CODEWITHSUNDEEP
c++language-lawyerobject-lifetimeplacement-new
Quentin
Quentin

Reputation: 63154

Manually constructing a trivial base class via placement-new

Beware, we're skirting the dragon's lair.

Consider the following two classes:

struct Base {
    std::string const *str;
};

struct Foo : Base {
    Foo() { std::cout << *str << "\n"; }
};

As you can see, I'm accessing an uninitialized pointer. Or am I?

Let's assume I'm only working with Base classes that are trivial, nothing more than (potentially nested) bags of pointers.

static_assert(std::is_trivial<Base>{}, "!");

I would like to construct Foo in three steps:

  1. Allocate raw storage for a Foo

  2. Initialize a suitably-placed Base subobject via placement-new

  3. Construct Foo via placement-new.

My implementation is as follows:

std::unique_ptr<Foo> makeFooWithBase(std::string const &str) {

    static_assert(std::is_trivial<Base>{}, "!");

    // (1)
    auto storage = std::make_unique<
        std::aligned_storage_t<sizeof(Foo), alignof(Foo)>
    >();

    Foo * const object = reinterpret_cast<Foo *>(storage.get());
    Base * const base = object;

    // (2)
    new (base) Base{&str};

    // (3)
    new (object) Foo(); 

    storage.release();
    return std::unique_ptr<Foo>{object};
}

Since Base is trivial, my understanding is that:

And so Foo receives an initialized pointer, and all is well.

Of course, this is what happens in practice, even at -O3 (see for yourself!).
But is it safe, or will the dragon snatch and eat me one day?

Upvotes: 13

Views: 575

Answers (1)

sp2danny
sp2danny

Reputation: 7687

This seems to be explicitly disallowed by the standard. Ending an objects lifetime, and starting a new objects lifetime in the same location is explicitly allowed, unless it's a base class:

§3.8 Object Lifetime

§3.8.7 - If, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, a new object is created at the storage location which the original object occupied, a pointer that pointed to the original object, a reference that referred to the original object, or the name of the original object will automatically refer to the new object and, once the lifetime of the new object has started, can be used to manipulate the new object, if:

  • the storage for the new object exactly overlays the storage location which the original object occupied, and

  • the new object is of the same type as the original object (ignoring the top-level cv-qualifiers), and

  • [snip] and

  • the original object was a most derived object (1.8) of type T and the new object is a most derived object of type T (that is, they are not base class subobjects).

Upvotes: 8

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