CODEWITHSUNDEEP
c++option-typenullptr
Dan
Dan

Reputation: 7737

convert nullptr to std::optional

I have a library defining method() that returns type V* and can return nullptr. What's the best way to wrap that method to turn the value into a std::optional<V>?

The naive way would be something like:

std::optional<V> maybe_value;
V* value = method();
if (value != nullptr) {
    maybe_value = *value;    // assuming the type is even copyable
}

But I was hoping there'd already be some STL function I could use to do this as a one-liner wrapper around method().

Upvotes: 1

Views: 2196

Answers (2)

3CxEZiVlQ
3CxEZiVlQ

Reputation: 38708

There is no such function. Assignment of any value makes the optional containing a value. But you can provide your own

template <typename T>
std::optional<T*> optional_ptr(T* ptr) {
    return ptr ? std::optional<T*>(ptr) : std::optional<T*>();
}

Upvotes: 1

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385224

Your way looks basically fine to me, and is roughly what I'd do. You don't want a one-liner here as it would not be very clear. Ultimately you can create a function for it if you like.

One concern: be very careful with ownership semantics here. Who owns *value? Can it be copied? Is that meaningful? Can you move it instead? Who frees the original object?

Always ask these questions when passed a pointer, even if it appeared that the pointer was only chosen in order to add nullability.

To be quite honest, although std::optional is preferable in new code (or code you're refactoring), I'm not convinced that wrapping a function like this is worth the potential confusion, not to mention the cost of a copy (if, indeed, that is necessary).

Upvotes: 4

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