I have a struct defined in my program.
struct A{ int arr[10]; } Lets say I have a pointer to it. A * a = new A;
I can zero it in two ways:
memset(&a->arr,0,sizeof(A)); memset(a->arr,0,sizeof(A)); both work and look the same!
which one is more correct?
which one is more correct?
I'd argue neither. The easiest way would be to value initialize the allocated object:
A * a = new A(); Of course, this assumes that you actually have a good reason to new this object.
Since you are using C++ I would take advantage of C++11 features and use:
#include <iostream> #include <cmath> using namespace std; struct A{ int arr[10]{}; // initializes array with all 0's }; int main() { A * a = new A; for (auto e : a->arr) // ranged based for loop to show all of the 0's cout << e << "\n"; return 0; } You can see it running with this Live Example
While the type of each expression is different, the actual result, the pointer you pass to memset, will be equal in both cases.
Personally I would probably use std::fill instead of memset in a C++ program:
std::fill(std::begin(a->arr), std::end(a->arr), 0); Also note that if you have more members in the structure, sizeof(A) will be different from sizeof(a->arr) (or sizeof(A::arr)).
you can define a default construct function
struct A{ int arr[10]; A():arr(){} }; 
A () : arr() {}.
This is the correct way for just the array
memset(a->arr,0,sizeof(a->arr)) Picked out the arr member just in case there are other structure members that do not need to be touched. Makes no difference in your example the following will do likewise
memset(a->arr,0,sizeof(A)); 