指针什么时候惯用的？

Question

I come from languages which don't have explicit pointers, so I don't really understand the point of their existence (no pun intended).

The problem is that I have no idea, most of the time, why I am passing a pointer to a function. I do understand that when you pass in a pointer, modifications to the variable are done to the value everywhere, but what's the point? Why not just modify the value and return the result?

For example, http.HandlerFunc is a function that receives http.ResponseWriter and *http.Request as arguments. I have read that interfaces are in fact pointers (is that right?), but what I'm not getting is, why?

Why am I getting a pointer to a writer? I'm not modifying it, I'm merely writing to it. And, why am I getting a pointer to a request? I am doing stuff like request.FormValue().

What I'm trying to determine here, through these examples, is the answer to the question, "when do I need to pass in a pointer?"

The way I do it right now is to write my code, try to compile it, fix the errors that say I must pass in a pointer by adding an ampersand and asterisk, until the errors pass. However, I feel this half-understood concept of pointers is going to bite me in the backside some day very soon.

Answer 1

You can think of a pointer as a value that points to the memory address of an object. Pointers are small (say 8 bytes) compared to most data structures.

A lot of times you will get a pointer to an object because it is much faster to pass those 8 bytes than to create a copy of the entire object that you want to pass.

In the case of a Request object it would be very expensive to create a copy of everything under the request (the payload, headers, and whatnot) compared to just passing a pointer that has access to the original data.