从引用创建 std::shared_ptr

Question

I'm running into an object lifetime issue when I write

std::ifstream input("/Users/d.a.hosek/CLionProjects/gftodvi/data/cmr10.2602gf");
GFReader reader {std::shared_ptr<std::ifstream>(&input)};

When the program concludes, it runs into an issue destroying things in my chain, saying that it's trying to free a pointer that wasn't allocated.

But if I do

GFReader reader {std::make_shared<std::ifstream>("/Users/d.a.hosek/CLionProjects/gftodvi/data/cmr10.2602gf")};

then the problem goes away. I'm assuming I'm doing something wrong in the first call sequence when I create my std::shared_ptr but I don't really understand what it is. Can someone give me clear explanation of why the first call sequence fails?

Answer 1

The first way is intended to take an existing heap-allocated resource and manage its lifetime. You pass it a non heap allocated resource, violating its rules.

The second creates a new heap allocated resource and immediately manages it. It even allocates the ref counting block adjacent to the object, to save on calls to ::new .