Does the defaulted default constructor initialize variables to zero?

Question

I'm updating a class to C++14, and trying to figure out the simplest way to initialize all of the instance variables to zero on construction. Here's what I have so far:

class MyClass {
public:
    int var;
    float* ptr;
    double array[3];
    MyStruct data;
    unique_ptr<MyStruct> smart_ptr;

    MyClass() = default;
    ~MyClass() = default;
}

Is setting the constructor to default the equivalent of doing:

MyClass() : var{}, ptr{}, array{}, data{}, smart_ptr{} {}

... or do I need to init each variable? (I've tried both in Visual Studio and I get zeros either way, but I'm not sure if that's luck or not.)

I'm instancing the class without brackets: MyClass obj;

Man, I was going to give you smug answer but now I'm not certain. See also http://stackoverflow.com/questions/2417065/does-the-default-constructor-initialize-built-in-types — Mikhail, Feb 05 '17 at 05:52
Hahah :) I know, right! I actually saw that post, but it was really this comment that left me feeling uncertain: http://stackoverflow.com/questions/2417065/does-the-default-constructor-initialize-built-in-types#comment36188015_2418195. So it zero-inits if you don't have a constructor at all, and call `MyClass instance()` with brackets? :| — Chris Nolet, Feb 05 '17 at 06:02
Logically, setting a constructor to `default` would be expected to default-initialise members. Default-initialisation is not always equivalent to zero-initialisation. — Peter, Feb 05 '17 at 07:20

score 19 · Accepted Answer · edited Jun 20 '20 at 09:12

19

Is setting the constructor to default the equivalent of doing:
MyClass() : var{}, ptr{}, array{}, data{}, smart_ptr{} {}

No. It is not.

The line

MyClass() = default;

is more akin to but not exactly equivalent to:

 MyClass() {}

In either case, using

 MyClass obj;

results in a default-initialized object whose members are default initialized.

However, the difference between them when using

 MyClass obj{};

is that obj will be zero-initialized with the defaulted default constructor while it will be still default initialized with the user provided default constructor.

edited Jun 20 '20 at 09:12

Community

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answered Feb 05 '17 at 06:25

R Sahu

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typo: int{} should be var{} – AndersK Feb 05 '17 at 06:35
Fixed, thanks! I appreciate your answer, too. Am I right in thinking that `MyClass() {}` is a user-provided constructor, whereas `MyClass = default;` is a default constructor, so it would be zero-initialized in some circumstances? http://stackoverflow.com/questions/26699720/value-initialization-default-initialization-or-zero-initialization – Chris Nolet Feb 05 '17 at 07:04
@ChrisNolet "default constructor" means a constructor that can be called with no arguments – M.M Feb 08 '17 at 06:28
Ahh, right you are, sorry. I meant defaulted constructor. – Chris Nolet Feb 08 '17 at 06:38

score 4 · Answer 2 · answered Feb 08 '17 at 06:30

4

To make all of your variables zero-initialized on construction, even if the creator did not request it, one way is:

struct MyClassMembers
{
    int var;
    float* ptr;
    double array[3];
    MyStruct data;
    unique_ptr<MyStruct> smart_ptr;
};

struct MyClass : MyClassMembers
{
    MyClass(): MyClassMembers{} {}
};

Then MyClass m; will use MyClassMembers{} to initialize the members.

answered Feb 08 '17 at 06:30

M.M

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unless one of the members has a non defaulted default constructor, as detailed in R Sahu's answer – Danra Feb 08 '17 at 21:57
@Danra I'm assuming that OP is fine with a member using its own constructor as defined – M.M Feb 08 '17 at 21:58

Hari · Answer 3 · 2023-04-08T07:47:36.323

It is worth clarifying a few points. For a class with members as follows,

class MyClass {
public:
  int var;
  float* ptr;
  double array[3];
  MyStruct data;
  unique_ptr<MyStruct> smart_ptr;
  ...
}

A default constructor defined by the user as follows,

MyClass() : var{}, ptr{}, array{}, data{}, smart_ptr{} {}

will lead to value initialization of the member variables. If a member variable is a class type then its value initialization happens under certain conditions as described in that link.

While defining an object with or without braces (MyClass obj{}; or MyClass obj;), this default constructor will lead to value initialization of the class members.

Suppose, MyClass has a default constructor in one of the following forms:

MyClass() {};
MyClass() = default;

(See here about further information on these two forms for the default constructor.) As far as defining and initializing objects of MyClass is concerned, MyClass() {}; leads to default initialization, while MyClass() = default; leads to value initialization when defining an object with braces (MyClass obj{};). This is because MyClass() {}; is a user defined default constructor with an empty initialization list and an empty body, which will lead to default initialization of the members. On the other hand, MyClass() = default; leads to a compiler defined default constructor, which will correctly lead to value initialization when defining an object with braces. See here about how even the = default; could go wrong.

Does the defaulted default constructor initialize variables to zero?

3 Answers3

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