CODEWITHSUNDEEP
c++shared-ptrc-apiraw-pointer
Lars Reimer
Lars Reimer

Reputation: 51

How to prevent deletion of a raw pointer that is used as shared pointer?

I implemented a C-API for a C++ class which uses shared-pointers of other objects to access them. In my C-API I can of course only get raw pointers. So I "convert" the raw pointer in my C-API to a shared-pointer and use this then with my C++ class methods:

method(std::shared_ptr<dataType>(raw-pointer));

Now I have the problem that at the end of "method" always the shared-pointer destructor is called and it unfortunately kills the object my raw-pointer is pointing at (which I don't want). So, how can I prevent the raw-pointer from being killed?

I already tried shared-pointer functions like reset() or swap(), but they all didn't let my raw-pointer go...

bool Traffic_doStep(traffic_handle t, environment_handle e, double cycletime) {
    if (!valid(t, __FUNCTION__)) return false;
    if (!valid(e, __FUNCTION__)) return false;
    if (!valid(cycletime, __FUNCTION__)) return false;

    try {
        t->doStep(std::shared_ptr<Environment>(e), cycletime);
        return true;
    }
    catch (const std::exception& e) {
        std::cerr << e.what() << std::endl;
        return false;
    }
}

Expected result would be that the raw-pointer e is still pointing to a valid object after this function returned. Actually the raw-pointer points then to a deleted object.

Upvotes: 4

Views: 2709

Answers (2)

Nestor
Nestor

Reputation: 747

You may provide a custom deleter of a shared pointer.

Here is a viable constructor:

template< class Y, class Deleter > 
shared_ptr( Y* ptr, Deleter d );

But I would rather use a unique pointer and then release it.

Upvotes: 2

Xirema
Xirema

Reputation: 20396

Don't put the pointer in a std::shared_ptr

The purpose of Smart Pointers in C++ is to provide automatic Lifetime management. When you write std::shared_ptr<int> ptr{raw_ptr};, the expectation is that when ptr goes out of scope, the object pointed to by raw_ptr will be delete'd. If this is not the intent, then you should not be placing the pointer in a smart pointer.

So if your application does not manage the lifetime of a pointer, then it is perfectly acceptable to store a raw pointer.

If the API behind the function cannot be altered, you will instead need to construct the std::shared_ptr with a no-op deleter function, so that when clean-up is called, nothing will happen to the pointer.

try {
    std::shared_ptr<Environment> temp_ptr{e, [](int *) {}/*No-Op Deleter*/};
    t->doStep(temp_ptr, cycletime);
    return true;
}

This will solve your problem, but this is, of course, an Antipattern; don't do this unless you've been forced to by API design constraints that you cannot control.

Upvotes: 15

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