C++ How does order of instance creation matter?

Question

I have a class that creates instances of other classes, and when I call them, compiler gives me warning about order of the instances. Why does it matter? It does same job, regardless of the order.

E.g. I have this in my core class header file (core class handles game loop):

HUD hud;
World myWorld;

Like this they do all they need to. But compiler gives a warning:

'Core::myWorld' will be initialized after [-Wreorder]|

Then if I put myWorld instance above the hud instance, it doesn't give me warning anymore. I was just wondering, how on earth does it matter which order they are in?

Answer 1

Warning is since, in constructor initializer-list you initialize World before HUD, but actually members will be initialized in order they are declared in class.

Just litle example, where it can be worse:

class B
{
public:
   B(int i) : value(i) {}
private:
   int value;
};

class A
{
public:
   A() : value(10), b(value)
   {
   }
private:
   B b;
   int value;
};

Here b will be initialized before value and so, something will be sended to b constructor, but not 10 as programmer want.