Qt C++ QVector QLine QPoint drawingtool - c++

Question

I have QVector m_vertices and QVector m_lines in my drawingWidget.h
I currently trying to do simple drawing tool, where i can manipulate with vertices and lines. I managed to draw multiple vertices on my "MainWindow", now i am trying to implement line between two vertices
The main idea is, that i have a pop-up menu, where i can choose a "tool". I can add vertex, remove vertex, move vertex, add line, delete line. The idea is , when i choose for example "Add Vertex" then the "m_state" will change to "Adding" so i can only add vertices. When i choose "Add Line" for example, then the "m_state" again will change, so that i can only choose two created vertices, that will result a line between them.
enum DrawingWidgetState {
NO_TOOL_SELECTED,
ADD_VERTEX_SELECTED,
MOVE_VERTEX_SELECTED,
DELETE_VERTEX_SELECTED,
ADD_LINE_SELECTED,
DELETE_LINE_SELECTED
};
Here, in the end, it turns out that if I click "Add Vertex" then when I put the second vertex, then it automatically puts a line between them
void DrawingWidget::paintEvent(QPaintEvent *event) {
QPainter painter(this);
painter.fillRect(event->rect(), Qt::blue);
painter.setBrush(Qt::black);
for(int i = 0; i < m_vertices.size() ; i++) {
painter.drawEllipse(m_vertices[i], 20, 20);
}
for(int i = 0; i < m_vertices.size()-1 ; i++)
{
const QPoint& point1 = m_vertices[i];
const QPoint& point2 = m_vertices[i+1];
painter.drawLine(point1, point2);
}
}
void DrawingWidget::mousePressEvent(QMouseEvent *event) {
if(m_state == ADD_VERTEX_SELECTED) {
if(event->button() == Qt::LeftButton) {
//m_x = event->x();
//m_y = event->y();
//update();
QPoint point = event->pos();
m_vertices.append(point);
update();
}
}
if(m_state == ADD_LINE_SELECTED) {
if(event->button() == Qt::LeftButton) {
for(int i = 0; i < m_vertices.size()-1 ; i++) {
const auto &point1 = m_vertices[i];
const auto &point2 = m_vertices[i+1];
}
}
}
}
Could someone explain to me why this happens and what should i do to improve my code ?
If something requires explanation, i will tell.