Pointers to object as class member in "modern C++"

Question

So one rule of thumb I've heard with respect to modern C++ style is that one shouldn't need to use new or delete, and one should instead use smart pointers. So how to go about this when I have a class where one of the members is a pointer to another object. By using a smart pointer I can avoid the need to delete, but I still need to create the object with new. E.g. is the below "canonical" modern C++ style, or how should one go about this?

#include 
#include 

class B {
public:
    B () { std::printf("constructing B\n");}
    ~B () { std::printf("destroying B\n");}
};

class A {
public:
    A () 
    { 
        std::printf("constructing A\n");
        b = std::unique_ptr(new B());
    }
    ~A () { std::printf("destroying A\n");}

private:
    std::unique_ptr b;
};

int main()
{
    A a;
    return 0;
}

Answer 1

You use new. There's nothing wrong with using new, it should just be used as rarely as possible.

(delete on the other hand, should almost never be used, since its usage should always be encapsulated in some sort of RAII handle like a smart pointer or a container.)

---

Note that when using smart pointers, you should always assign the result of new to a named smart pointer or use reset. In your case, you'd want to use:

A() : b(new B()) { }

or:

A()
{
    std::unique_ptr<B> x(new B());
    b = std::move(x);
}

or:

A() { b.reset(new B()); }

(For why this is important, see the "Best Practices" section of the boost::shared_ptr documentation.)