I recently noticed some strange behavior when using a returned empty shared_ptr. To illustrate the problem consider this example:
struct A {
A() { }
void foo() {
std::cout << "A::foo" << std::endl;
}
};
struct B {
B() :i(42) { }
void foo() {
std::cout << "B:foo with i: " << i << std::endl;
}
int i;
};
template<typename T>
std::shared_ptr<T> create_empty() {
return std::shared_ptr<T>();
}
Then calling:
std::shared_ptr<A> pa(create_empty<A>());
pa->foo(); // #1: Works fine and prints: "A::foo".
std::shared_ptr<B> pb(create_empty<B>());
pb->foo(); // #2: Throws an exception.
Now my question is why call #1 works (I expected to get an exception as well) and if this is the correct behavior how to prevent #1 from working. Am I supposed to check if the return value is empty? Are there some other ways to return null or empty shared_ptr? I'm using MSVC++ 11 if that matters...
The code in both cases dereferences a null pointer. The behavior is undefined, so anything can happen, including things that seem to make sense. Don't try to make sense out of them. Undefined is undefined.