why do I need a constructor function?

Question

#include<iostream>
using namespace std;

class A {
public:
    int i;
};

int main() {
  const A aa;  //This is wrong, I can't compile it! The implicitly-defined constructor does not initialize ‘int A::i’
}

when I use

class A {
public:
  A() {}
  int i;
};

this is ok! I can compile it! why I can't compile it when I use the implicitly-defined constructor?

Answer 1

why the implicit-defined constructor does not work?

It does work, but one of the language rules is that it can't be used to initialise a const object unless it initialises all the members; and it doesn't initialise members with trivial types like int. That usually makes sense, since being const there's no way to give them a value later.

_{(That's a slight simplification; see the comments for chapter and verse from the language standard.)}

If you define your own constructor, then you're saying that you know what you're doing and don't want that member initialised. The compiler will let you use that even for a const object.

If you want to set it to zero, then you could value-initialise the object:

const A aa {};    // C++11 or later
const A aa = A(); // historic C++

If you want to set it to another value, or set it to zero without the user having to specify value-initialisation, then you'll need a constructor that initialises the member:

A() : i(whatever) {}