I have a class named Jellyfish who use the singleton design pattern :
jellyfish.h
#ifndef JELLYFISH_H
#define JELLYFISH_H
#include <QHash>
#include <QScriptEngine>
class QScriptValue;
class Jellyfish : public QObject
{
public:
static Jellyfish * getInstance();
static Jellyfish * instance;
private:
Jellyfish();
};
#ifndef
jellyfish.cpp
Jellyfish * Jellyfish::instance = NULL;
Jellyfish * Jellyfish::getInstance()
{
if ( !Jellyfish::instance )
{
Jellyfish::instance = new Jellyfish();
}
return Jellyfish::instance;
}
When I am in main.cpp and make tests, I have no errors:
main.cpp
#include <QApplication>
#include "jellyfish.h"
class Jellyfish;
int main( int argc, char *argv[] )
{
QApplication app( argc, argv );
Jellyfish *toto = Jellyfish::getInstance();
Jellyfish *toto2 = Jellyfish::getInstance();
Jellyfish *toto3 = Jellyfish::getInstance();
return app.exec();
}
But I want to use some static methods in Jellyfish
in external QScript files :
jellyfish.h
private:
static QScriptValue set( QScriptContext *context, QScriptEngine *engine );
QScriptEngine *script_engine;
jellyfish.cpp
Jellyfish::Jellyfish()
{
script_engine = new QScriptEngine;
/* ... */
initScriptEngine();
}
void Jellyfish::initScriptEngine()
{
QScriptValue object = script_engine->newQObject( this );
object.setProperty( "set", script_engine->newFunction( set ) );
script_engine->globalObject().setProperty( "jellyfish", object );
}
QScriptValue Jellyfish::set( QScriptContext *context, QScriptEngine *engine )
{
// I have to load instance because I am in a static method.
// But this is where the application loop endlessly.
Jellyfish *jellyfish = Jellyfish::getInstance();
return true;
}
And finaly the parsed script:
jellyfish.set( "line_numbers", true );
Problem
When I run the application, getInstance()
always creates a new Jellyfish instance. But the real problem is that on the output with debug (qDebug() << "test";
) I can see that the application loop on Jellyfish::getInstance();
until I get a sgmentation fault
.
Could someone help me to understand this ?
It is because your Jellyfish constructor call itself recursively!
Break your program in debugger and you will see backtrace like this:
...
Jellyfish::Jellyfish()
Jellyfish::getInstance()
Jellyfish::set
Jellyfish::initScriptEngine()
Jellyfish::Jellyfish()
Jellyfish::getInstance()
The problem is that your constructor calls Jellyfish::getInstance()
, but when object is constructed the Jellyfish::instance
is still NULL:
Jellyfish * Jellyfish::getInstance()
{
if ( !Jellyfish::instance )
{
// this line
// Jellyfish::instance = new Jellyfish();
// is equivalent to
Jellyfish * temp = new Jellyfish(); // *
Jellyfish::instance = temp; // not NULL after construction
}
return Jellyfish::instance;
}
If you call getInstance() from your constructor then it will always result in infinite recursion (until segfault). The only one solution is to call Jellyfish::initScriptEngine() not from your constructor - but explicit:
Jellyfish::getInstance()->initScriptEngine();