CODEWITHSUNDEEP
c++c++11clang
chrisaverage
chrisaverage

Reputation: 254

rvalue hello world missing constructor

I'm trying to know more about rvalue references but I got stuck on this simplest example:

#include <iostream>
using namespace std;

struct C {
    C() { cout << "C()\n"; }
    ~C() { cout << "~C()\n"; }
    C(const C&) { cout << "C(const C&)\n"; }
    C& operator=(const C&) { cout << "operator=(const C&)\n"; return *this; }

    C(C&&) { cout << "C(C&&)\n"; }
    C& operator=(C&&) { cout << "operator=(C&&)\n"; return *this; }
};

C foo() { C c; return c; }

int main()
{
    const C c = foo();
    return 0;
}

I've compiled it with Clang 3.2 and -std=c++11 -fno-elide-constructors (to avoid (N)RVO) but the result is surprising to me:

C()
~C()    // huh?
C(C&&)
~C()
~C()

I expected exactly that except for the first ~C(). Where did it came from and what am I missing because there are 2 constructions and 3 destructions? Is the && constructor called with a destroyed object reference??

Upvotes: 9

Views: 169

Answers (1)

Andy Prowl
Andy Prowl

Reputation: 126522

This must be a bug. The destructor for the local object constructed in foo() is being invoked before the move constructor of the receiving object. In particular, it seems a temporary is allocated but not (move-)constructed when returning by value. The following program shows this:

#include <iostream>
using namespace std;

struct C 
{
    C(int z) { id = z; cout << "C():" << id << endl; }
    ~C() { cout << "~C():" << id << endl; }
    C(const C& c) { id = c.id + 1; cout << "C(const C&):" << id << endl; }
    C& operator=(const C&) { cout << "operator=(const C&)\n"; return *this; }
    C(C&& c) { id = c.id + 1; cout << "C(C&&):" << id << endl;}
    C& operator=(C&&) { cout << "operator=(C&&)\n"; return *this; }
    int id;
};

C foo() { C c(10); return c; }

int main()
{
    const C c = foo();
    return 0;
}

Output:

C():10
// THE TEMPORARY OBJECT IS PROBABLY ALLOCATED BUT *NOT CONSTRUCTED* HERE...
~C():10 // DESTRUCTOR CALLED BEFORE ANY OTHER OBJECT IS CONSTRUCTED!
C(C&&):4198993
~C():4198992
~C():4198993

Creating two objects inside of foo() seems to shed some more light on the issue:

C foo() { C c(10); C d(14); return c; }

Output:

C():10
C():14
~C():14
// HERE, THE CONSTRUCTOR OF THE TEMPORARY SHOULD BE INVOKED!
~C():10
C(C&&):1 // THE OBJECT IN main() IS CONSTRUCTED FROM A NON-CONSTRUCTED TEMPORARY
~C():0 // THE NON-CONSTRUCTED TEMPORARY IS BEING DESTROYED HERE
~C():1

Interestingly, this seems to depend on how the object is constructed in foo(). If foo() is written this way: 

C foo() { C c(10); return c; }

Then the error appears. If it is written this way it does not: 

C foo() { return C(10); }

Output with this last definition of foo():

C():10 // CONSTRUCTION OF LOCAL OBJECT
C(C&&):11 // CONSTRUCTION OF TEMPORARY
~C():10 // DESTRUCTION OF LOCAL OBJECT
C(C&&):12 // CONSTRUCTION OF RECEIVING OBJECT
~C():11 // DESTRUCTION OF TEMPORARY
~C():12 // DESTRUCTION OF RECEIVING OBJECT

Upvotes: 3

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