How to make QDockWidget and QOpenGLWidget combination?

[learningOpenGL]

Hello, everyone. My mother tongue is not English, but I will try my best to describe question clearly.

I want to use OpenGL render something(for example spin box or an easy triangle) in QDockWidget.
I learn that OpenGL can be used by QDockWidget, and QDockWidget can make the floating or dock subwidow in window(like visual studio or 3dmax)
After search I reference to https://learnopengl.com/Getting-started/Hello-Triangle and tutorial and Qt Assistant to write below code:

#include "mainwindow.h"
#include <QDockWidget>
#include <QApplication>
#include "core3dockwidget.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    QDockWidget view3d2("3DView2", &w,Qt::Widget);
    view3d2.show();
    w.show();
    w.resize(800,600);
    Core3DockWidget view3d;
    view3d2.setWidget(&view3d);
    view3d2.show();
    view3d2.resize(400,300);
    return a.exec();
}

class Core3DockWidget : public QOpenGLWidget , protected QOpenGLFunctions_3_3_Core{
{
    Q_OBJECT          
public:
    explicit Core3DockWidget(QWidget *parent = nullptr);
    //..................
}

#include "core3dockwidget.h"
#include <QDebug>
#include <QDockWidget>

static GLuint VBO, VAO, EBO;

Core3DockWidget::Core3DockWidget(QWidget *parent):QOpenGLWidget(parent)
{
    //view3dp = parent;
    //parent->setFeatures(QDockWidget::AllDockWidgetFeatures);
}

Core3DockWidget::~Core3DockWidget()
{
    glDeleteVertexArrays(1, &VAO);
    glDeleteBuffers(1, &VBO);
//    glDeleteBuffers(1, &EBO);
}

QDockWidget* Core3DockWidget::returnParent(){
    return view3dp;
}
void Core3DockWidget::initializeGL(){
    this->initializeOpenGLFunctions();

    bool success = shaderProgram.addShaderFromSourceFile(QOpenGLShader::Vertex, ":/shader/triangle.vert");
    if (!success) {
        qDebug() << "shaderProgram addShaderFromSourceFile failed!" << shaderProgram.log();
        return;
    }

    success = shaderProgram.addShaderFromSourceFile(QOpenGLShader::Fragment, ":/shader/triangle.frag");
    if (!success) {
        qDebug() << "shaderProgram addShaderFromSourceFile failed!" << shaderProgram.log();
        return;
    }

    success = shaderProgram.link();
    if(!success) {
        qDebug() << "shaderProgram link failed!" << shaderProgram.log();
    }

    float vertices[] = {
        0.5f,  0.5f, 0.0f,  // top right
         0.5f, -0.5f, 0.0f,  // bottom right
        -0.5f, -0.5f, 0.0f,  // bottom left
        -0.5f,  0.5f, 0.0f   // top left
    };
    unsigned int indices[] = {  // note that we start from 0!
        0, 1, 3,  // first Triangle
        1, 2, 3   // second Triangle
    };

    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glGenBuffers(1, &EBO);
    glBindVertexArray(VAO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);  

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(GLfloat), (void*)0);/
    glEnableVertexAttribArray(0);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindVertexArray(0); 
}


void Core3DockWidget::resizeGL(int w, int h){
    glViewport(0, 0, w, h);
}


void Core3DockWidget::paintGL(){
    glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT);

    shaderProgram.bind();
    glBindVertexArray(VAO); 
    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

    shaderProgram.release();
}

After I build and run, there is the window within 1/4 size window titled '3DView2' in which is right render context.
However, when I drag '3DView2' floating(there is black '3DView2') or switch to dock (there is a white rectangle without color but have glClearColor(0.2f, 0.3f, 0.3f, 1.0f)), there is:
shaderProgram link failed! "ERROR: Duplicate function definitions for \"main\"; prototype: \"main()\" found.\n"
when i resize '3DView2' dock in mainwindow, there is:

QOpenGLShader::link: ERROR: Duplicate function definitions for "main"; prototype: "main()" found.
QOpenGLShader::link: ERROR: Duplicate function definitions for "main"; prototype: "main()" found.
QOpenGLShader::link: ERROR: Duplicate function definitions for "main"; prototype: "main()" found.

I search google and still no solution.
Should I use QDockWidget and QOpenGLWidget combination to render?
if yes what I need to learn about QDockWidget and QOpenGLWidget?
if no what technology I can use....

Finally, thanks for reading, please give me some advice.
thanks again.