Vulkan: What is the point of sType in vk*CreateInf

Question

In all of the create info structs (vk*CreateInfo) in the new Vulkan API, there is ALWAYS a .sType member. Why is this there if the value can only be one thing? Also the Vulkan specification is very explicit that you can only use vk*CreateInfo structs as parameters for their corresponding vkCreate* function. It seems a little redundant. I can see that if the driver was passing this struct straight to the GPU, you might need to have it (I did notice it is always the first member). But this seems like a really bad idea for the app to do it because if the driver did it, apps would be much less error prone, and prepending an int to a struct doesn't seems like an extremely computational inefficient operation. I just don't see why it exists.

TL;DR
    Why do the vk*CreateInfo structs have the .sType member?

Answer 1

So that the API can be changed without breaking backwards compatibility.

If version 1.1 of Vulkan wants to expand on the creation of, for example, command buffer pools, how would it do that? Well, they could add a whole new entrypoint: vkCreateCommandPool2. But this function would have almost the exact same signature as vkCreateCommandPool; the only difference is that they take different pCreateInfo structures.

So instead, all you have to do is declare a VkCommandPoolCreateInfo2 structure. And then declare that vkCreateCommandPool can take either one. How would the implementation tell which one you passed in?

Because the first 4 bytes of any such structure is sType. They can test that value. If the value is VK_STRUCTURE_TYPE_COMMAND_POOL_CREATE_INFO, then it's the old structure. If it's VK_STRUCTURE_TYPE_COMMAND_POOL_CREATE_INFO_2, then it's the new one.

This also makes it easier for extensions to fully override a CreateInfo structure. The pNext field is for augmenting an API with additional parameters. With sType, an extension can change existing parameters.