What is the Identity function in C++? How is it useful?

In general, an ID is an abbreviation for identification. Identification is the act of being identified so a system can verify whom you say you really are. For example, if a service or system is protected with a username and password and the correct username and password is entered you are identified and can log in to the service or system. Implementing an identity function is usually with the goal of substituting every occurrence of an expression in the code by an identity function without changing the semantics of the program at all. However, the identity function is supposedly nothing more than a cast and it doesn’t affect the performance of the program. Lifetime extension applies to temporaries whose construction is visible. Interposing the call to the identity function disables lifetime extension.