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Runtime Flow

Members initialization difference between C# and C++

Joe Duffy presented an interesting example where order of members initialization is different between C# and C++ in his On partially-constructed objects blog post. "In C#, member initializers run from most derived first, to least derived. In C++, however, member initializers run alongside the ordinary construction process." I've traced his examples in Runtime Flow.

This C# program will print E_init, D_init, C_init, C_ctor, D_ctor, and then E_ctor:

using System;
class C {
    int x = M();
    public C() {
        Console.WriteLine("C_ctor");
    }
    private static int M() {
        Console.WriteLine("C_init");
        return 42;
    }
}
class D : C {
    int x = M();
    public D() : base() {
        Console.WriteLine("D_ctor");
    }
    private static int M() {
        Console.WriteLine("D_init");
        return 42;
    }
}
class E : D {
    int x = M();
    public E() : base() {
        Console.WriteLine("E_ctor");
    }
    private static int M() {
        Console.WriteLine("E_init");
        return 42;
    }
}
class Program {
    public static void Main() {
        new E();
    }
}

This C++ program will print C_init, C_ctor, D_init, D_ctor, E_init, E_ctor, ~E, ~D, and finally ~C:

#include 
using namespace std;
struct C {
    int x;
    C() : x(M()) { cout << "C_ctor" << endl;   }
    ~C() { cout << "~C" << endl; }
    static int M() { cout << "C_init" << endl; return 42; }
};
struct D : C {
   int x;
    D(): x(M()) { cout << "D_ctor" << endl; }
    ~D() { cout << "~D" << endl; }
    static int M() { cout << "D_init" << endl; return 42; }
};
struct E : D {
   int x;
    E() : x(M()) { cout << "E_ctor" << endl; }
    ~E() { cout << "~E" << endl; }
    static int M() { cout << "E_init" << endl; return 42; }
};
static void main() {
    E e;
}

You see how in C# E's member is constructed even before the D object, while in C++ E's member is constructed the last. It is interesting to note that while logically members are initialized before constructors, technically constructors start first and then immediately initialize members.