Compiling a lambda using a signal thinks signal is undefined?

[fcarney]

In the constructor I define a lambda. That lambda when called needs to trigger a signal defined later in the class. Why would it think it is undefined? The compiler linker does not like this.

#include <QAbstractProxyModel>
#include <QAbstractItemModel>
#include <QModelIndex>
#include <QFileSystemModel>

#ifndef TABLEPROXYMODEL_H
#define TABLEPROXYMODEL_H

class FileSystemTableProxyModel : public QAbstractProxyModel
{

    Q_PROPERTY(QAbstractItemModel* model READ sourceModel WRITE setSourceModel NOTIFY sourceModelChanged)
    Q_PROPERTY(QModelIndex rootIndex READ rootIndex WRITE setRootIndex NOTIFY rootIndexChanged)
public:
    FileSystemTableProxyModel(){
        connect(this, &QAbstractProxyModel::sourceModelChanged, [this](){
            m_rootIndex = sourceModel()->index(0,0);
            emit rootIndexChanged(m_rootIndex);
        });
    }

    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override
    {
        return QVariant();
    }
    QModelIndex index(int row, int column, const QModelIndex &parent = QModelIndex()) const override
    {
        Q_UNUSED(parent)

        if(row < 0 || row >= rowCount())
            return QModelIndex();
        if(column < 0 || column >= columnCount())
            return QModelIndex();

        return createIndex(row, column);
    }
    QModelIndex parent(const QModelIndex &index) const override
    {
        Q_UNUSED(index)

        return QModelIndex();
    }
    int rowCount(const QModelIndex &parent = QModelIndex()) const override
    {
        Q_UNUSED(parent)

        return sourceModel()->rowCount(m_rootIndex);
    }
    int columnCount(const QModelIndex &parent = QModelIndex()) const override
    {
        Q_UNUSED(parent)

        return 4;
    }

    QModelIndex mapFromSource(const QModelIndex &sourceIndex) const override
    {
        if(sourceIndex.model() != sourceModel() || !sourceIndex.isValid())
            return QModelIndex();

        auto proxyindex = index(sourceIndex.row(), sourceIndex.column());
        if(!proxyindex.isValid())
            return QModelIndex();

        return proxyindex;
    }
    QModelIndex mapToSource(const QModelIndex &proxyIndex) const override
    {
        if(proxyIndex.model() != this || !proxyIndex.isValid())
            return QModelIndex();

        auto sourceindex = sourceModel()->index(proxyIndex.row(), proxyIndex.column());
        if(!sourceindex.isValid())
            return QModelIndex();

        return sourceindex;
    }

    QModelIndex rootIndex() const
    {
        return m_rootIndex;
    }

public slots:
    void setRootIndex(QModelIndex rootindex)
    {
        if (m_rootIndex == rootindex)
            return;

        if(sourceModel() != rootindex.model())
            return;

        m_rootIndex = rootindex;
        emit rootIndexChanged(m_rootIndex);
    }

signals:
    void rootIndexChanged(QModelIndex rootIndex);

private:

    QModelIndex m_rootIndex;
};

#endif // TABLEPROXYMODEL_H

Edit:
The error:

undefined reference to Makefile:263: recipe for target 'ProxyModelTesting' failed
`FileSystemTableProxyModel::rootIndexChanged(QModelIndex)'
collect2: error: ld returned 1 exit status

[Chris Kawa]

@fcarney You're missing Q_OBJECT macro so moc did not generate signal's body.

Unrelated - You should use const QModelIndex& parameters and make your signals const.