key-repeat | lightweight beat repeat sampler | Audio Utils library

AFAIK there is no built-in syntax for keydown check. But you can check the state with getkeystate in a loop:

DirectInput is an outdated, async abstraction layer, that does exactly the same thing: processes raw input. It is not recommended to use it unless you need to support joystick or anything legacy, for gamepads XInput is being recommended.

Windows is not a real time OS, the best option is to stick to WM_INPUT messages. This requires maintaining an array of key states (bool keyState[256]) and basing your logic as if(keyState[VK_BACKSPACE] == true){}. If you want to also catch the press start and release events, you will have to maintain an array of last key state, and analyzing WM_INPUT check for the change, and produce the press start event only if last state of key was false and WM_INPUT message says key is pressed now.

The other option is to use GetAsyncKeyState to manually check all the input regularly. But that will leave you without the ability to catch key press if it happened between your two calls for GetAsyncKeyState. The documentation of the function says that the lower bit tells exactly that, but that bit is shared among all applications and can be reset by other app, which is sad.

If I understand you correctly, what you want is just the momentary key state, which can be easily obtained through helper classes like Keyboard, it does not use WM_INPUT though, so a minor latency may occur due to window check layer.