c++ assigning value to struct member through shared pointer causes SIGSEGV

Question

Good day. I'm refreshing my C++ knowledge after not using it since the late 90s and am getting up-to-speed on new things. Today, my question is on shared pointers and structs.

Here's my code:

#include <memory>

struct A {
    int x;
};

int main() {
  shared_ptr<struct A> y;
  y->x =0;

  return 0;
}

compile string:

g++ -Wall -Wpedantic --std=gnu++11 -g test.cpp -o test

valgrind memcheck notes there's an "Invalid write of size 4" on line 11, which is y->x = 0

So, my question is: why is there an invalid write? My understanding--obviously faulty--is that the shared_ptr idiom will handle allocation and de-allocation of memory for the thing it's pointed to, and indeed this happens when I use it to point to primitive types and classes as far as I can tell, but this case of using it with a struct is tripping me up.

The real use case I'm trying to figure out is using a smart pointer to store a struct sockaddr_in, but I figure I should (re)learn to crawl before I learn to build smart pointers to objects defined in a time before smart pointers were a widely-accepted thing.

Thanks.

Answer 1

shared_ptr<struct A> y;

That's an empty pointer, which doesn't point to (or own) any object. You can't dereference it.

You'll need to create an object to be managed by the pointer, for example:

auto y = std::make_shared<A>();
std::shared_ptr<A> y {new A};

The first is usually preferred, since it can allocate memory more efficiently.

Answer 2

You still have to allocate your object with new or with std::make_shared