What is the best smart pointer return type for a factory function?

Question

With respect to smart pointers and new C++11/14 features, I am wondering what the best-practice return values and function parameter types would be for classes that have these facilities:

1. A factory function (outside of the class) that creates objects and returns them to users of the class. (For example opening a document and returning an object that can be used to access the content.)

2. Utility functions that accept objects from the factory functions, use them, but do not take ownership. (For example a function that counts the number of words in the document.)

3. Functions that keep a reference to the object after they return (like a UI component that takes a copy of the object so it can draw the content on the screen as needed.)

What would the best return type be for the factory function?

• If it's a raw pointer the user will have to delete it correctly which is problematic.
• If it returns a unique_ptr<> then the user can't share it if they want to.
• If it's a shared_ptr<> then will I have to pass around shared_ptr<> types everywhere? This is what I'm doing now and it's causing problems as I'm getting cyclic references, preventing objects from being destroyed automatically.

What is the best parameter type for the utility function?

• I imagine passing by reference will avoid incrementing a smart pointer reference count unnecessarily, but are there any drawbacks of this? The main one that comes to mind is that it prevents me from passing derived classes to functions taking parameters of the base-class type.
• Is there some way that I can make it clear to the caller that it will NOT copy the object? (Ideally so that the code will not compile if the function body does try to copy the object.)
• Is there a way to make it independent of the type of smart pointer in use? (Maybe taking a raw pointer?)
• Is it possible to have a const parameter to make it clear the function will not modify the object, without breaking smart pointer compatibility?

What is the best parameter type for the function that keeps a reference to the object?

• I'm guessing shared_ptr<> is the only option here, which probably means the factory class must return a shared_ptr<> also, right?

Here is some code that compiles and hopefully illustrates the main points.

#include <iostream>
#include <memory>

struct Document {
    std::string content;
};

struct UI {
    std::shared_ptr<Document> doc;

    // This function is not copying the object, but holding a
    // reference to it to make sure it doesn't get destroyed.
    void setDocument(std::shared_ptr<Document> newDoc) {
        this->doc = newDoc;
    }
    void redraw() {
        // do something with this->doc
    }
};

// This function does not need to take a copy of the Document, so it
// should access it as efficiently as possible.  At the moment it
// creates a whole new shared_ptr object which I feel is inefficient,
// but passing by reference does not work.
// It should also take a const parameter as it isn't modifying the
// object.
int charCount(std::shared_ptr<Document> doc)
{
    // I realise this should be a member function inside Document, but
    // this is for illustrative purposes.
    return doc->content.length();
}

// This function is the same as charCount() but it does modify the
// object.
void appendText(std::shared_ptr<Document> doc)
{
    doc->content.append("hello");
    return;
}

// Create a derived type that the code above does not know about.
struct TextDocument: public Document {};

std::shared_ptr<TextDocument> createTextDocument()
{
    return std::shared_ptr<TextDocument>(new TextDocument());
}

int main(void)
{
    UI display;

    // Use the factory function to create an instance.  As a user of
    // this class I don't want to have to worry about deleting the
    // instance, but I don't really care what type it is, as long as
    // it doesn't stop me from using it the way I need to.
    auto doc = createTextDocument();

    // Share the instance with the UI, which takes a copy of it for
    // later use.
    display.setDocument(doc);

    // Use a free function which modifies the object.
    appendText(doc);

    // Use a free function which doesn't modify the object.
    std::cout << "Your document has " << charCount(doc)
        << " characters.\n";

    return 0;
}

Solution 1

I'll answer in reverse order so to begin with the simple cases.

Utility functions that accept objects from the factory functions, use them, but do not take ownership. (For example a function that counts the number of words in the document.)

If you are calling a factory function, you are always taking ownership of the created object by the very definition of a factory function. I think what you mean is that some other client first obtains an object from the factory and then wishes to pass it to the utility function that does not take ownership itself.

In this case, the utility function should not care at all how ownership of the object it operates on is managed. It should simply accept a (probably const) reference or – if “no object” is a valid condition – a non-owning raw pointer. This will minimize the coupling between your interfaces and make the utility function most flexible.

Functions that keep a reference to the object after they return (like a UI component that takes a copy of the object so it can draw the content on the screen as needed.)

These should take a std::shared_ptr by value. This makes it clear from the function's signature that they take shared ownership of the argument.

Sometimes, it can also be meaningful to have a function that takes unique ownership of its argument (constructors come to mind). Those should take a std::unique_ptr by value (or by rvalue reference) which will also make the semantics clear from the signature.

A factory function (outside of the class) that creates objects and returns them to users of the class. (For example opening a document and returning an object that can be used to access the content.)

This is the difficult one as there are good arguments for both, std::unique_ptr and std::shared_ptr. The only thing clear is that returning an owning raw pointer is no good.

Returning a std::unique_ptr is lightweight (no overhead compared to returning a raw pointer) and conveys the correct semantics of a factory function. Whoever called the function obtains exclusive ownership over the fabricated object. If needed, the client can construct a std::shared_ptr out of a std::unique_ptr at the cost of a dynamic memory allocation.

On the other hand, if the client is going to need a std::shared_ptr anyway, it would be more efficient to have the factory use std::make_shared to avoid the additional dynamic memory allocation. Also, there are situations where you simply must use a std::shared_ptr for example, if the destructor of the managed object is non-virtual and the smart pointer is to be converted to a smart pointer to a base class. But a std::shared_ptr has more overhead than a std::unique_ptr so if the latter is sufficient, we would rather avoid that if possible.

So in conclusion, I'd come up with the following guideline:

• If you need a custom deleter, return a std::shared_ptr.
• Else, if you think that most of your clients are going to need a std::shared_ptr anyway, utilize the optimization potential of std::make_shared.
• Else, return a std::unique_ptr.

Of course, you could avoid the problem by providing two factory functions, one that returns a std::unique_ptr and one that returns a std::shared_ptr so each client can use what best fits its needs. If you need this frequently, I guess you can abstract most of the redundancy away with some clever template meta-programming.

Solution 2

What would the best return type be for the factory function?

unique_ptr would be best. It prevents accidental leaks, and the user can release ownership from the pointer, or transfer ownership to a shared_ptr (which has a constructor for that very purpose), if they want to use a different ownership scheme.

What is the best parameter type for the utility function?

A reference, unless the program flow is so convoluted that the object might be destroyed during the function call, in which case shared_ptr or weak_ptr. (In either case, it can refer to a base class, and add const qualifiers, if you want that.)

What is the best parameter type for the function that keeps a reference to the object?

shared_ptr or unique_ptr, if you want it to take responsibility for the object's lifetime and not otherwise worry about it. A raw pointer or reference, if you can (simply and reliably) arrange for the object to outlive everything that uses it.

Solution 3

Most of the other answers cover this, but @T.C. linked to a few really good guidelines which I'd like to summarise here:

Factory function

A factory that produces a reference type should return a unique_ptr by default, or a shared_ptr if ownership is to be shared with the factory. -- GotW #90

As others have pointed out, you as the recipient of the unique_ptr can convert it to a shared_ptr if you wish.

Function parameters

Don’t pass a smart pointer as a function parameter unless you want to use or manipulate the smart pointer itself, such as to share or transfer ownership. Prefer passing objects by value, *, or &, not by smart pointer. -- GotW #91

This is because when you pass by smart pointer, you increment the reference counter at the start of the function, and decrement it at the end. These are atomic operations, which require synchronisation across multiple threads/processors, so in heavily multithreaded code the speed penalty can be quite high.

When you're in the function the object is not going to disappear because the caller still holds a reference to it (and can't do anything with the object until your function returns) so incrementing the reference count is pointless if you're not going to keep a copy of the object after the function returns.

For functions that don't take ownership of the object:

Use a * if you need to express null (no object), otherwise prefer to use a &; and if the object is input-only, write const widget* or const widget&. -- GotW #91

This doesn't force your caller to use a particular smart pointer type - any smart pointer can be converted into a normal pointer or a reference. So if your function doesn't need to keep a copy of the object or take ownership of it, use a raw pointer. As above, the object won't disappear in the middle of your function because the caller is still holding on to it (except in special circumstances, which you would already be aware of if this is an issue for you.)

For functions that do take ownership of the object:

Express a “sink” function using a by-value unique_ptr parameter.

void f( unique_ptr<widget> );

-- GotW #91

This makes it clear the function takes ownership of the object, and it's possible to pass raw pointers to it that you might have from legacy code.

For functions that take shared ownership of the object:

Express that a function will store and share ownership of a heap object using a by-value shared_ptr parameter. -- GotW #91

I think these guidelines are very useful. Read the pages the quotes came from for more background and in-depth explanation, it's worth it.

Solution 4

I would return a unique_ptr by value in most situations. Most resources shouldn't be shared, since that makes it hard to reason about their lifetimes. You can usually write your code in such a way to avoid shared ownership. In any case, you can make a shared_ptr from the unique_ptr, so it's not like you're limiting your options.

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Comments

[user657267]

If it returns a unique_ptr<> then the user can't share it if they want to. Check out ctor 13.

[Praetorian]

The user of the factory function can always create a shared_ptr from a unique_ptr, but not the other way around. And you need to use weak_ptr to avoid cyclic references when using shared_ptr, it's difficult to say exactly when you should do so without knowing your use cases that result in these cyclic references.

[Some programmer dude]

You shouldn't really look at the smart pointers as pointers, instead look at them in terms of ownership. Like if you want a resource to only have one owner (std::unique_ptr) or shared between multiple owners (std::shared_ptr).

[T.C.]

And don't pass shared_ptrs around, unless you need to do something about the shared_ptr itself, and not just the thing it points to.

[ApproachingDarknessFish]

" The main one that comes to mind is that [using a reference] prevents me from passing derived classes to functions taking parameters of the base-class type." This is false. class A {}; class B : public A {}; ... B b; A& ar = b; compiles fine.

[T.C.]

Also, check out GotW #90 and #91.

[Malvineous]

@ApproachingDarknessFish: Try to compile my example and change charCount() to take a reference-to-smart-pointer. It won't compile because the reference is for the smart pointer, not the pointed-to type. Sorry if this was unclear in my explanation.

[T.C.]

@Malvineous Such a function should take a const Document& and be called with *doc.

[Puppy]

+1- the key issue here is that you completely can share from a unique pointer if you want to and therefore there's no reason not to return a unique_ptr.

[5gon12eder]

@Puppy Except for performance if you want to avoid the additional memory allocation and know that you are going to share.

[kiranpradeep]

@Puppy I was not able to understand what you meant by sharing from aunique_ptr. Is that to pass unique_ptr by reference ? It would be helpful, if you could give a bit more clarity.

[Mike Seymour]

@Kiran: It means you can transfer ownership from the unique_ptr to a shared_ptr, and share ownership thereafter. You can't transfer ownership the other way - once it's shared, it remains shared - so returning shared_ptr forces shared ownership on the user, while returning unique_ptr leaves the choice open.

[Amir]

Thanks for the references, very nicely written and informative.