shared pointer and raw pointer lifetime

Question

Could someone explain simply the reason why this does not work:

std::shared_pointer<Bar> getSharedPointer() {
    return std::make_shared<Bar>();
}

...

auto foo = getSharedPointer().get();

Apparently using the raw pointer foo will cause a segfault because the lifetime of the shared pointer returned by getSharedPointer() will have run out. Somehow I would expect it to last until the end of its scope (like whatever block it is inside).

Is this correct and are there any analogous examples to this situation?

Answer 1

For getSharedPointer().get();, getSharedPointer() returns a temporary std::shared_ptr which will be destroyed after the expression immediately, and the pointer managed by it will be deleted too. After that foo will become dangled, any dereference on it causes UB.

auto foo = getSharedPointer().get();
// foo have become dangled from here

You could use a named variable instead:

auto spb = getSharedPointer();
auto foo = spb.get();
// It's fine to use foo now, but still need to note its lifetime
// because spb will be destroyed when get out of its scope
// and the pointer being managed will be deleted too