All called functions are defined before being called. What's wrong with this statement?

This sentence is naturally wrong, and the two upstairs are right. I repeat: when calling a function, you only need to know its declaration, that is, the return type of the function, the number of parameters and the type of each parameter, but you don't need to know the function body of the function, that is, you don't need to know the definition of the function. There are two examples to explain why we should do this: 1, recursive call, that is, a function may need to call itself, and the definition of the function must not be completed when it is called; 2. Using function pointer or callback mechanism, we know nothing about the definition of the actual incoming function when calling.