How to Use Custom Deleters with `std::unique_ptr` Class Members?

Using Custom Deleters with std::unique_ptr Members

In programming, utilizing a custom deleter with a std::unique_ptr member can provide greater control over the destruction of objects. This article aims to address the question of how to incorporate a custom deleter within a class structure.

Consider a scenario where a class, Foo, contains a member of type std::unique_ptr. Bar is a third-party class that offers both a create() and destroy() function. When working with std::unique_ptr in a stand-alone function, the following syntax can be employed:

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

This approach allows for the specification of a custom deleter using a lambda expression. The question arises: is there a way to replicate this behavior when std::unique_ptr is utilized as a member of a class?

Solution: Custom Deleter for Class Member

Assuming that create and destroy are free functions, the following solution can be implemented:

class Foo {

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

    // ...

public:

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

    // ...
};

In this scenario, the custom deleter is provided directly within the class definition. The destroy function is utilized as the deleter, negating the need for a lambda expression. This approach ensures that the Bar object is properly destroyed when the Foo object goes out of scope.

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