what will be the default value of an uninitialized boolean value in c++

Question

Suppose I have a struct called foo_boolean that contains some boolean values:

struct foo_boolean {
    bool b1;
    bool b2;
};

If I define a variable of type foo_boolean without initializing it, what will the default value of the member variables be? (i.e., true, false, or a random value of the two.)

Answer 1

It depends on how you create it. If the struct is constructed by default-initialization e.g.

void foo () {
  fool_boolen x;   // <---

then the values will be undefined (bad things will happen if you read it before setting a value).

On the other hand, if the struct is constructed by value-initialization or zero-initialization e.g.

fool_boolen x;   // <--

void foo2 () {
  static fool_boolen y; // <--
  fool_boolen z = fool_boolen();  // <--

then the values will be zero, i.e. false.

Answer 2

The value of the bool will is undefined. It will be whatever else was on the stack before it, which is sometimes zeroed out if nothing has used it previously.

But again, it is undefined, which means it can be either true or false.

If you need a default value, you can do:

struct fool_bool {
  bool b1;
  bool b2;
  fool_bool() {
    b1 = true;
    b2 = false;
  }
};

This makes b1 true by default, and b2 false.