warning C4355: 'this' : used in base member initializer list

Hi,

I'm passing a reference to an object to another object which is hold by composition like this:

Code:

---

class A;

class B : boost::noncopyable
{
  A& mParent;
public:
  B(A& parent)
    :mParent(parent)
  {}
};

class A : boost::noncopyable
{
  B mB;
public:
  A()                // warning
    :mB(*this)
  {}
};

int main() {}

---

and I'm getting the following warning, which was never apparent to me before (while using composition regulary):

Quote:

---

warning C4355: 'this' : used in base member initializer list

---

What I'm doing wrong this monday?

THank you in advance!

*this is not valid BEFORE the object has been created, so before you are INSIDE the constructor. The initializer list is performed "outside" the constructor, so *this is (technically) not valid.

--
Mats

Thank you. What style is more common as a solution: Giving B a method

void setParent(A& parent);

to allow some form of 2-step-creation, or holding the handle to the parent as const pointer?

EDIT:
I see, passing this leads to the same warning as passing *this, so the first one is the only option, is it?

I think a two-step solution is the right thing here - after all, you want to construct the mB from an existing A object - you can't do that until A has been constructed properly, which would be at the end of the constructor.

--
Mats

"this" is perfectly valid in the constructor - it's just that the object it points to is not fully constructed, so using it is undefined behaviour.

However, if all B does in its constructor is store the reference, passing *this in the initializer list is perfectly safe, and you can ignore the warning.

Two-step construction is almost never the right solution, in my opinion.

Hi,

Quote:

---

Originally Posted by CornedBee

Two-step construction is almost never the right solution, in my opinion.

---

What is the reason for this?

I found out why I never saw this warning: g++ doesn't produce it even with -Wall (It seems to be more causal regarding warnings in general)

Some code review guys are a bit pedantic about warnings, maybe I'll just merge the two classes. They are strongly coupled anyway

Quote:

---

What is the reason for this?

---

It makes the object considerably more complicated, both in usage and implementation. As a user, you have to worry about whether the object is fully constructed - it's very possible to forget the second step. With a single stage, you never have to worry about this. As an implementor, you also have to worry about the partially constructed stage, namely you have to insert some sort of check into every member function that the object is fully constructed - even if it's just an assert and the release behaviour is undefined.

is there a way to suppress this warning?

Code:

---

#pragma warning( disable : 4355 )

---

#pragma warning(push)
#pragma warning(disable: 4355)
#pragma warning(pop)

This will disable a warning for a range of code only. It's up to you if you want to permanently disable it or not. It's also possible to ignore warnings in the project options under C/C++ -> Advanced.

thank you.