Question

I have Qt libraries 4.8.1 for Windows (VS 2010 ultimate) with Qt Visual Studio Add-in. This is my very simple app :

#include<qobject.h>
#include<qstring.h>
#include<memory>

class MyClass : public QObject{
Q_OBJECT

public:
MyClass( const QString &text, QObject *parent = 0 ) : m_text(text) {}

public slots:
void setText( const QString &text );

signals:
void textChanged( const QString& );

private:
QString m_text;
};

void MyClass::setText( const QString &text ){
if( m_text == text ) return;
m_text = text;
emit textChanged( m_text );
}

int main(int argc, char *argv[]){
    std::shared_ptr<MyClass> a(new MyClass("foo"));
    std::shared_ptr<MyClass> b(new MyClass("bar"));

QObject::connect( a.get(), SIGNAL(textChanged(const QString&)),
b.get(), SLOT(setText(const QString&)) );
a->setText("changed");
}

Errors I get related to unresolved externals : Error 6 error LNK2001:
unresolved external symbol “public: virtual int __thiscall
MyClass::qt_metacall(enum QMetaObject::Call,int,void * *)”
(?qt_metacall@MyClass@@UAEHW4Call@QMetaObject@@HPAPAX@Z)

Error 4 error LNK2001: unresolved external symbol “public: virtual
struct QMetaObject const * __thiscall MyClass::metaObject(void)const ”
(?metaObject@MyClass@@UBEPBUQMetaObject@@XZ)

Error 5 error LNK2001: unresolved external symbol “public: virtual
void * __thiscall MyClass::qt_metacast(char const *)”
(?qt_metacast@MyClass@@UAEPAXPBD@Z)

Error 3 error LNK2019: unresolved external symbol “protected: void
__thiscall MyClass::textChanged(class QString const &)” (?textChanged@MyClass@@IAEXABVQString@@@Z) referenced in function
“public: void __thiscall MyClass::setText(class QString const &)”
(?setText@MyClass@@QAEXABVQString@@@Z)

and two warnings :

Warning 1 warning MSB8017: A circular dependency has been detected
while executing custom build commands for item
“GeneratedFiles\Debug\main.moc”. This may cause incremental build to
work incorrectly.

Warning 2 warning : No resources in
‘C:\Users\Anonymous\documents\visual studio
2010\Projects\qtWorld\qtWorld\qtworld.qrc’.

I didn’t use any qmake / nmake . They ain’t required when you get latest Qt Visual Studio Add-in 1.1.11 (even Intellisense recognizes the keywords slots: signals:), right?

Now my questions :

1. I heard many errors are resolved by just rebuilding the whole solution , why is that ?
2. Please explain why I get these errors in deep and their possible solutions.

p.s please be good at explanation, don’t be rep whorer

Answer 1

If you have QObject class defined in cpp file then you should also include moc for that file.
For example your MyClass is defined in main.cpp then you should add

#include "main.moc"

to end of your main.cpp file.

Thanks to @Tomma for providing the unexplained answer 😛

Explanation why above code didn’t work :

However, the moc is typically applied to header files only. Since your class definition is part of the main program file, the IDE does not recognize it.

Thanks to @koahnig for providing nice explanation