Connect signal to slot from different class

[Leutzig]

Hi,

I have a signal that I want to connect to a slot found in a different class. How do I do that?
What I thought I could do:

Class A header:

#include "classB.h"
 
class ClassA
{
 
Q_Object
 
public:
ClassA();
...
...
 
signals:
void stuffChange( const QString & );
 
private:
 
ClassB classb
};

#include "classB.h"

class ClassA
{

Q_Object

public:
ClassA();
...
...

signals:
void stuffChange( const QString & );

private:

ClassB classb
};

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And in Class A constructor:

connect(this, SIGNAL( stuffChange( const QString & ) ), &classb, SLOT( setStuff( const QString & ) ) );

connect(this, SIGNAL( stuffChange( const QString & ) ), &classb, SLOT( setStuff( const QString & ) ) );

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Class B header:

class ClassB
{
 
Q_Object
 
public:
ClassB();
...
...
 
public slots:
void setStuff( const QString & );
 
private:
QString stuff;
 
};

class ClassB
{

Q_Object

public:
ClassB();
...
...

public slots:
void setStuff( const QString & );

private:
QString stuff;

};

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This gives me the error: LNK2019: unresolved external symbol "public: __ ...
What is the right way to do this?