IERC165

IERC165 is a standard interface used in Ethereum smart contracts to enable interface detection. Defined by EIP-165, it provides a uniform way for contracts to declare which interfaces they implement, allowing other contracts to query capabilities without inspecting code.

The IERC165 interface consists of a single function: supportsInterface(bytes4 interfaceId) external view returns (bool). A contract that implements IERC165 should return true from supportsInterface when queried with the interface identifier of an interface it supports. The interface identifier for IERC165 itself is 0x01ffc9a7, and a compliant contract must return true for this value.

Interface identifiers are calculated as the XOR of the function selectors of all functions in an interface. The supportsInterface function enables callers to check support for any interface by passing its identifier. For example, a contract that implements additional standards such as ERC721 or ERC1155 would return true for their respective interface IDs as well, enabling dynamic capability checks by other contracts and tools.

Adoption and usage of IERC165 are widespread in the Ethereum ecosystem. Many standards extend IERC165 to advertise compatibility; for instance, ERC721 and ERC1155 include support for interface detection via ERC165. Developers often implement a base ERC165 contract or use libraries (such as OpenZeppelin ERC165) to provide a robust, standards-compliant supportsInterface implementation.

Limitations include that supportsInterface only signals interface support and does not guarantee full behavioral compliance beyond the declared interface. It is a detection mechanism, not a security or access control feature.