Why I couldn't get gravity value in Cocos2d-x 3.2 - c++

Question

I'm new with cocos2d-x. I was trying to use cocos2d-x 3.2 to build a simple physics game, but I got a problem.
At the beginning, I followed the tutorial and added these in HelloWorld.h:
private:
PhysicsWorld* m_world;
public:
void setPhyWorld(PhysicsWorld* world){ m_world = world; }
Then, I added these in HelloWorld.cpp:
Scene* HelloWorld::createScene()
{
auto scene = Scene::createWithPhysics();
auto layer = HelloWorld::create();
layer->setPhyWorld(scene->getPhysicsWorld());
scene->addChild(layer);
return scene;
}
Then, I tried to get the gravity value in function init() like this:
bool HelloWorld::init()
{
Vect g=m_world->getGravity();
if ( !Layer::init() )
{
return false;
}
Size visibleSize = Director::getInstance()->getVisibleSize();
return true;
}
Then, I ran it, but the program stopped at "Vect g=m_world->getGravity();"
It said that "0x00C53B44 had an Unhandled Exceptions", and I had to interrupt it.
Did anybody have the same problem before? I really appreciate if anyone can help me.

Answer 1

Please observe your code
auto layer = HelloWorld::create(); //Calling init Method of Hello World Layer
layer->setPhyWorld(scene->getPhysicsWorld()); // Setting PhysicsWorld instance to the layer
the init() method is called first and then you are setting setPhyWorld(scene->getPhysicsWorld()) so m_world = null.
If you really want physicsWorld instance in the init() method, you should customize the create & init method of HelloWorld layer and send physicsWorld instance with create() method.
//This code above header class of HelloWorld
#define CUSTOM_CREATE_FUNC(__TYPE__) \
static __TYPE__* create(PhysicsWorld* world) \
{ \
__TYPE__ *pRet = new __TYPE__(); \
if (pRet && pRet->init(world)) \
{ \
pRet->autorelease(); \
return pRet; \
} \
else \
{ \
delete pRet; \
pRet = NULL; \
return NULL; \
} \
}
and then
CUSTOM_CREATE_FUNC(HelloWorld); // in the header class, instead of CREATE_FUNC(HelloWorld)
virtual bool init(PhysicsWorld* world);
and
auto layer = HelloWorld::create(scene->getPhysicsWorld()); // in the createScene() Method
and finally
bool HelloWorld::init(PhysicsWorld* world)
{
m_world = world;
Vect g=m_world->getGravity();
if ( !Layer::init() )
{
return false;
}
Size visibleSize = Director::getInstance()->getVisibleSize();
return true;
}