moc cpp generation fails with "No relevant classes found. No output generated."

[jsulm]

@vikramg Can you show your D11 class?

[vikramg]

Not sure if it will help, but here is an obfuscated version. D11.h:

#ifndef D11_H
#define D11_H

#include "b1.h"

class B2;

class D11 : public B1
{
    Q_OBJECT  // <-- this is causing "undefined reference to vtable for D11" WHY!??
public:
    D11(const B2& s);
    ~D11() override;
    void processRawData(const QByteArray& etba) override;

protected:
    void initFSMs() override;
    void setLock(byte prefix) override;
    void dropLock() override;
    void updateFsm1(byte prefix) override;
    void updateFsm2(byte prefix) override;
    bool updateFsm3(byte dataByte) override;
    bool extractVals(byte dataByte) override;
    void signExtend(int& val) override;

signals:
    void signal1();
    void signal2();

private:
    const int CONST1    = 0b1000'0000;    
    const int CONST2    = 0b0100'0000;     
    const int MASK      = 0b0010'0000;     

    // About 15-20 int and bool private members here...

    enum class Fsm1State    { ...names here... };
    enum class Fsm2State    { ...names here... };
    enum class Fsm3State    { ...names here... };

    Fsm1State   mFsm1State;
    Fsm2State   mFsm2State;
    Fsm3State   mFsm3State;

    bool fn1(byte dataByte);
    bool fn2(byte dataByte);
    bool fn3(byte dataByte);
    bool fn4(byte dataByte);

};

#endif // D11_H

D11.cpp:

#include "d11.h"
#include "B2dir/b2.h"

D11::D11(const B2& s) : B1(s)
{
    initFSMs();
}

D11::~D11()
{
    qDebug("D11 dtor");
}

void D11::initFSMs()
{
    mFsm1State  = Fsm1State::NAME1;
    mFsm2State  = Fsm2State::NAME1;
}

void D11::processRawData(const QByteArray &etba)
{
    for (byte dataByte : etba) {
        
        . . .

    }
    emit mBase2.b2signal();

}

// Other vanilla member function definitions here...

B1.h:

#ifndef B1_H
#define B1_H

#include <QObject>
...other QIncludes here, like QByteArray etc...

class B2;

class B1 : public QObject
{
    Q_OBJECT
public:
    explicit B1(const B2& s, QObject *parent = nullptr);
    virtual ~B1();

    using byte = unsigned char;

signals:    

public slots:

public:
    virtual void processRawData(const QByteArray& etba) = 0;

protected:

    // some int/bool members here

    const B2& mBase2;

    virtual void initFSMs() = 0;
    virtual void setLock(byte dataByte) = 0;
    virtual void dropLock() = 0;
    virtual void updateFsm1(byte dataByte) = 0;
    virtual void updateFsm2(byte dataByte) = 0;
    virtual bool updateFsm3(byte dataByte) = 0;
    virtual bool extractVals(byte dataByte) = 0;
    virtual void signExtend(int& val) = 0;


};

#endif // B1_H

B1.cpp:

#include "b1.h"
#include "B2dir/b2.h"

B1::B1(const B2& s, QObject *parent) :
                QObject(parent),
                mBase2(s)
{
    qDebug("B1 abstract base class ctor");
}

B1::~B1()
{
    qDebug("B1 abstract base class dtor");
}

I'm happy to provide any specific additional information. I can comment out the Q_OBJECT in D11 and of course it builds just fine.

[SGaist]

Hi,

Did you re-run qmake after adding the Q_OBJECT macro ?

[vikramg]

@sgaist yes, see my first post. In particular, I have tried deleting the build directory, rerunning qmake and rerunning the entire build after that.

[vikramg]

Is there a verbose mode to the moc command that will elaborate on the No relevant classes found?

[vikramg]

Quite incredibly, it turned out that the moc error was because of moc parsing not being able to comprehend C++14 digit separators in the constants I had in the private section of my class! Removing the digit separators enabled the moc compilation to go through without errors! So this doesn't work:

private:
    const int CONST1    = 0b1000'0000;

This works:

private:
    const int CONST1    = 0b10000000;

I have found C++14 digit separator support to be abysmal within the Qt ecosystem, from Qt Creator to now moc. I will personally just stop using it till Qt 6 at least; hopefully it will improve by then :P

[SGaist]

Opening a feature request will be a better idea, it will make the issue known to moc's developers.

[vikramg]

@sgaist How/where can I do that?

[aha_1980]

Hi @vikramg,

At bugreports.qt.io

Please post a link to the report here too, so others can follow later. Thanks!

[Homer2000]

Remove (no clean) build directory and build project.

[vikramg]

@SGaist @aha_1980
Bug report here: https://bugreports.qt.io/browse/QTBUG-77421

I also found other bugs filed against moc that one would need to watch out for:

• moc does not support raw string literals: https://bugreports.qt.io/browse/QTBUG-62632
• moc does not support (certain?) unicode files: https://bugreports.qt.io/browse/QTBUG-26843

These will also result in the No relevant classes found error.

[SGaist]

Thanks for sharing your additional findings !