What is [] in a struct pointer?

Question

I'm trying to use a callback function in C and I got into this piece of code:

memcpy(&(mem->response[mem->size]), data, realsize);

I understand everything except this part :

&(mem->response[mem->size]

I understand that we're trying to get the address of mem but what is [] in this context ?

Answer 1

If mem->response is a pointer to an array of elements, then

mem->response[mem->size]

refers to the element at position mem->size in that array and & is used to get a pointer to that element.

You get the same result with mem->response + mem->size if that makes it clearer:

memcpy(mem->response + mem->size, data, realsize);