Chapter 5 Advanced Concepts of Modern C++

No Ownership, but Secure Access with std::weak_ptr<T>

Sometimes it is necessary to have a non-owning pointer to a resource that is owned by one or more shared pointers. At first you might say, okay, but what’s the problem? I

simply can obtain the raw pointer from an instance of std::shared_ptr<T> at any time by calling its get() member function. See Listing 5-5.

Listing 5-5.  Retrieving the Regular Pointer from an Instance of std::shared_ptr<T>

std::shared_ptr<ResourceType> resource { std::make_shared<ResourceType>() };

// ...

ResourceType* rawPointerToResource { resource.get() };

Watch your step! This could be dangerous. What will happen if the last instance of std::shared_ptr<ResourceType> gets destroyed somewhere in your program and this raw pointer is still in usage somewhere? The raw pointer will point to No-Man’s-­

Land and using it can cause serious problems (remember my warning about undefined behavior in the previous chapter). You have absolutely no chance to determine that the raw pointer points to a valid address of a resource, or to an arbitrary location in memory. If you need a pointer to the resource without having ownership, you should use

std::weak_ptr<T> (defined in the <memory> header), which has no influence on the resource’s lifetime. std::weak_ptr<T> merely “observes” the managed resource and can be interrogated that it is valid. See Listing 5-6.

Listing 5-6.  Using std::weak_ptr<T> to Deal with Resources That Are Not Owned

01 #include <memory>

02

03 void doSomething(const std::weak_ptr<ResourceType>& weakResource) {

04 if (! weakResource.expired()) {

05 // Now we know that weakResource contains a pointer to a valid object 06 std::shared_ptr<ResourceType> sharedResource = weakResource.lock(); 07 // Use sharedResource...

08 }

09 }

10

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11int main() {

12auto sharedResource{ std::make_shared<ResourceType>() };

13std::weak_ptr<ResourceType> weakResource{ sharedResource };

15doSomething(weakResource);

16sharedResource.reset(); // Deletes the managed instance of ResourceType

17doSomething(weakResource);

18

19return 0;

20}

As you can see on Line 4 in Listing 5-6, we can interrogate the weak pointer object if it manages a valid resource. This is done by calling its expired() member function. std::weak_ptr<T> does not provide dereference operators, like *, or ->. If we want to use the resource, we first must call the lock() function (see line 6) to obtain a shared pointer object from it.

You might be asking yourself now what the use cases of this smart pointer type are. Why is it necessary, because you could readily also take a std::shared_ptr<T> everywhere a resource is needed?

First of all, with std::shared_ptr<T> and std::weak_ptr<T>, you are able to distinguish between owners of a resource and users of a resource in a software design. Not every software unit that requires a resource just for a certain and time-limited task wants to become its owner. As we can see in the function doSomething() in the previous example, sometimes it is sufficient just to “promote” a weak pointer to a strong pointer for a limited amount of time.

A good example would be an object cache that for the purpose of performance efficiency keeps recently accessed objects in memory for a certain amount of time. The objects in the cache are held with std::shared_ptr<T> instances, together with a last-­used timestamp. Periodically, a kind of garbage collector process is running that scans the cache and decides to destroy those objects that have not been used for a defined time span.

At those places where the cached objects are used, instances of std::weak_ptr<T> are used to hold non-owning pointers to these objects. If the expired() member function of those std::weak_ptr<T> instances returns true, the garbage collector process has cleared the objects from the cache. In the other case, the std::weak_ptr<T>::lock() function can be used to retrieve a std::shared_ptr<T> from it. Now the object can be

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safely used, even if the garbage collector process gets active. The process could evaluate the usage counter of the std::shared_ptr<T> and ascertain that the object has currently at least one user outside the cache. As a consequence, the object’s lifetime would be extended. Or the process could delete the object from the cache, which does not interfere with its users.

Another example is to deal with circular dependencies. For instance, if you have a class A that needs a pointer to another class B and vice versa, you will end up with a circular dependency. If you use std::shared_ptr<T> to point to the respective class, as shown in Listing 5-7, you can end up with a memory leak. The reason for this is that

the usage counter in the respective shared pointer instance will never count down to 0. Thus, the objects will never be deleted.

Listing 5-7.  The Problem with Circular Dependencies Caused Through Careless Use of std::shared_ptr<T>

#include <memory>

class B; // Forward declaration

class A { public:

void setB(std::shared_ptr<B>& pointerToB) { myPointerToB = pointerToB;

}

private:

std::shared_ptr<B> myPointerToB;

};

class B { public:

void setA(std::shared_ptr<A>& pointerToA) { myPointerToA = pointerToA;

}

private:

std::shared_ptr<A> myPointerToA;

};

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int main() {

{// Curly braces build a scope

auto pointerToA = std::make_shared<A>(); auto pointerToB = std::make_shared<B>(); pointerToA->setB(pointerToB); pointerToB->setA(pointerToA);

}

//At this point, one instance each of A and B is "lost in space" (memory leak!)

return 0;

}

If the std::shared_ptr<T> member variables in the classes are replaced with non-­owning weak pointers (std::weak_ptr<T>) to the respective other class, the issue with the memory leak is solved. See Listing 5-8.

Listing 5-8.  Circular Dependencies Implemented the Right Way with std::weak_ptr<T>

class B; // Forward declaration

class A { public:

void setB(std::shared_ptr<B>& pointerToB) { myPointerToB = pointerToB;

}

private:

std::weak_ptr<B> myPointerToB;

};

class B { public:

void setA(std::shared_ptr<A>& pointerToA) { myPointerToA = pointerToA;

}

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