Renaming variable yields different results - c++

Question

I wrote a simple function which finds the oldest person in an array of structs. The structure stores information about age and name.
#include <iostream>
struct Person
{
int age;
char name[16];
};
char* oldest(Person* arr, int len)
{
int max = 0;
char* ptr = nullptr;
Person elem;
for (int i = 0; i < len; i++)
{
elem = arr[i];
if (max < elem.age)
{
max = elem.age;
ptr = arr[i].name;
}
}
return ptr;
}
int main()
{
Person list[3] = {
{20, "Alice"},
{70, "Bob"},
{25, "James"}
};
std::cout << oldest(list, 3) << '\n';
}
It yields correct result (namely I see Bob on the screen), but when I use elem instead of arr[i] in the line ptr = arr[i].name; (which is nothing but giving another name to arr[i], right??) the program suddenly starts giving some weird results (unprintable characters). I have no idea why it behaves this way.
For your reference, this is the code that doesn't work:
char* oldest(Person* arr, int len)
{
int max = 0;
char* ptr = nullptr;
Person elem;
for (int i = 0; i < len; i++)
{
elem = arr[i];
if (max < elem.age)
{
max = elem.age;
ptr = elem.name;
}
}
return ptr;
}

Answer 1

ptr = elem.name; assigns to ptr the address of the first element of elem.name (because the arary elem.name is automatically converted to a pointer to its first element). elem.name is of course an array inside elem, and elem is an object with automatic storage duration, meaning it is created automatically in the block it is defined in and it is destroyed when execution of that block terminates. So, when the function return, elem ceases to exist in the C++ model of computing, and a pointer to part of it becomes invalid.
… which is nothing but giving another name to arr[i], right??
No, the statement elem = arr[i]; makes a copy of arr[i] in elem. It does not make elem an alternate name for arr[i]. That copy ceases to exist when the function returns.
If you removed the Person elem; declaration and, inside the loop, used Person &elem = arr[i]; instead of elem = arr[i];, that would define elem to be a reference to arr[i]. Then it would be effectively an alternate name for arr[i], and ptr = elem.name; would set ptr to point to the first element of arr[i].name.

Answer 2

(which is nothing but giving another name to arr[i], right??)
Not right. elem is a distinct object. It is not a name of arr[i].
but when I use elem instead of arr[i] in the line ptr = arr[i].name; ... the program suddenly starts giving some weird results
With that change, you are returning a pointer to (a member of) an automatic variable. When the function returns, the automatic variable is destroyed and the returned pointer will be invalid. When you indirect through the invalid pointer and attempt to access deallocated memory, the beahviour of the program is undefined.