Vulkan is a C API and as such inherits all common pitfalls of using a general C programming library. The motivation of a low-level Vulkan C++ API is to avoid these common pitfalls by applying commonly known C++ features while keeping the overall structure of a Vulkan program and preserving the full freedom it provides as low-level graphics API. An additional guideline we followed was not to introduce additional runtime overhead by providing a header-only library with inline functions.
Have a look at the following piece of code which creates a VkImage:
```c++
VkImageCreateInfo ci;
ci.sType = VK_STRUCTURE_TYPE_IMAGE_CREATE_INFO;
ci.pNext = nullptr;
ci.flags = ...some flags...;
ci.imageType = VK_IMAGE_TYPE_2D;
ci.format = VK_FORMAT_R8G8B8A8_UNORM;
ci.extent = VkExtent3D { width, height, 1 };
ci.mipLevels = 1;
ci.arrayLayers = 1;
ci.samples = VK_SAMPLE_COUNT_1_BIT;
ci.tiling = VK_IMAGE_TILING_OPTIMAL;
ci.usage = VK_IMAGE_USAGE_COLOR_ATTACHMENT_BIT;
ci.sharingMode = VK_SHARING_MODE_EXCLUSIVE;
ci.queueFamilyIndexCount = 0;
ci.pQueueFamilyIndices = 0;
ci.initialLayout = VK_IMAGE_LAYOUT_UNDEFINED;
vkCreateImage(device, &ci, allocator, &image));
```
There may be some issues that can happen when filling the structure which cannot be caught at compile time:
* initialization of ```ci.sType``` using wrong enum values
* uninitialized data fields (e.g. missing initialization of ```ci.mipLevels```)
* use of invalid bits for ```ci.flags``` (no type-safety for bits)
* use of incorrect enums for fields (no type-safety for enums)
These initializations will most likely show up as random runtime errors, which usually are nasty and time-consuming to debug.
Our auto-generated, C++ 11-conform layer uses commonly known C++ features like implicit initialization through constructors
to avoid incorrect or missing initializations and introduces type-safety with scoped enums to turn explicit initialization
errors into compile errors. Following is a list of features and conventions introduced by our Vulkan C++ layer:
* works along the official C version of the API
* defines all symbols within the 'vk' namespace and to avoid redundancy the vk/Vk/VK_ prefixes have been removed from all symbols, i.e. ```vk::ImageCreateInfo``` for VkImageCreateInfo.
* camel case syntax with an 'e' prefix has been introduced for all enums, i.e. ```vk::ImageType::e2D``` (the prefix was a compromise, more about that later) removes the 'BIT' suffix from all flag related enums, i.e. ```vk::ImageUsage::eColorAttachment```.
* introduces constructors for all structs, which by default set the appropriate ```sType``` and all other values to zero.
* introduces wrapper classes around the vulkan handles, i.e. ```vk::CommandBuffer``` for VkCommandBuffer
* introduces member functions of those wrapper classes, that map to vulkan functions getting the corresponding vulkan handle as its first argument. The type of that handle is stripped from the function name, i.e. ```vk::Device::getProcAddr``` for vkGetDeviceProcAddr. Note the special handling for the class CommandBuffer, where most of the vulkan functions would just include "Cmd", instead of "CommandBuffer", i.e. ```vk::CommandBuffer::bindPipeline``` for vkCmdBindPipeline.
With those changes applied, the updated code snippet looks like this:
At development time it can be quite handy to have a utility function that can convert an enum or flags to a string for debugging purposes. To achieve this,
we have implemented ```to_string(type)``` functions for all enums and flags. Calling ```to_string(vk::SharingMode::eExclusive)``` will return 'Exclusive' and calling
```to_string(vk::QueueFlagBits::eGraphics | vk::QueueFlagBits::eCompute)``` will return the concatenated string 'Graphics | Compute'.
# Alternative Initialization of Structs
Another nice feature of those constructors is that sType is being initialized internally and thus is always correct.
Finally, we have added a default constructor to each struct which initializes all values to 0 to allow setting the values with a variant of the named parameter idiom which is similar to the designated initializer list of C99.
* ```(count, T*)``` has been replaced by ```vk::ArrayProxy<T>```, which can be created out of a single T, a (count, T*) pair, a std::array<T,N>, a vector<T>, or an initializer_list<T>.
* ```const char *``` has been replaced by ```const std::string &```
* ```const T *``` has been replaced by ```const T &``` to allow temporary objects. This is useful to pass small structures like ```vk::ClearColorValue``` or ```vk::Extent*```
Optional parameters are being replaced by ```Optional<T>``` which accept a type of ```const T```, ```T```, or ```const std::string```. ```nullptr``` can be used to initialize an empty ```Optional<T>```.
* no output value -> return type is ```vk::Result```
* one output value -> return type is a structure ```vk::ResultValue<T>``` with a member ```result``` of type ```vk::Result``` holding the actual result code, and a member ```value``` of type T, which is the type of the output value, holding that output value.
