomPlot(this);
    customPlot->addGraph();
    QVector<double> x(101), y(101); // initialize with entries 0..100
    for (int i=0; i<101; ++i)
    {
      x[i] = i/50.0 - 1; // x goes from -1 to 1
      y[i] = x[i]*x[i]; // let's plot a quadratic function
    }
    customPlot->graph(0)->setData(x, y);

    // set axes ranges, so we see all data:
    customPlot->xAxis->setRange(-1, 1);
    customPlot->yAxis->setRange(0, 1);

    // create selection rect:
    selectionRect = new QCPSelectionRect(customPlot);
    selectionRect->setPen(QPen(Qt::blue, 2, Qt::DashLine));

    // connect signals of selection rect:
    connect(selectionRect, SIGNAL(selectionStart(QMouseEvent*)), this, SLOT(selectionRectStarted(QMouseEvent*)));
    connect(selectionRect, SIGNAL(selectionChanged(QRect,QMouseEvent*)), this, SLOT(selectionRectChanged(QRect,QMouseEvent*)));
    connect(selectionRect, SIGNAL(selectionEnd(QMouseEvent*)), this, SLOT(selectionRectEnded(QMouseEvent*)));

    // set central widget:
    setCentralWidget(customPlot);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::selectionRectStarted(QMouseEvent *event)
{
    customPlot->graph(0)->clearData();
}

void MainWindow::selectionRectChanged(const QRect &rect, QMouseEvent *event)
{
    // do something with rect, e.g. translate to QCPAxisRect coordinates, then set this as clipRect for the respective axis rect:
    customPlot->xAxis->setRange(customPlot->xAxis->pixelToCoord(rect.left()), customPlot->xAxis->pixelToCoord(rect.right()));
    customPlot->yAxis->setRange(customPlot->yAxis->pixelToCoord(rect.bottom()), customPlot->yAxis->pixelToCoord(rect.top()));
    customPlot->replot();
}

void MainWindow::selectionRectEnded(QMouseEvent *event)
{
    // reset data and axes ranges:
    QVector<double> x(101), y(101);
    for (int i=0; i<101; ++i)
    {
      x[i] = i/50.0 - 1;
      y[i] = x[i]*x[i];
    }
    customPlot->graph(0)->setData(x, y);
    customPlot->xAxis->setRange(-1, 1);
    customPlot->yAxis->setRange(0, 1);
    customPlot->replot();
}

// make MainWindow a friend of QCPSelectionRect
class QCPSelectionRect
{
    friend class MainWindow;
protected:
    void startSelection(QMouseEvent *event) { QCPAbstractItem::startSelection(event); }
};