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Mark_L_1
Beginner
86 Views

In-class initializer bug, destructors incorrectly called, compiler 2015

I've got a test case where I have a class with 3 subobjects (AB and C), and the 2nd subobject Bthrows an exception during construction. As I understand C++, the compiler should rewind the construction of the big class and destroy the 1st object A, but not the 2nd (B) or 3rd (C) objects.

What I see is that if I use "In-class initialization" of the first object A, then instead of the first object Agetting destroyed, the 3rd object C gets destroyed. Of course it is VERY BAD to destroy an object that has not been constructed! If, for example, C was a std:unique_ptr<T>, it will probably signal a segmentation violation when it tries to free a garbage pointer.

If I use old school "member initialization", then this problem doesn't happen.

I don't see this with gcc 4.8

Here's the code. The class D exposes the bug. The class E should have identical function, but it does not expose the bug.

#include <iostream>

using namespace std;


struct A {
    A(const string& x) : x_(x) { cout << "A::A()" << (void*)this <<endl; }
    ~A() { cout << "A::~A() " << (void*)this<< endl;}
    string x_;
};

struct B {
    B(const A& a)  { cout << "B::B()" << endl; throw "dead"; }
    ~B() { cout << "B::~B()" << endl;}
};

struct C {
    C()  { cout << "C::C()" << endl; }
    ~C() { cout << "C::~C()" << endl;}
};


struct D  {
    A a{"foo"}; // "new school In-class initialization"
    B b{a};
    C c;
    D() { cout <<"D::D()" << endl; }
    ~D() { cout <<"D::~D()" << endl; }
};

struct E {
    A a;
    B b;
    C c;
    E()
        :a{"foo"}  // "old school member initialization"
        ,b(a)
        { cout <<"E::E()" << endl; }
    ~E() { cout <<"E::~E()" << endl; }
};

int main()
{
   try {
       D d;
   }
   catch(...)
   {
       cout << "got exception" << endl;
   }

   try {
       E e;
   }
   catch(...)
   {
       cout << "got exception" << endl;
   }

   return 0;
}

Here is the output. I expect to see A constructed, B partially constructed then throws, then Adestroyed, but that is not what I see for the D case.

$ icpc -std=c++11 test.cpp
$ ./a.out
A::A()0x7fffe0a5ee90
B::B()
C::~C()
got exception

A::A()0x7fffe0a5eea0
B::B()
A::~A() 0x7fffe0a5eea0
got exception

-- update --

The section of the standard that describes what should happen is 15.2.3

For an object of class type of any storage duration whose initialization or destruction is terminated by an exception, the destructor is invoked for each of the object’s fully constructed subobjects, that is, for each subobject for which the principal constructor (12.6.2) has completed execution and the destructor has not yet begun execution, except that in the case of destruction, the variant members of a union-like class are not destroyed. The subobjects are destroyed in the reverse order of the completion of their construction. Such destruction is sequenced before entering a handler of the function-try-block of the constructor or destructor, if any.

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1 Reply
Kittur_G_Intel
Employee
86 Views

Hi Mark,
I tried your test case and it does seem to be an issue. I'll file this with the developers and will touch base with you as soon as I've an update. Appreciate your patience till then.
Thanks,
Kittur 

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