Pass a function pointer to a method of another class - c++

Question

I'm working on my first C++ project. I have 2 classes: 1 for the interaction with the sqlite db, the other one for the qt main window. In the main I create a new window. In the window constructor I would like to load the content of the db and display it in a QtWidget.
So if I understand well sqlite callback function will be called for each row that the sqlite3_exec returns. I made a select_all function in the database class which takes a callback function as an argument so I'll be able to do use the same sql function to display/use the data in different ways.
#include <cstdio>
#include <iostream>
#include <QtCore>
#include <QtGui>
#include <QtWidgets>
#include <qmainwindow.h>
#include <qstandarditemmodel.h>
#include <sqlite3.h>
#include <string>
using namespace std;
class Database {
public:
sqlite3* db;
Database() {db = create_or_open_database();}
sqlite3* create_or_open_database()
{
sqlite3 *db = NULL;
const char *query;
int ret = 0;
char *errorMsg = 0;
ret = sqlite3_open("expense.db", &db);
query = "CREATE TABLE IF NOT EXISTS EXPENSES(NAME TEXT KEY NOT NULL, AMOUNT INT NOT NULL, TAG TEXT, SUBTAG TEXT, DATE CHAR(10) NOT NULL);";
ret = sqlite3_exec(db, query, callback, 0, &errorMsg);
return db;
}
void select_all(int (*f)(void*, int, char**, char**)){
int ret = 0;
char *errorMsg = 0;
const char *query = "SELECT * FROM EXPENSES";
ret = sqlite3_exec(db, query, (*f), 0, &errorMsg);
}
};
class MainWindow
{
public:
QWidget window;
Database expenses;
QTreeView *navigateView = new QTreeView;
QTreeView *expensesList = new QTreeView;
QPushButton *newButton = new QPushButton;
QVBoxLayout *mainVLayout = new QVBoxLayout;
QHBoxLayout *listHLayout = new QHBoxLayout;
QStandardItemModel *expensesModel = new QStandardItemModel;
MainWindow()
{
QSizePolicy navigateSize(QSizePolicy::Preferred, QSizePolicy::Preferred);
QSizePolicy expenseListSize(QSizePolicy::Preferred, QSizePolicy::Preferred);
navigateSize.setHorizontalStretch(1);
navigateView->setSizePolicy(navigateSize);
expenseListSize.setHorizontalStretch(2);
expensesList->setSizePolicy(expenseListSize);
newButton->setText("New");
listHLayout->addWidget(navigateView);
listHLayout->addWidget(expensesList);
mainVLayout->addLayout(listHLayout);
mainVLayout->addWidget(newButton);
window.setLayout(mainVLayout);
// int (MainWindow::*foo)(void*, int, char**, char**) = &MainWindow::display_expenses_in_list;
// expenses.select_all(foo);
expenses.select_all(this->display_expenses_in_list);
}
int display_expenses_in_list(void *NotUsed, int argc, char **argv, char **azColName)
{
QStringList list = {"Name", "Amount (€)", "Tag", "Subtag", "Date"};
this->expensesModel->setVerticalHeaderLabels(list);
// here I'll create items and add them to the QTreeView
return 0;
}
};
int main(int argc, char* argv[])
{
QApplication app(argc, argv);
MainWindow ui;
ui.window.show();
return app.exec();
}
With this code I get reference to a non-static member function must be called [bound_member_function]
If googled it and tried, I guess, to create a function pointer foo that point to the callback function (the lines that are currently commented). I get this : Cannot initialize a parameter of type 'int (*)(void *, int, char **, char **)' with an lvalue of type 'int (MainWindow::*)(void *, int, char **, char **)' [init_conversion_failed]
If I make display_expenses_in_list static then I can't edit the expensesModel...

Answer 1

The key here is that void* argument to sqlite3_exec. You now pass 0, but you need to pass this instead.
You can now make display_expenses_in_list static. That NotUsed parameter then becomes used. You just need to cast it back to MainWindow* and use it instead of this.

Answer 2

The problem:
The problem is your display_expenses_in_list(...) function is a class member function. Therefore your need to use a pointer to a member function rather than a pointer to a function, because it must always be called on an instance of the class it's a member of - however the sqlite library will only take a void* function pointer.
Check out this article: https://isocpp.org/wiki/faq/pointers-to-members
The fix:
Modern C++ to the rescue here. Wrap up your whole class member function call up in a lambda with the class instance in scope, then pass a pointer to this new, anonymous function as the callback.
Check this stackoverflow answer out showing how to do it (copied below): https://stackoverflow.com/a/31461997/410072
An example (copied from the linked answer):
if (sqlite3_exec(this->db, this->SQL_SELECT_READINGS_QUERY,
+[](void* instance, int x, char** y, char** z) {
return static_cast<dataSend_task*>(instance)->callback(x, y, z);
},
this,
&err))
{
/* whatever */
}