The GameCube Controller Adapter is a add-on for the Wii U, released in 2014. The device supports connection with up to four GameCube controllers. It was exclusively made for use with Super Smash Bros. for Wii U. It uses two of the four USB ports on the console though only one is required as one is for controllers' rumble function. Due to the accessory's high demand, Nintendo released more Adapters into 2015 though it has not seen an official release since then. Numerous clones have come out in the meantime generally replicating the Adapter's function.
The Nintendo Switch can support the accessory. For a limited time, Nintendo Switch owners could plug in the GameCube Controller Adapter to the Nintendo Switch's dock and it could work as a USB controller. Due to some missing buttons, not all the controls were mapped correctly. Nintendo disabled this function in an update soon after this discovery.