c++ - Does the order of base-class initializers and member variable initializers matter?

Question

Is the order of the initializers for a class' constructor significant?

So say I have:

class MyClass : BaseClass
{
      int a, b, c;

   public:
      MyClass(int);
}

e.g. 1:

MyClass::MyClass(int forBase) :
  a(7),
  b(14),
  c(28),
  BaseClass(forBase) { }

e.g. 2:

MyClass::MyClass(int forBase) :
  BaseClass(forBase),
  a(7),
  b(14),
  c(28) { }

Would example 1 do something different to example 2?

Answer 1

Would example 1 do something different to example 2?

No. Initialisation order is dictated by the standard, not by the order in which you write the initialisers:

[C++11: 12.6.2/10]: In a non-delegating constructor, initialization proceeds in the following order:

• First, and only for the constructor of the most derived class (1.8), virtual base classes are initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where “left-to-right” is the order of appearance of the base classes in the derived class base-specifier-list.
• Then, direct base classes are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).
• Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).
• Finally, the compound-statement of the constructor body is executed.

In fact, if you write them in any other order and one depends on the other, you may well be warned about it:

struct T {
   std::vector<int> v;
   int w;

   T(int w) : w(w), v(0, w) {}
};

int main() {
   T t(3);
}

// g++ 4.1.2:
// t.cpp: In constructor 'T::T(int)':
// Line 3: warning: 'T::w' will be initialized after
// Line 2: warning:   '__gnu_debug_def::vector<int, std::allocator<int> > T::v'
// Line 5: warning:   when initialized here