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c++pointersc++11stdshared-ptr

Using shared_from_this() without managed shared pointer in C++11

Let's say I have a class which is a child class of enable_shared_from_this. The documentation of this base class says there should be a shared pointer which owns this class before calling shared_from_this. Is it safe to allocate the class with new and call shared_from_this to manage the object?

Solution

  • No, it's not safe. You should only call shared_from_this if the object is managed by a shared_ptr, not allocated via new (without an associated shared_ptr). For example this code

    struct Test: std::enable_shared_from_this<Test> {
  std::shared_ptr<Test> getptr() {
    return shared_from_this();
  }
};

Test *test = new Test;
std::shared_ptr<Test> test2 = test->getptr();

    will throw std::bad_weak_ptr (at least when using libstdc++). But this is OK:

    std::shared_ptr<Test> test(new Test);
std::shared_ptr<Test> test2 = test->getptr();