Is it safe to use the “this” pointer in an initialization list?

Question

I have two classes with a parent-child relationship (the Parent class \"has-a\" Child class), and the Child class has a pointer back t

Answer 1

The parent this pointer, in "pointer terms", is well-defined (otherwise how would the parent constructor know on which instance is it operating?), but:

• the fields that are declared after the Child object aren't initialized yet;
• the code in the constructor hasn't run yet;
• also, the usual warnings about using virtual members from the constructor apply¹.

So, the parent object in general is still in an inconsistent state; everything the child object will do on construction on the parent object, will be done on a half-constructed object, and this in general isn't a good thing (e.g. if it calls "normal" methods - that rely on the fact that the object is fully constructed - you may get in "impossible" code paths).

Still, if all the child object do with the parent pointer in its constructor is to store it to be use it later (=> when it will be actually constructed), there's nothing wrong with it.

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1. I.e., virtual dispatch doesn't work in constructors, because the vtable hasn't been updated yet by the derived class constructor. See e.g. here.