CODEWITHSUNDEEP
c++graphicsgpuvulkan
Makogan
Makogan

Reputation: 9538

What does vk::DeviceQueueCreateFlags() actually do?

The C like way to initialize this structure is:

VkDeviceQueueCreateInfo queueCreateInfo = {};
    queueCreateInfo.sType = VK_STRUCTURE_TYPE_DEVICE_QUEUE_CREATE_INFO;
    queueCreateInfo.queueFamilyIndex = queueFamily;
    queueCreateInfo.queueCount = 1;
    queueCreateInfo.pQueuePriorities = &queuePriority;

The C++ way, using and abusing the vulkan.hpp header, is:

vk::DeviceQueueCreateInfo deviceQueueCreateInfo(vk::DeviceQueueCreateFlags(), static_cast<uint32_t>(graphicsQueueFamilyIndex), 1, &queuePriority);

It seems that a lot of work is encapsulated in the function vk::DeviceQueueCreateFlags().

However looking through the source with my editor is not revealing anything useful. I was hoping someone with more experience could provide some information as to what the function is doing.

Upvotes: 2

Views: 374

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473537

It's an alias for vk::Flags, which is a template for dealing with Vulkan bitfields in a type-safe manor.

You want enumerations to be type-safe; you don't want implicit conversions to/from integers. So in C++, you do this by defining the enumeration to be an enum class.

That's fine for regular enumerations. But bitfields are special; they're combinations of enumerators from a specific enumeration. The typical post-C++11 solution is to just give the enumeration type overloaded operators, so that you can apply & and | to the enumeration type itself.

Personally, this solution always rubbed me the wrong way. To me, a strong enumeration type should hold one of the enum values, not several of them. So using operator overloading to effectively allow an enumeration value to lie didn't sit well with me.

Apparently I'm not alone in that, because the writers of vulkan.hpp opted for a different solution. They created a template class vk::Flags, which takes two template parameters. One of them is the enumeration that contains all of the valid bitflags. The second parameter is the "enumeration" that represents the concatenation of multiple flags. Which is something that the vulkan.xml specification file actually understands: the difference between the field containing the possible bits and the field which is an aggregation of bits.

vk::Flags can be given bits from the bitfield, and you can manipulate it with bits from that bitfield via some of the bitwise operators. And it is implicitly convertible to the type that represents the aggregation of bits.

So DeviceQueueCreateInfo is just an alias for Flags<DeviceQueueCreateFlagBits, VkDeviceQueueCreateFlags>, which is a bitfield that takes device creation bits and aggregates them into the flags aggregate.

Upvotes: 4

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