Qt C ++：在动态库的标头中，Connect调用失败

Question

I have the following code which fails to connect for signals in header file for dynamic library(TestTemplateInheritance) but does work fine for static library

//TestTemplateInheritance.pro

QT       += core
QT       -= gui
TARGET = $$qtLibraryTarget($$TARGET)
TEMPLATE = lib
SOURCES += \
    base.cpp \  

HEADERS += \
    base.h \

//base.h

#ifndef BASE_H
#define BASE_H
#include <QObject>
#include <QDebug>
class Q_DECL_EXPORT Base : public QObject
{
    Q_OBJECT
public:
    explicit Base(QObject *parent = 0);

signals:
    void testSignal();
  public:
    template <class T>
    void callInHeader(T arg)
    {
        qCritical()<<"Does it connect here? in " << arg;
        connect(this, &Base::testSignal, this, &Base::testSlot);
    }
    void callInDefinition();
public slots:
    void testSlot()
    {
        qCritical() << "Slot called";
    }
};
#endif // BASE_H

//base.cpp

#include "base.h"

Base::Base(QObject *parent) : QObject(parent)
{    
}
void Base:: callInDefinition()
{
    qCritical()<<"Does it connect here?";
    connect(this, &Base::testSignal, this, &Base::testSlot);
    qCritical()<<"Yes it does!";
}

Now the application

//TestTemplateFail.pro

QT       += core
QT       -= gui
TARGET = TestTemplateFail
CONFIG   += console
CONFIG   -= app_bundle
LIBS += -L$$PWD/../release/release -lTestTemplateInheritance
TEMPLATE = app
SOURCES += main.cpp

//main.cpp

#include <QCoreApplication>
#include "../TestTemplateInheritance/base.h"
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    QObject *obj = new QObject();
    Base* d = new Base(obj);
    d->callInHeader<QString>("header");
    d->callInDefinition();
    return a.exec();
}

RESULT:

Does it connect here? in  "header"
QObject::connect: signal not found in Base
Does it connect here?
Yes it does!

If I build a static library(CONFIG += staticlib) instead of a dynamic library. There is no connect error

RESULT:

Does it connect here? in  "header"
Does it connect here?
Yes it does!

I also observed that its not just function template, the connect statement in any function which is defined in the header file fails. Function templates are more obvious ones to have definition in header file. So why does the connect statement fail in the functions defined in the header file in dynamic library fail?

Answer 1

You have a problem with this line:

class Q_DECL_EXPORT Base : public QObject

When the header is included from another program (as opposed to when you compile your library) it should be :

class Q_DECL_IMPORT Base : public QObject

The easy solution is to replace it by:

#if defined(MYLIB_LIBRARY)
#  define MYLIBSHARED_EXPORT Q_DECL_EXPORT
#else
#  define MYLIBSHARED_EXPORT Q_DECL_IMPORT
#endif
class MYLIBSHARED_EXPORT Base : public QObject

and add

DEFINES += MYLIB_LIBRARY

ONLY in the .pro file of your library.