C++: Direct initialization of class with no value or void

Question

Consider this:

#include <iostream>
class Base {
public:
    Base()=default;   // though ctor,dctor are never called
    ~Base()=default;
};
int main()
{
    Base b();   //1 or Base b(void) produces same result
    return 0;   //2 Base b = Base(); please don't consider this
}

Now consider snippet from debugger:

Reading symbols from /home/a.out...done.
(gdb) run
Starting program: /home/a.out

Breakpoint 1, main () at main.cpp:12
12          return 0;

(gdb) ptype b
type = const union {
    int i[2];
    double d;
}
(gdb) print b
$1 = {i = {0, 1068498944}, d = 0.0625}

(gdb) ptype Base
type = class Base {
  public:
    Base(void);
    ~Base(int);
}

My point of Interest:

Yes, the above snippet is pretty basic but I fail to understand the data revealed by debugger(not the cmds itself).

Why this method fails to initialize class object?
Why 'ptype b' reveals a type union?
What does the values in 'b' symbolizes?

What have been done so far.

I have executed above snippet in MS visual 2015 - dbg fails to watch object 'b'. I never stepped into class Base b (as it should be, class Base is never instantiated).
Anyhow, GCC comes next - now we have above snippet to think about, gcov profiler shows no traces of code coverage for class Base, it validates our above MS visual's axiom.

code/execute- onlinegdb.com

Vexing parse. `Base b();` is seen by the compiler as a declaration of a function, `b`, that returns a `Base`. Will go find duplicate. — user4581301, Aug 17 '18 at 16:19
"Why this method fails to initialize class object?" where do you take that from? — 463035818_is_not_an_ai, Aug 17 '18 at 16:20
@user463035818: strictly speaking this _not_ the Most Vexing Parse as that involves arguments. This just a plain function declaration. Reading about the Most Vexing Parse does explain the problem, though. — Dietmar Kühl, Aug 17 '18 at 16:24
@KillzoneKid I was headed in that direction, but I can't find anything that addresses the side problems. — user4581301, Aug 17 '18 at 16:24
@user4581301 too many questions in one question unfortunately — Killzone Kid, Aug 17 '18 at 16:26
@DietmarKühl I think you were aiming that one at me, not my fellow anonymous user. — user4581301, Aug 17 '18 at 16:26
@user4581301: indeed - in the mobile browser I’m using no completion is suggested and I didn’t notice the digits are different... — Dietmar Kühl, Aug 17 '18 at 16:29
"Why 'ptype b' reveals a type union?" best I can come up with is out there somewhere in wide scope exists a variable `b` of the given type that is defined somewhere deep in the implementation. Or perhaps the debugger just screwed up. — user4581301, Aug 17 '18 at 16:33
@DietmarKühl is there something as the "almost most vexing parse" ? ;) — 463035818_is_not_an_ai, Aug 17 '18 at 16:33
@user463035818 I left off the "most" pretty much for that reason. There is no denying it's vexing, but not most vexing. — user4581301, Aug 17 '18 at 16:37

C++: Direct initialization of class with no value or void

0 Answers0