Qt 删除边距

I am currently using a QMainWindow widget and I would like to remove margins around the widget inside. I success to remove margins for window borders but not for the widgets inside my window.

这是我的代码，例如:

this->mainWidget = new QWidget(this);
this->mainLayout = new QHBoxLayout;
QLabel *foo = new QLabel("foo", this);
QLabel *bar = new QLabel("bar", this);

mainLayout->setContentsMargins(0, 0, 0, 0); // Remove margins for window borders

this->setWindowFlags(Qt::FramelessWindowHint);

foo->setStyleSheet("background-color: green");
bar->setStyleSheet("background-color: red");
foo->setContentsMargins(0, 0, 0, 0); // Has no effect
bar->setContentsMargins(0, 0, 0, 0); // Has no effect

this->mainLayout->addWidget(foo);
this->mainLayout->addWidget(bar);

this->mainWidget->setLayout(mainLayout);
this->setCentralWidget(mainWidget);

这是它呈现的内容:

我想去掉两个小部件之间的白色部分.

 this->mainLayout->setSpacing(0);