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A Look at C++11 Shared Pointers in Leadwerks 5


Josh

1,801 views

C++11 introduces shared pointers, a powerful language feature that provides easy memory management without the overhead of garbage collection.  The example below should look familiar to you:

#include "Leadwerks.h"

using namespace Leadwerks;

int main(int argc, const char *argv[])
{
	shared_ptr<Leadwerks::Window> window = Leadwerks::Window::Create();

	shared_ptr<Context> context = Context::Create(window);

	shared_ptr<World> world = World::Create();

	shared_ptr<Camera> camera = Camera::Create();
	camera->SetRotation(35, 0, 0);
	camera->Move(0, 0, -6);

	shared_ptr<Light> light = DirectionalLight::Create();
	light->SetRotation(35, 35, 0);

	shared_ptr<Model> model = Model::Box();
	model->SetColor(1.0, 0.0, 0.0);
	model->SetPosition(-4, 0, 0);

	while (true)
	{
		if (window->Closed() || window->KeyDown(Key::Escape)) return false;
		
		Leadwerks::Time::Update();
		world->Update();
		world->Render();
		context->Sync();
	}
  
	//Everything will get automatically deleted when we return from this function
	return 0;
}

Using the auto keyword simplifies everything (and it makes this code compatible with Leadwerks 4):

#include "Leadwerks.h"

using namespace Leadwerks;

int main(int argc, const char *argv[])
{
	auto window = Leadwerks::Window::Create();

	auto context = Context::Create(window);

	auto world = World::Create();

	auto camera = Camera::Create();
	camera->SetRotation(35, 0, 0);
	camera->Move(0, 0, -6);

	auto light = DirectionalLight::Create();
	light->SetRotation(35, 35, 0);

	auto model = Model::Box();
	model->SetColor(1.0, 0.0, 0.0);
	model->SetPosition(-4, 0, 0);
	
	while (true)
	{
		if (window->Closed() || window->KeyDown(Key::Escape)) return false;
		
		Leadwerks::Time::Update();
		world->Update();
		world->Render();
		context->Sync();
	}
  
  	//Everything will get automatically deleted when we return from this function
	return 0;
}

Now things get interesting.  This function would normally cause a horrible memory leak, but with shared pointers everything is fine:

void SaveTexture(shared_ptr<Texture> tex)
{
	auto bank = Bank::Create(tex->GetMipmapSize());
	tex->GetPixels(bank->buf);
	bank->Save("pixeldata.dat");
}

Yet shared pointers can still equal nullptr:

auto bank = Bank::Create();
bank.reset();
Debug::Assert(bank==nullptr);

You can even simply set a shared pointer to nullptr, and if that was the last pointer that referenced it, it gets deleted!

auto bank = Bank::Create();
bank = nullptr;// auto deletion here!

How to Delete an Entity
The code below will not delete the entity, because a shared pointer is still stored in the world.

auto entity = Pivot::Create();
entity = nullptr;

The entity must be have its world set to NULL, and the shared pointer must be set to NULL or go out of scope:

entity = Pivot::Create();
entity->SetWorld(nullptr);
entity = nullptr;

Children use a weak pointer to the parent, so they will not cause self-referencing.

Asset Management
You no longer have to worry about calling Release() when loading assets:

auto material = Material::Create();
auto texture = Texture::Load("mytex.tex");
material->SetTexture(texture);
texture = nullptr;

Unused assets will automatically be deleted if they go out of scope:

auto material = Material::Create();
auto texture = Texture::Load("mytex.tex");
material->SetTexture(texture);
texture = nullptr;
material = nullptr;

But if they are in use, they will be retained in memory:

auto material = Material::Create();
auto texture = Texture::Load("mytex.tex");
material->SetTexture(texture);
model->SetMaterial(material);
texture = nullptr;
material = nullptr;

In conclusion, shared pointers automate many of the tasks we have been doing manually with the Leadwerks reference counting system and the AddRef and Release commands.

4 Comments


Recommended Comments

I take it the above code examples means you've already started transitioning LE to shared pointers?

What happens if you call delete on a shared pointer? Will I also have to transition my code when migrating to LE 5.5?

Link to comment
13 minutes ago, martyj said:

I take it the above code examples means you've already started transitioning LE to shared pointers?

I am adding defines in the code so the Leadwerks 4 source can be compiled in both modes.  It's nowhere near functional, I just started experimenting with it.  The LEADWERKS_5 define will keep the v5 stuff from messing with the current version.

13 minutes ago, martyj said:

What happens if you call delete on a shared pointer? Will I also have to transition my code when migrating to LE 5.5?

They did a nice job with the implementation of this.  It looks like delete will just call the reset() function:
https://stackoverflow.com/questions/12321949/explicitly-deleting-a-shared-ptr

That means if this is the only handle to the object, the object will be deleted.  Otherwise, the shared pointer will just have its ref count decremented.  (In Leadwerks 4 you should be calling Release() on Leadwerks objects, not delete.)

Note that the Release() and AddRef() functions will not exist in Leadwerks 5.

Link to comment
23 minutes ago, jen said:

This is optional right? We can still use the traditional method of assigning a type for each variable in our projects? 

I prefer the traditional way to be honest. Especially when developing multiplayer.

The type is a shared pointer (instead of a pointer).  The first example above shows how it is done.  I prefer the auto keyword because it is less to type.

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