c++ - Why would multiple shared pointers pointing to the same memory cause memory leak?

Question

I am reading about shared pointers and the book say that it is bad idea having multiple shared pointers point to the same memory because if one shared_ptr reference count decrease to 0, it will deallocate that memory, making the other shared_ptr pointing to garbage.

int *z = new int;
shared_ptr<int> bad1(z); 
shared_ptr<int> bad2(z);

However, when the reference count for z becomes 0, wouldn't both shared_ptr know this?

Answer 1

That won't cause a leak. It's actually much worse, since you have two unrelated shared pointers pointing to the same memory. Which means both will think they have ownership of the memory, and each will try to free it on their own.

If you want two shared memory object pointing to the same memory, then use the std::shared_ptr initialization (or assignment):

shared_ptr<int> good1(new int);
shared_ptr<int> good2 = good1;