Compiler error: memset was not declared in this scope

You should include <string.h> (or its C++ equivalent, <cstring>).

Whevever you get a problem like this just go to the man page for the function in question and it will tell you what header you are missing, e.g.

$ man memset

MEMSET(3)                BSD Library Functions Manual                MEMSET(3)

NAME
     memset -- fill a byte string with a byte value

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <string.h>

     void *
     memset(void *b, int c, size_t len);

Note that for C++ it's generally preferable to use the proper equivalent C++ headers, <cstring>/<cstdio>/<cstdlib>/etc, rather than C's <string.h>/<stdio.h>/<stdlib.h>/etc.