How Can I Use Custom Deleters with std::unique_ptr Members in a C Class?

Custom Deleters with std::unique_ptr Members

In C , the std::unique_ptr class offers a convenient way to manage ownership of pointer-based objects. However, if you're working with third-party objects that require a customized deletion process, you may face challenges when using std::unique_ptr as a member of a class.

Consider the following scenario: you have a class with a std::unique_ptr member. The Bar class is from a third-party library and defines its own create() and destroy() functions for object instantiation and destruction.

To utilize std::unique_ptr with such a scenario in a standalone function, you can use a custom deleter:

void foo() {
    std::unique_ptr<Bar, void (*)(Bar*)> bar(create(), [](Bar* b) { destroy(b); });
    ...
}

But how can you achieve this when the std::unique_ptr is a member of a class?

Custom Deleters in Class Members

Assuming that create and destroy are free functions with the following signatures:

Bar* create();
void destroy(Bar*);

You can define your Foo class as follows:

class Foo {

    std::unique_ptr<Bar, void (*)(Bar*)> ptr_;

    // ...

public:

    Foo() : ptr_(create(), destroy) { /* ... */ }

    // ...
};

In this implementation, you're directly providing the destroy function as the deleter for the std::unique_ptr. By using a free function as a deleter, you avoid the need for lambdas or custom deleter classes.

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