How to convert a `id<MTLCommandQueue>` to a `MTL::CommandQueue` of Metal-Cpp?

Question

The situation: I'm developing an iOS Metal App in Objective-C. I need to use a 3rd-party library VkFFT that provides Metal FFTs based on Metal-Cpp.

What I need: pass a id<MTLCommandQueue> instance from Objective-C world to C++ world as a MTL::CommandQueue object so that I can push compute commands to it. I don't want to create a seperate commmand queue in Metal-Cpp because using a single command queue can let me keep pushing all commands to it without waitUntilCompleted.

Answer 1

With the help of both Hamid Yusifli's answer and this GitHub Issue, I solved my problem.

1. From Objective-C code (.m), cast the id<MTLCommandQueue> into void*:

   id<MTLCommandQueue> objcQueue = /* Your Objective-c queue */;
   void* commandQueuePtr = (__bridge void*)objcQueue;
   

2. Pass this opaque pointer to C++ world.

3. In C++ code, "decode" the void* back to a MTL::CommandQueue*:

   void foo(void* commandQueuePtr) {
     MTL::CommandQueue* commandQueue = (MTL::CommandQueue*) commandQueuePtr;
     // submit GPU works to commandQueue...
   }