CODEWITHSUNDEEP
c++
user3208430
user3208430

Reputation: 676

What happens when you define an empty default constructor?

I have searched previous questions, and have not found a satisfying answer to my question:

If I define an empty default constructor for a class, as for example

class my_class{
public:
    myclass(){}
private:
    int a;
    int* b;
    std::vector<int> c;
}

my understanding is that if I define an object using the default constructor, say

my_class my_object;

then my_object.a will be a random value, the pointer my_object.b will also be a random value, however the vector c will be a well-behaved, empty vector.

In other words, the default constructor of c is called while the default constructors of a and b is not. Am I understanding this correctly? What is the reason for this?

Thank you!

Upvotes: 1

Views: 647

Answers (2)

AnT stands with Russia
AnT stands with Russia

Reputation: 320381

a and b have non-class types, meaning that they have no constructors at all. Otherwise, your description is correct: my_object.a and my_object.b will have indeterminate values, while my_object.c will be properly constructed.

As for why... by writing a user-defined constructor and not mentioning a and b in the initializer list (and not using C++11 in-class member initializers) you explicitly asked the compiler to leave these members uninitialized.

Note that if your class did not have a user-defined constructor, you'd be able to control the initial values of my_object.a and my_object.b from outside, by specifying initializers at the point of object declaration

my_class my_object1;
// Garbage in `my_object1.a` and `my_object1.b`

my_class my_object2{};
// Zero in `my_object2.a` and null pointer in `my_object2.b`

But when you wrote your own default constructor, you effectively told the compiler that you want to "override" this initialization behavior and do everything yourself.

Upvotes: 7

dbush
dbush

Reputation: 223699

Since a and b are not objects but primitive datatypes, there is no constructor to call. In contrast, c is an object, so its default constructor is called.

Upvotes: 2

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