Initialization of boolean variables

Question

This question might seem naive (hell, I think it is) but I am unable to find an answer that satisfies me.

Take this simple C++ program:

#include<iostream>
using namespace std;
int main ()
{
    bool b;
    cout << b;
    return 0;
}

When compiled and executed, it always prints 0.

The problem is that is not what I'm expecting it to do: as far as I know, a local variable has no initialization value, and I believe that a random byte has more chances of being different rather than equal to 0.

What am I missing?

Answer 1

That is undefined behavior, because you are using the value of an uninitialized variable. You cannot expect anything out of a program with undefined behavior.

In particular, your program necessitates a so-called lvalue-to-rvalue conversion when initializing the parameter of operator << from b. Paragraph 4.1/1 of the C++11 Standard specifies:

A glvalue (3.10) of a non-function, non-array type T can be converted to a prvalue. If T is an incomplete type, a program that necessitates this conversion is ill-formed. If the object to which the glvalue refers is not an object of type T and is not an object of a type derived from T, or if the object is uninitialized, a program that necessitates this conversion has undefined behavior. If T is a non-class type, the type of the prvalue is the cv-unqualified version of T. Otherwise, the type of the prvalue is T.