C++: QGridLayout - Whitespace between widgets - c++

Question

I currently have a problem with a QGridLayout.
Each square is a widget and I have a loop like this
for(int i = 0; i < 100; i++;)
{
ui->layout->addWidget(new Square(this),rowNr,colNr);
}
The QGridLayout is part of a QFrame.
My question is: Why is there so much whitespace between each square (horizontally)
This is the code for a square
QPainter painter(this);
painter.setRenderHint(QPainter::Antialiasing);
painter.setBrush(QBrush("#c56c00"));
painter.drawRect(0, 0, 30, 30);
Where is my problem? I want to have each cell 1 by 1 without any space between them. I dont know why it is vertically correct.. Im completely new to C++ and Qt..

Answer 1

Try that:
#include "mainwindow.h"
#include <QGridLayout>
#include <QPushButton>
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
{
resize(800, 800);
auto widget = new QWidget(this);
setCentralWidget(widget);
auto gl = new QGridLayout(widget);
gl->setSpacing(0);
gl->setAlignment(Qt::AlignTop | Qt::AlignLeft);
widget->setLayout(gl);
for (int i = 0; i < 10; i++)
for (int j = 0; j < 10; j++)
gl->addWidget(new QPushButton(QString::number(i*10 + j), this), i, j);
}

Answer 2

Since you have fixed size widgets (30x30), each widget is exactly 30x30 pixels. If your layout is larger horizontally/vertically, the spacing is increased to allow even distribution.
Example:
Let's say you have a frame with a width of 100px with 3 squares aligned horizontally. 3 times 30 equals 90 so you have 10px remaining. Since layouts in general try to evenly distribute the components that they align you will get approx. 3px spacing between each square.
You either have to play with the sizing of your QFrame (my guess is that it is not fixed size and increases/decreases in size when you resize it) or avoid using fixed size widgets inside it.
In general I would recommend either sticking with fixed size for all of your components or make the children (here: squares) to properly resize.
PS: For the task at hand it's quite easy to provide a minimal working example. ;)