Qt window closes abruptally after changing dropdown option C++

[weverson]

Hi, I am new in Gt and am using it to learn and improve my skills in c++. I am develping a GUI to show database information. The window opens fine, but as soon as I try to change 'Source' on the dropdown menu, the window abruptally closes without showing errors.

What I've tried:
Add:

blocksignals(true)
code
blocksignals(false)

On the QObject::connect (connect signals),
replaced:

QObject::connect(sourceCombo, &QComboBox::currentTextChanged, window, updateCategories); 
QObject::connect(categoryCombo, &QComboBox::currentTextChanged, window, updateSubjects);  
QObject::connect(subjectCombo, &QComboBox::currentTextChanged, window, updateDetails);

For:

QObject::connect(sourceCombo, &QComboBox::currentTextChanged, sourceCombo, updateCategories); 
QObject::connect(categoryCombo, &QComboBox::currentTextChanged, sourceCombo, updateSubjects);  
QObject::connect(subjectCombo, &QComboBox::currentTextChanged, sourceCombo, updateDetails);

dbwindow.h:

#include <QWidget>      // Qt base class for all UI objects
#include <QComboBox>    // Qt widget for drop-down lists
#include <QVBoxLayout>  // Qt layout for vertical arrangement of widgets
#include <QTextEdit>    // Qt for text edit field
#include <QStringList>  // Qt container for lists of strings
#include <QString>      // Qt string class
#include <QLabel>       // Qt widget for text labels
#include <QSet>         // Qt container for text labels
#include <vector>       // Standard C++ vector container
#include <iostream>


struct Record {
    QString source;     // Source name
    QString category;   // Category name
    QString subject;    // Subject name
    QString details;    // Details text
};

// Function to create and show the main window with dependent dropdowns
inline QWidget* createDbWindow(const std::vector<Record>& records) {


    try {

        QWidget* window = new QWidget;                  // Create the main window widget
        QVBoxLayout* layout = new QVBoxLayout(window);  // Create a vertical layout and set it for the window

        // Create dropdowns and text field
        QComboBox* sourceCombo = new QComboBox(window);         // Dropdown for source
        QComboBox* categoryCombo = new QComboBox(window);       // Dropdown for categories
        QComboBox* subjectCombo = new QComboBox(window);        // Dropdown for subjects
        QTextEdit* detailsEdit = new QTextEdit(window);         // Multi-line text field for details
        detailsEdit->setReadOnly(true);                         // Make details field read-only
        detailsEdit->setLineWrapMode(QTextEdit::WidgetWidth);   // Enable line wrapping to fit the widget width

        // Collect unique source from records
        QSet<QString> sources;               // Set to store unique brands
        for (const auto& rec : records) {
            sources.insert(rec.source);     // Insert brand into set
        }

        sourceCombo->addItems(QStringList(sources.begin(), sources.end()));     // Add sources to dropdown
        layout->addWidget(new QLabel("Source:", window));    // Add label for source
        layout->addWidget(sourceCombo);                     // Add source dropdown to layout
        layout->addWidget(new QLabel("Category:", window)); // Add label for category
        layout->addWidget(categoryCombo);                   // Add category dropdown to layout
        layout->addWidget(new QLabel("Subject:", window));  // Add label for subject
        layout->addWidget(subjectCombo);                    // Add subject dropdown to layout
        layout->addWidget(new QLabel("Details:", window));  // Add label for details
        layout->addWidget(detailsEdit);                     // Add details text field to layout


        // Helper function to update details field based on selected subject
        auto updateDetails = [&]() {
            QString selectedSource = sourceCombo->currentText();        // Get selected source
            QString selectedCategory = categoryCombo->currentText();    // Get selected category
            QString selectedSubject = subjectCombo->currentText();      // Get selected subject
            for (const auto& rec : records) {
                if (rec.source == selectedSource && rec.category == selectedCategory && rec.subject == selectedSubject) {
                    detailsEdit->setText(rec.details);              // Set details text for matching record
                    return;
                }
            }
            detailsEdit->clear();                               // Clear details if no match found
        };

        // Helper function to update subject options based on selected category
        auto updateSubjects = [&]() {
            subjectCombo->blockSignals(true);
            subjectCombo->clear();               // Clear subject dropdown
            QSet<QString> subjects;              // Set to store unique subjects
            QString selectedSource = sourceCombo->currentText();    // Get selected source
            QString selectedCategory = categoryCombo->currentText(); // Get selected category
            for (const auto& rec : records) {
                if (rec.source == selectedSource && rec.category == selectedCategory)   // If record matches brand and category
                    subjects.insert(rec.subject);                   // Insert subject into set
            }
            subjectCombo->addItems(QStringList(subjects.begin(), subjects.end()));  // Add subjects to dropdown
            subjectCombo->blockSignals(false);  // Re-enable signals
            
            updateDetails();
        };

        // Helper function to update category options based on selected source
        auto updateCategories = [&] () {
            categoryCombo->blockSignals(true);
            categoryCombo->clear();                                // Clear category dropdown
            QSet<QString> categories;                              // Set to store unique categories
            QString selectedSource = sourceCombo->currentText();   // Get selected source
            for (const auto& rec: records) {
                if (rec.source == selectedSource)                  // If record matches selected brand
                    categories.insert(rec.category);               // Insert category into set 
            }
            categoryCombo->addItems(QStringList(categories.begin(), categories.end()));  // Add categories to dropdown
            categoryCombo->blockSignals(false);  // Re-enable signals
            
            updateSubjects();
        };

        
        // Connect signals to update dropdowns and details when selection changes
        QObject::connect(sourceCombo, &QComboBox::currentTextChanged, window, updateCategories);        // Update categories when source changes
        QObject::connect(categoryCombo, &QComboBox::currentTextChanged, window, updateSubjects);        // Update subjects when category changes
        QObject::connect(subjectCombo, &QComboBox::currentTextChanged, window, updateDetails);          // Update details when subject changes

        // Initialize dropdowns with first source selected
        if (sourceCombo->count() > 0) {
            sourceCombo->setCurrentIndex(0);    // Select first source
            updateCategories();                 // Update categories for initial source
        }

        window->setLayout(layout);                  // Set the layout for the window
        window->setWindowTitle("Database Viewer");  // Set the window title
        window->resize(755, 378);                   // Set window to 20 x 10
        window->show();                             // Show the window on the screen
        return window;

    } catch (const std::exception& e) {
        std::cerr << e.what() << "\n";
    }
    
}


#endif  //!DBWINDOW_H

dbacess.h:

#ifndef DBCONNECTION_WINDOW_H
#define DBCONNECTION_WINDOW_H

#include <pqxx/pqxx>
#include <iostream>
#include <vector>
#include <string>

// Structure to store each database record
struct DataEntry {
    // Fields for 'source', 'category', 'subject' and 'details' column 
    std::string source;
    std::string category;
    std::string subject;
    std::string details;
};

// Function to open a connection to the PostgreSQL database
inline pqxx::connection openDatabase() {
    // Connection string with db parameters (host, port, dbname, user, password)
    std::string conn_str = "host=localhost port=5432 dbname=prescreen_diag_data_api user=postgres password=shakey-10";
    return pqxx::connection(conn_str);  // Return a new pqxx connection object
}

// Function to fetch data from db
inline std::vector<DataEntry> fetchData(pqxx::connection& conn) {
    std::vector<DataEntry> data;    // Vector to store all fetched records
    try {
        pqxx::work txn(conn);   // Start a transaction on the connection
        // Execute teh SQL query to select the desired columns
        pqxx::result res = txn.exec("SELECT source, category, subject, details FROM diagnostic_assistance_db;");
        
        for (const auto& row : res) {
            // Create a new DataEntry struct
            DataEntry entry;
            // Assign each column value to the struct fields
            entry.source = row["source"].c_str();
            entry.category = row["category"].c_str();
            entry.subject = row["subject"].c_str();
            entry.details = row["details"].c_str();
            data.push_back(entry);  // Add the entry to the vector
        }
    } catch (const std::exception& e) {
        std::cerr << "Error: " << e.what() << std::endl;
    }

    return data;    // Return the vector with all records
}

#endif  //!DBCONNECTION_WINDOW_H

main.cpp

#include <QApplication>         // Qt application class
#include <QStringList>          // For QStringList
#include <pqxx/pqxx>
#include "dbwindow.h"           // For createDbWindow function
#include "dbaccess.h"           // For database access functions


int main(int argc, char *argv[]) {
    try {
        QApplication app(argc, argv);   // Create the Qt application

        pqxx::connection conn = openDatabase();   // Open database connection
    
        std::vector<DataEntry> data = fetchData(conn);    // Fetch all data from the database
    
        std::vector<Record> records;      // Vector to store all records for the GUI

        for (const auto& entry : data) {
            // Convert each DataEntry from the database to a Record for GUI
            records.push_back({
                QString::fromStdString(entry.source),       // Set source field
                QString::fromStdString(entry.category),     // Set category field    
                QString::fromStdString(entry.subject),      // Set subject field
                QString::fromStdString(entry.details)       // Set details field
            });
        }

        QWidget* window = createDbWindow(records);  // Create and show the window with dependent dropdowns
    
        return app.exec();      // Start the Qt event loop
    } catch(const std::exception& e) {
        std::cerr << e.what() << "\n";
    }

    return 1;

} 

[Axel Spoerl]

Hi @weverson and welcome to the Qt forum!
Are you running this in a debugger?
Maybe add debug statements in the updateSomething() lambdas.
The debugger of Qt Creator shows you where the program aborts (if it does).
My guess is that you still end up in an endless recursion and eventually the executable runs out of memory.
But that's hard to say, because the code is rather busy.
I'd recommend to drop all the try()ing.

[weverson]

Hi Axel,
Thank you very much for your help. I forgot to mention some important details.
I am using VS code, so the debbuger is MSVC.
Also, I am executing the code on cmd after creating the build files through Cmake.

[JonB]

@weverson
So you need to run/step through your code under the MSVC debugger. People don't know where in your hundreds of lines of code the issue lies. Not sure what your "Also, I am executing the code on cmd after creating the build files through Cmake." means, but as stated you need to run this under the debugger, not free running in some console window outside of debugger. Learning to debug with whatever debugger you use is the single most useful coding skill you will ever obtain, so it's worth spending some time to get familiar with it.