V603. Object was created but not used. If you wish to call constructor, use 'this->Foo::Foo(....)'.

The analyzer has detected a potential error: incorrect use of a constructor. Programmers often make mistakes trying to call a constructor explicitly to initialize an object.

Consider a typical sample taken from a real application:

class CSlideBarGroup
{
public:
  CSlideBarGroup(CString strName, INT iIconIndex,
                 CListBoxST* pListBox);
  CSlideBarGroup(CSlideBarGroup& Group);
  ...
};

CSlideBarGroup::CSlideBarGroup(CSlideBarGroup& Group)
{
  CSlideBarGroup(Group.GetName(), Group.GetIconIndex(),
                 Group.GetListBox());
}

There are two constructors in the class. To reduce the source code's size the programmer decided to call one constructor from the other. But this code does quite the other thing than intended.

The following happens: a new unnamed object of the CSlideBarGroup type is created and gets destroyed right after. As a result, the class fields remain uninitialized.

The correct way is to create an initialization function and call it from the constructors. This is the correct code:

class CSlideBarGroup
{
  void Init(CString strName, INT iIconIndex,
            CListBoxST* pListBox);
public:
  CSlideBarGroup(CString strName, INT iIconIndex,
                 CListBoxST* pListBox)
  {
    Init(strName, iIconIndex, pListBox);
  }
  CSlideBarGroup(CSlideBarGroup& Group)
  {
    Init(Group.GetName(), Group.GetIconIndex(),
         Group.GetListBox());
  }
  ...
};

If you still want to call the constructor, you may write it in this way:

CSlideBarGroup::CSlideBarGroup(CSlideBarGroup& Group)
{
  this->CSlideBarGroup::CSlideBarGroup(
    Group.GetName(), Group.GetIconIndex(), Group.GetListBox());
}

Another identical code:

CSlideBarGroup::CSlideBarGroup(CSlideBarGroup& Group)
{
  new (this) CSlideBarGroup(
    Group.GetName(), Group.GetIconIndex(),
    Group.GetListBox());
}

The code of the given samples is very dangerous and you should understand well how they work!

You may do more harm than good with this code. Consider the following samples showing where such a constructor call is admissible and where it is not.

class SomeClass
{
  int x,y;
public:
  SomeClass() { SomeClass(0,0); }
  SomeClass(int xx, int yy) : x(xx), y(yy) {}
};

The code contains an error. In the 'SomeClass() ' constructor, a temporary object is created. As a result, the 'x' and 'y' fields remain uninitialized. You can fix the code in this way:

class SomeClass
{
  int x,y;
public:
  SomeClass() { new (this) SomeClass(0,0); }
  SomeClass(int xx, int yy) : x(xx), y(yy) {}
};

This code will work well. It is safe and working because the class contains primary data types and is not a descendant of other classes. In this case the double constructor call is not harmful.

Consider another code where the explicit constructor call causes an error:

class Base 
{ 
public: 
 char *ptr; 
 std::vector vect; 
 Base() { ptr = new char[1000]; } 
 ~Base() { delete [] ptr; } 
}; 
 
class Derived : Base 
{ 
  Derived(Foo foo) { } 
  Derived(Bar bar) { 
     new (this) Derived(bar.foo); 
  } 
}

When we call the "new (this) Derived(bar.foo);" constructor, the Base object is already created and the fields are initialized. The repeated constructor call will lead to double initialization; we will write a pointer to the newly allocated memory area into 'ptr'. As a result we will get memory leak. And if you take double initialization of an object of the std::vector type, you cannot predict its result at all. But one thing is obvious: this code is inadmissible.

In conclusion, I want to note it once again that you'd better create an initialization function instead of explicitly calling a constructor. Explicit constructor call is needed only in very rare cases.

Explicit call of one constructor from the other in C++11 (delegation)

The new standard allows you to perform call of constructors from other constructors (known as delegation). It enables you to create constructors that use behavior of other constructors without added code.

This is an example of correct code:

class MyClass {
    int m_x;
 public:
    MyClass(int X) : m_x(X) {}
    MyClass() : MyClass(33) {}
};

The MyClass constructor without arguments calls a constructor of the same class with an integer argument.

C++03 considers an object to be constructed when its constructor finishes executing, but C++11 considers an object constructed once any constructor finishes execution. Since multiple constructors will be allowed to execute, this will mean that each delegate constructor will be executing on a fully constructed object of its own type. Derived class constructors will execute after all delegation in their base classes is complete.

Additional information

• Discussion on Stack Overflow. C++'s "placement new".
• Discussion on Stack Overflow. Using new (this) to reuse constructors.

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