Macro expansion in moc

Question

I'd like to store some class info using Q_CLASSINFO macro. However I would like to wrap it in my own macro, for example:

#define DB_TABLE( TABLE ) \
    Q_CLASSINFO( "db_table", #TABLE )

#define DB_FIELD( PROPERTY, COLUMN ) \
    Q_CLASSINFO( "dbcol_" #PROPERTY, #COLUMN )

class Foo : public QObject
{
    Q_OBJECT
    DB_TABLE( some_table )
    DB_FIELD( clientName, client_name )
}

Unfortunately, moc doesn't expand macros so the Q_CLASSINFO is not added.

I've tried to feed moc with already preprocessed source, but it failes on some included Qt classes.

Do you know any workaround for this?

Answer 1

Other than rolling your own pre-moc preprocessor, no. That is what MeeGo Touch does, for example. Since Nokia themselves are doing it, I believe there is no other way.

In your case, it would only involve translating your own declarations into Q_CLASSINFO, so it shouldn't be too hard. If you use qmake, it can be added to the build sequence, too.

Answer 2

The easy way to make that is modifing moc preprocessor.

1. Go to Qt source code to qtbase/src/tools/moc e.g. (C:\Qt\Qt5.0.1\5.0.1\Src\qtbase\src\tools\moc)
2. Create a new copy of moc project e.g. moc_modified
3. Open the copy of moc project with QtCreator (moc.pro file)
4. Open preprocessor.cpp file and go to Symbols Preprocessor::preprocessed(const QByteArray &filename, QIODevice *file) function

5. Search the line:

   // phase 1: get rid of backslash-newlines
   input = cleaned(input);
   
   // <- insert your code to modify input variable
   // input is a QByteArray object that contents the source code of .h file than moc is processing
   // I had created the replaceCustomMacros function, see next line
   replaceCustomMacros(input);
   ...
   

6. Compile the new source code. The moc executable file is generated to /bin folder (if you use windows look at c:/bin/moc.exe)

7. Go to Qt bin (C:\Qt\Qt5.0.1\5.0.1\msvc2010\bin) folder and rename moc executable file e.g. moc.exe.bak

8. Copy new moc executable file to Qt bin folder.

9. In your current app you need to create a Macro for example:

   #ifndef Q_MOC_RUN
   #define DB_FIELD( PROPERTY, COLUMN )
   #endif
   
   //or in my case
   
   #ifndef Q_MOC_RUN
   #define Q_SERVICE_INFO(method, path, type)
   #endif
   

Finally I let you my own source code of function replaceCustomMacros:

This function convert Q_SERVICE_INFO(method, path, type) macro to Q_CLASSINFO("srv://method", "type:path")

void Preprocessor::replaceCustomMacros(QByteArray &source)
{
    QString str(QLatin1String(source.data()));
    QString param_exp(QLatin1String("([^,\n]+)"));
    QByteArray expression("Q_SERVICE_INFO\\s*\\(");
    expression
        .append(param_exp.toLatin1())
        .append(",")
        .append(param_exp.toLatin1())
        .append("(,")
        .append(param_exp.toLatin1())
        .append(")?\\)");
    QRegExp *reg_ex = new QRegExp(QLatin1String(expression));
    int pos = -1, offset = -1, len = str.length();
    while ((offset = reg_ex->lastIndexIn(str, pos)) != -1)
    {
            reg_ex->cap(1);
            pos = -(len - offset) - 1;

            QString capturedString = reg_ex->capturedTexts().at(0);

            QString pattern = capturedString;
            pattern.remove(0, pattern.indexOf(QLatin1String("(")) + 1);
            pattern.remove(pattern.length() - 1, 1);
            QStringList params = pattern.split(QLatin1String(","));

            QString method = params.at(0).trimmed();
            method = method.mid(1, method.length() - 2);

            QString type;
            if (params.length() < 3)
            {
                type.append(QLatin1String("GET"));
            }
            else
            {
                type = params.at(2).trimmed();
                type = type.mid(1, type.length() - 2);
            }

            QString path = params.at(1).trimmed();
            path = path.mid(1, path.length() - 2);

            source.replace(offset, capturedString.length(), QString(QLatin1String("Q_CLASSINFO(\"srv://%1\",\"%2:%3\")")).arg(method, type, path).toLatin1());
    }
    delete reg_ex;

}

I have not found any specific solution on Internet then I have posted this solution.

Good Luck :)