Default value of enum declared in class

Question

I have a class whose member is an enum declared inside this class:

#include<iostream>

class test
{
public:
    enum TYPE{MAN, WOMAN};

    TYPE type;
};


int main()
{
    test x;
    if(x.type == test::MAN) std::cout<<"MAN"<<std::endl;
    if(x.type == test::WOMAN) std::cout<<"WOMAN"<<std::endl;
    std::cout<<"ok"<<std::endl;
    return 0;
}

I know that if an enum is declared at namespace scope, it has a default value 0 and when it's declared locally, it doesn't have any default values, which leads to undefined behavior.

My question is: what if I have an enum which belongs to a class? Is it undefined behavior as well?

I tested the above code and x.type is neither MAN nor WOMAN . However, I've done it for only one compiler and one operating system. I'm interested in a more general answer. I haven't found any information regarding this issue anywhere else.

Edit1: Can referring to this indeterminate value cause segmentation fault?

Edit2: I know this is not a well designed class- it's not mine and I'm trying to debug it. So telling me that I can default-initialize object doesn't solve my problem. Please, treat it as a theoretical question.

Answer 1

The default value of the first name in an enum is 0, regardless of the scope of the enum .

There is no guaranteed default value of an automatic local variable like test x; in main here. It has an indeterminate value . And it's Undefined Behavior to use that value.

You can ¹default-initialize it like this:

test x{};

---

^{¹ A subtle point is that at top level this gives a “value-initialization”.}

Answer 2

If your object don't have any constructors, then it depends on where you create your object. If it's created globally, then all variables are zero-initialized. If not, they are not initialized properly and reading from them results in UB.

You can force zero-initialization of a non-global variable with test x{}; syntax.

Answer 3

First: "Testing" for undefined behavior is almost never going to give you the right answer.

This is undefined behavior because you are reading from an uninitialized variable with automatic storage duration. Such a variable has an indeterminate value and must not be read from. Every non-static function scope variable has automatic storage duration.

I think you are confusing the definition of the enum type (which happens inside the class definition) with the declaration of a variable of this type (at function scope). In your example x is a variable with automatic storage duration no matter where the type TYPE has been defined.