C++ default constructor does not initialize pointer to nullptr?

Question

I wonder if I have a A* member in my class, shouldn't it we set to nullptr automatically if I have a constructor of my class in this form:

class MyCLass
{
    A* m_pointer;

public:
    MyCLass()
    {
    }
};

Does it matter if I do MyCLass* o = new MyCLass; or I do MyCLass* o = new MyCLass(); in C++11?

You don't need an explicit pointer in the first place, just use `std::unique_ptr`. — David G, Oct 01 '14 at 13:11
@0x499602D2 What makes you think that this class should be the exclusive owner of the pointed to object? — James Kanze, Oct 01 '14 at 13:15
@JamesKanze Unless there is evidence otherwise I would suggest he use that class. — David G, Oct 01 '14 at 13:30
It _will_ be set to `nullptr` if you just mention it in the initializer_list, no exlicit value needed: `MyCLass() : m_pointer() { }` — MSalters, Oct 01 '14 at 13:40
@0x499602D2 Why? It's very exceptional for an `std::unique_ptr` to be appropriate in a class. If the object lifetime should correspond exactly to that of the class, then just make the object a member, and don't use a pointer at all. — James Kanze, Oct 01 '14 at 13:48
@JamesKanze My suggestion was a response to his question about *pointers* inside of a class. If he wants to use a *pointer* inside of a class then my first suggestion would be to use a `std::unique_ptr`. I don't know his situation to say it's better to use an actual `A` object, but that's not what the question's about. — David G, Oct 01 '14 at 14:13
@0x499602D2 I don't know his actual situation either, but since the cases where `std::unique_ptr` would be appropriate are very rare, I certainly wouldn't recommend it. — James Kanze, Oct 01 '14 at 14:34
@JamesKanze You wouldn't recommend `std::unique_ptr` in place of a raw pointer? Not sure I follow. — David G, Oct 01 '14 at 14:43
@0x499602D2 Not in the general case. `std::unique_ptr` is very useful for certain specific cases, almost always as a local variable, but it certainly isn't appropriate for generic use. — James Kanze, Oct 01 '14 at 15:06

score 19 · Accepted Answer · edited May 23 '17 at 12:13

19

Pointers are "POD types"...a.k.a. "Plain Old Data". The rules for when and where they are default-initialized are summarized here:

Default initialization of POD types in C++

So no. It doesn't matter what your constructor for a class is, if it's a raw pointer as a member of the class. You aren't actually instantiating the class. So members like Foo * or std::vector<Foo> * or anything ending in * will not be initialized to nullptr.

The smart pointer classes are not POD. So if you use a unique_ptr<Foo> or a shared_ptr<Foo> that is creating instances of classes, that do have a constructor that makes them effectively null if you do not initialize them.

Does it matter if I do MyCLass* o = new MyCLass; or I do MyCLass* o = new MyCLass(); in C++11?

One question per question, please.

Do the parentheses after the type name make a difference with new?

edited May 23 '17 at 12:13

Community

1
1

answered Oct 01 '14 at 13:17

HostileFork says dont trust SE

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But if I don't define a constructor at all, I guess it will be initialized to `nullptr` by compiler generated default CTOR, right? – Narek Oct 01 '14 at 13:20
@Narek In that case you would need `MyClass m{};` or `MyCLass* m = new MyClass();` to ensure data members such as pointers are zero-initialized, i.e. you need to *value-initialize* the object. – juanchopanza Oct 01 '14 at 13:21
@Narek Nope. You could easily read with a search like "what all exactly does a default constructor do, and when is it invoked?" and find the likes of [this answer](http://stackoverflow.com/questions/2417065/does-the-default-constructor-initialize-built-in-types) – HostileFork says dont trust SE Oct 01 '14 at 13:21

score 3 · Answer 2 · answered Oct 01 '14 at 13:24

The default constructor, if compiler-generated or defaulted, will default-initialize all members. Any user-defined constructor will similarly default-initialize all members that don't have an explicit initializer in the initializer-list.

To default-initialize means "call the default constructor for classes, leave everything else uninitialized".

C++ default constructor does not initialize pointer to nullptr?

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