Re: class cross-reference
gergomeister@gmail.com ha scritto:
class A {
public:
A(B& b) : mB(b) {}
private:
B& mB;
};
class B {
public:
B(A& a) : mA(a) {}
private:
A& mA;
};
class C {
public:
C() : mA(mB), mB(mA) {}
private:
A mA;
B mB;
};
int main() {
C myC;
return 0;
}
It is guaranteed to work by 3.8/6: "[...] before the lifetime of an
object has started but after the storage which the object will occupy
has been allocated [...] any lvalue which refers to the original object
may be used but only in limited ways. [...]"
The storage for an object and all its subobjects is allocated before the
constructor is invoked and the use you make of mB is compatible with the
"limited ways" described in the paragraph.
If you change the code like this:
class A {
public:
A(B& b) : mB(b) { mB.foo(); } // uses mB!
private:
B& mB;
};
then you would be violating the "limited ways" as you would be calling a
non-static member function of a object whose lifetime hasn't started and
code would incur in undefined behaviour.
Notice that the restriction is only on mB, as mA is fully constructed
once you get into B constructor.
HTH,
Ganesh
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