C++ enum class default value without initialization

Question

What is the default value of enum class variable without initialization? The following code prints weird result.

#include <iostream>
#include <cstdio>

using namespace std;

struct C {
    C() {
        m_a = 1;
    }

    enum class E:bool {
        a = false,
        b = true,
    };

    int m_a;
    E m_e;
};

int main()
{
    C c;
    cin >> c.m_a;
    printf("m_e: %d; compare to C::E::a: %d\n", static_cast<int>(c.m_e), c.m_e == C::E::a);
    return 0;
}

Build with gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.1)

g++ -g -o c c.cpp
# run, input 1 and get the result.
./c
1
m_e: 252; compare to C::E::a: 253
# run c, input 2 and get the result.
./c
2
m_e: 254; compare to C::E::a: 255

Q1: Why m_e is not initialized as C::E::a (value false == 0?) or C::E::b (value true == 1?)

Q2: Why dose the comparison expression c.m_e == C::E::a return 253/255, but not false (value 0) or true (value 1)?

Answer 1

What is the default value of enum class variable without initialization?

It's indeterminate since you never initialized it. Reading from c.m_e will therefore make your program have undefined behavior and the program could show just about any value - or no value and crash (or worse).

Change E m_e; to

E m_e{};

and it'll be zero initialized which will mean false in your case.

Note: Zero initializing an enum class may lead to confusing results if 0 isn't actually the value of one of the defined enumerators. It'll zero initialize the enum class's base type nevertheless.

Better be explicit:

E m_e{E::a};

Q1: Why m_e is not initialized as C::E::a (value false == 0?) or C::E::b (value true == 1?)

Because, it's not initialized and anything may show up - or not show up at all.

Q2: Why does the comparison expression c.m_e == C::E::a return 253/255, but not false (value 0) or true (value 1)?

Again. The behavior of the program is undefined.

Note: In the current MSVC, when you do cin >> c.m_a; and later print the value of c.m_e, it'll show the value you entered into c.m_a. Strange? No, that's just one way undefined behavior may show up.