Modifying value of object pointed by a shared pointer - c++

Question

I have recently started working with shared pointers and need some help. I have a vector 1 of shared pointers to some objects. I need to construct another vector 2 of shared pointers to the same objects, so that modifying vector 2 would result in modification of vector 2.
This is how my code looks like:
This works fine
class A
{
public:
int a;
A (int x) {
a = x;
}
int print() {
return a;
}
};
int main()
{
shared_ptr<A> ab = make_shared<A>(100);
cout<< ab->print();
shared_ptr<vector<shared_ptr<A>>> vec1 = make_shared<vector<shared_ptr<A>>>(1);
shared_ptr<vector<shared_ptr<A>*>> vec2 = make_shared<vector<shared_ptr<A>*>>();
vec2->push_back(&(*vec1)[0]);
for (shared_ptr<A>* obj : *vec2) {
*obj = make_shared<A>(100);
}
cout << (*((*vec1)[0])).a; // Prints 100
return 0;
}
But this gives a SEGV at the last line since vec1 is not populated:
class A
{
public:
int a;
A (int x) {
a = x;
}
int print() {
return a;
}
};
int main()
{
shared_ptr<vector<shared_ptr<A>>> vec1 = make_shared<vector<shared_ptr<A>>>(1);
shared_ptr<vector<shared_ptr<A>>> vec2 = make_shared<vector<shared_ptr<A>>>();
vec2->push_back((*vec1)[0]);
for (shared_ptr<A> obj : *vec2) {
obj = make_shared<A>(100);
}
cout << (*((*vec1)[0])).a; // SIGSEGV
return 0;
}
I want to understand why vec1 was not populated in the 2nd one and also would like to know if there is any other way of doing this. Thanks!

Answer 1

The code for the setup described in the comments could be:
#include <vector>
#include <memory>
#include <iostream>
using namespace std;
struct A
{
int a;
A(int a): a(a) {}
};
int main()
{
auto p_vec1 = make_shared<vector<shared_ptr<A>>>();
auto p_vec2 = make_shared<vector<shared_ptr<A>>>();
for (int i = 0; i < 100; ++i)
p_vec1->push_back( make_shared<A>(i) );
for (int i = 0; i < 50; ++i)
p_vec2->push_back( (*p_vec1)[i * 2] );
(*p_vec1)[2]->a = 213;
std::cout << (*p_vec2)[1]->a << '\n'; // print 213
return 0;
}
In case you are unaware, the "outer" shared_ptr is unnecessary, you could just use two vectors .