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)?

CodePudding user response：

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.