Working of nested class objects?

Question

I am trying to understand the order of exectution of a class which has nested objects of another class inside it. Here's my simple program :

#include<iostream>
#include <string>
using namespace std;
class Alpha
{
    int a;
    public:
    Alpha(int x)
    {
        a=x;
    }
};
class Beta
{   int b;
    public:
    Beta(int y)
    {
        b=y;
    }

};
class Gamma
{
    Alpha A;
    Beta B;
    int c;
    public:
    Gamma(int a,int b, int d): A(a), B(b)
    {
        c=d;
    }
};
void main()
{
    Gamma g(5,6,7);
}

As you can see, Gamma has 2 nested objects. Now when the first line of main() is executed, how does the execution start inside the class Gamma? The constructer is called first or the objects/data-members are created first?

Answer 1

The question of constructor execution order is simple: first, Gamma constructor starts, but then it immediately proceeds to initializing Alpha and Beta, as specified in your initialier list. Once the intializer list is done, the body of Gamma's constructor is executed.

There is an important twist to this: C++ will initialize nested objects in the order in which they are declared in the class, not in the order in which they are listed in the initializer list. In other words, Alpha will be initialized ahead of Beta even if you reverse A(a) and B(b):

// The compiler will issue a warning for this
Gamma(int a,int b, int d): B(b), A(a)
{
    c=d;
}