Lazy initialization of std::unique_ptr

Question

I have a class with std::unique_ptr members which are initially initialized to nullptr and later assigned an actual object. They also use custom deleters.

I have two questions regarding this scenario: Should I use std::unique_ptr::reset() or copy assignment operator to assign a value to a unique pointer which was first initialized to nullptr? And if the latter is the case, then how can I make the code below work?

class MyClass {
private:
    std::unique_ptr<Foo, void(*)(Foo*)> m_foo;
    std::unique_ptr<Bar, void(*)(Bar*)> m_bar;
}

MyClass::MyClass() : m_foo(nullptr, ReleaseFoo), m_bar(nullptr, ReleaseBar) { }

bool MyClass::init()
{
    m_foo.reset(CreateFoo()); // works ok
    m_bar = std::unique_ptr<Bar, ReleaseBar>(CreateBar()); // 'ReleaseBar' is not a valid template type argument for parameter '_Dx'
}

Also, would the answer change if no custom deleters were involved (i.e. use reset with custom deleters and copy assignment otherwise)?

Answer 1

You can use resetor the move assignment operator to assign a new value.

The problem with your statement

m_bar = std::unique_ptr<Bar, ReleaseBar>(CreateBar()); // 'ReleaseBar' is not a valid template type argument for parameter '_Dx'

is, as the error message you quote indicates, that ReleaseBar is not a valid template argument. Earlier code uses ReleaseBar in a way consistent with this being a function name. In the earlier code the corresponding template argument was void(*)(Bar*).