invoking copy constructor inside other constructor

Question

#include <stdlib.h>
#include <iostream>
#include <vector>
#include <string>
class A
{
public:
    std::string s;
    A()
    {
        s = "string";
        new(this)A(*this);
    }
};
int main()
{
    A a;
    std::cout<<a.s;
    return 0;
}

I get empty string in output. What does the C++ standard say about such behaviour?

is this: `new(this)A(*this);`invoking copy constructor inside other constructor? — Rohit Vipin Mathews, Mar 19 '12 at 10:56
`stdlib.h` => `cstdlib` but the header is unnecessary here anyway. — Konrad Rudolph, Mar 19 '12 at 10:56
possible duplicate of [placement new to defer to a different constructor](http://stackoverflow.com/questions/2668144/placement-new-to-defer-to-a-different-constructor) — Alexandre C., Mar 19 '12 at 12:16

score 4 · Accepted Answer · answered Mar 19 '12 at 11:01

4

There must be at least two problems here:

You try to initialize A with a copy of itself
Inside the constructor, A isn't yet fully constructed, so you cannot really copy it

Not to mention that new(this) is suspect in itself.

answered Mar 19 '12 at 11:01

Bo Persson

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`new(this)` is fine. And assigning to something with a copy of itself is usually also guaranteed to work (a copy assignment operator needs to guard against self-assignment) – copy construction, on the other hand, is a different matter. – Konrad Rudolph Mar 19 '12 at 11:03
@Konrad: So default (generated) copy ctor is guarded against self-assignment and therefore actual copying isn't performed? Seems that intermediate variable A a(*this); new(this)(a); solves the "problem" ? – user396672 Mar 19 '12 at 12:47
@user396672 Nothing of the sort. The default *copy assignment operator* guards against self-assignment. I don’t think the default copy constructor does, simply because this case should normally never occur. – Konrad Rudolph Mar 19 '12 at 13:15

score 0 · Answer 2 · answered Mar 19 '12 at 12:16

0

You are calling s's constructor twice in a row by doing this, ergo, the behavior is undefined (and most likely some memory is leaked).

answered Mar 19 '12 at 12:16

Alexandre C.

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invoking copy constructor inside other constructor

2 Answers2