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;

Answer 1

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.

Answer 2

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.