Why does my attempt at connecting a pushbutton to a lambda fail? - c++

Question

I have some issues while trying to use lambda expression to make connections between a pushbutton and a function I want to call when I click the button.
I am using Qt 5.6, with the compiler MinGW 4.9.2 (the default one). My code is the following :
In mainwindow.cpp :
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
initBuildings();
initPage();
for (int i(0); i<buildings.size(); ++i) {
connect(static_cast<QAbstractButton*>(widgetlist.at(i).at(2)), &QAbstractButton::clicked, [this, i]() {
buildings.at(i).buy(amountMultiplier);});
}
}
void MainWindow::initBuildings()
{
Building b1 = Building("Building 1",100,1,200);
Building b2 = Building("Building 2",1000,10,2000);
buildings.append(b1);
buildings.append(b2);
}
void MainWindow::initPage()
{
for (int i(0); i<buildings.size(); i++) {
QList<QWidget *> buttons;
QLabel *namelabel = new QLabel(buildings.at(i).getName());
QLabel *amountlabel = new QLabel;
QPushButton *buybutton = new QPushButton(this);
QPushButton *upgradebutton = new QPushButton(this);
amountlabel->setFixedSize(50,40);
buybutton->setFixedSize(100,40);
upgradebutton->setFixedSize(100,40);
buttons.append(namelabel);
buttons.append(amountlabel);
buttons.append(buybutton);
buttons.append(upgradebutton);
widgetlist.append(buttons);
}
}
In mainwindow.h :
#include <QMainWindow>
#include <QScrollArea>
#include <QList>
#include <building.h>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
private:
void initBuildings();
void initPage();
Ui::MainWindow *ui;
int amountMultiplier;
QList<Building> buildings;
QList<QList<QWidget*>> widgetlist;
};
And "Building" is a class I have created which does not inherit from another class. The function I want to use is a public funtion of this class:
void buy(int amount) const;
It doesn't compile and I get several errors :
no matching function for call to 'MainWindow::connect(QAbstractButton*, void (QAbstractButton::*)(bool), MainWindow::MainWindow(QWidget*)::<lambda()>)
invalid use of incomplete type 'struct QtPrivate::QEnableIf< false, QMetaObject::Connection>
cannot convert '<lambda closure object>MainWindow::MainWindow(QWidget*)::< lambda()>{((MainWindow*)this), i}' (type 'MainWindow::MainWindow(QWidget*)::< lambda()>') to type 'const QObject*
I tried to change the lambda capture list, or to change the way I get the value in the lists but it doesn't change anything and I don't figure what is the problem. Maybe I am wrong in the use of the lambda ?

Answer 1

Two problems:
buildings.at() returns a const Building &, and the buy method is not const. You must index the buildings using [] instead.
The type returned from widgetlist.at(i).at(2) is definitely not QPushButton* - if it was, the code would compile. Even the error message indicates what the issue is:
no matching function for call to 'MainWindow::connect(QWidget* const&, void (QAbstractButton::*)(bool), [...])
This compiles:
// https://github.com/KubaO/stackoverflown/tree/master/questions/lambda-list-37615204
#include <QtWidgets>
struct Building {
void buy() {}
};
class Class : public QObject {
QList<Building> m_buildings;
QList<QList<QWidget*>> m_widgets;
public:
Class() {
for (int i = 0; i<m_buildings.size(); ++i)
connect(static_cast<QAbstractButton*>(m_widgets.at(i).at(2)), &QAbstractButton::clicked, [=] {
m_buildings[i].buy();
});
}
};
int main() {}
If you wish an extra measure of safety in face of programming errors on your part, replace the static_cast with a qobject_cast, it'll then abort if you cast a non-button instead of doing something possibly misleading.

Answer 2

According to the documentation, your lambda should accept a bool argument.