Item 4:  Avoid gratuitous default constructors.

A default constructor (i.e., a constructor that can be called with no arguments) is the C++ way of saying you can get something for nothing. Constructors initialize objects, so default constructors initialize objects without any information from the place where the object is being created. Sometimes this makes perfect sense. Objects that act like numbers, for example, may reasonably be initialized to zero or to undefined values. Objects that act like pointers ( Item 28) may reasonably be initialized to null or to undefined values. Data structures like linked lists, hash tables, maps, and the like may reasonably be initialized to empty containers.

Not all objects fall into this category. For many objects, there is no reasonable way to perform a complete initialization in the absence of outside information. For example, an object representing an entry in an address book makes no sense unless the name of the thing being entered is provided. In some companies, all equipment must be tagged with a corporate ID number, and creating an object to model a piece of equipment in such companies is nonsensical unless the appropriate ID number is provided.

In a perfect world, classes in which objects could reasonably be created from nothing would contain default constructors and classes in which information was required for object construction would not. Alas, ours is not the best of all possible worlds, so we must take additional concerns into account. In particular, if a class lacks a default constructor, there are restrictions on how you can use that class.

Consider a class for company equipment in which the corporate ID number of the equipment is a mandatory constructor argument:

class EquipmentPiece {
public:
  EquipmentPiece(int IDNumber);
  ...
};

Because EquipmentPiece lacks a default constructor, its use may be problematic in three contexts. The first is the creation of arrays. There is, in general, no way to specify constructor arguments for objects in arrays, so it is not usually possible to create arrays of EquipmentPiece objects:

EquipmentPiece bestPieces[10];           // error! No way to call
                                         // EquipmentPiece ctors

EquipmentPiece *bestPieces =
  new EquipmentPiece[10];                // error! same problem

There are three ways to get around this restriction. A solution for non-heap arrays is to provide the necessary arguments at the point where the array is defined:

int ID1, ID2, ID3, ..., ID10;            // variables to hold
                                         // equipment ID numbers
...

EquipmentPiece bestPieces[] = {          // fine, ctor arguments
  EquipmentPiece(ID1),                   // are provided
  EquipmentPiece(ID2),
  EquipmentPiece(ID3),
  ...,
  EquipmentPiece(ID10)
};

Unfortunately, there is no way to extend this strategy to heap arrays.

A more general approach is to use an array of pointers instead of an array of objects:

typedef EquipmentPiece* PEP;             // a PEP is a pointer to
                                         // an EquipmentPiece

PEP bestPieces[10];                      // fine, no ctors called

PEP *bestPieces = new PEP[10];           // also fine

Each pointer in the array can then be made to point to a different EquipmentPiece object:

for (int i = 0; i < 10; ++i)
  bestPieces[i] = new EquipmentPiece( ID Number );

There are two disadvantages to this approach. First, you have to remember to delete all the objects pointed to by the array. If you forget, you have a resource leak. Second, the total amount of memory you need increases, because you need the space for the pointers as well as the space for the EquipmentPiece objects.

You can avoid the space penalty if you allocate the raw memory for the array, then use "placement new" (see Item 8) to construct the EquipmentPiece objects in the memory:

// allocate enough raw memory for an array of 10
// EquipmentPiece objects; see Item 8 for details on
// the operator new[] function
void *rawMemory =
  operator new[](10*sizeof(EquipmentPiece));

// make bestPieces point to it so it can be treated as an
// EquipmentPiece array
EquipmentPiece *bestPieces =
  static_cast<EquipmentPiece*>(rawMemory);

// construct the EquipmentPiece objects in the memory
// using "placement new" (see Item 8)
for (int i = 0; i < 10; ++i)
  new (&bestPieces[i]) EquipmentPiece( ID Number );

Notice that you still have to provide a constructor argument for each EquipmentPiece object. This technique (as well as the array-of-pointers idea) allows you to create arrays of objects when a class lacks a default constructor; it doesn't show you how to bypass required constructor arguments. There is no way to do that. If there were, it would defeat the purpose of constructors, which is to guarantee that objects are initialized.

The downside to using placement new, aside from the fact that most programmers are unfamiliar with it (which will make maintenance more difficult), is that you must manually call destructors on the objects in the array when you want them to go out of existence, then you must manually deallocate the raw memory by calling operator delete[] (again, see Item 8):

// destruct the objects in bestPieces in the inverse
// order in which they were constructed
for (int i = 9; i >= 0; --i)
  bestPieces[i].~EquipmentPiece();

// deallocate the raw memory
operator delete[](rawMemory);

If you forget this requirement and use the normal array-deletion syntax, your program will behave unpredictably. That's because the result of deleting a pointer that didn't come from the new operator is undefined:

delete [] bestPieces;                    // undefined! bestPieces
                                         // didn't come from the new
                                         // operator

For more information on the new operator, placement new and how they interact with constructors and destructors, see Item 8.

The second problem with classes lacking default constructors is that they are ineligible for use with many template-based container classes. That's because it's a common requirement for such templates that the type used to instantiate the template provide a default constructor. This requirement almost always grows out of the fact that inside the template, an array of the template parameter type is being created. For example, a template for an Array class might look something like this:

template<class T>
class Array {
public:
  Array(int size);
  ...

private:
  T *data;
};

template<class T>
Array<T>::Array(int size)
{
  data = new T[size];                    // calls T::T() for each
  ...                                    // element of the array
}

In most cases, careful template design can eliminate the need for a default constructor. For example, the standard vector template (which generates classes that act like extensible arrays) has no requirement that its type parameter have a default constructor. Unfortunately, many templates are designed in a manner that is anything but careful. That being the case, classes without default constructors will be incompatible with many templates. As C++ programmers learn more about template design, this problem should recede in significance. How long it will take for that to happen, however, is anyone's guess.

The final consideration in the to-provide-a-default-constructor-or-not-to-provide-a-default-constructor dilemma has to do with virtual base classes (see Item E43). Virtual base classes lacking default constructors are a pain to work with. That's because the arguments for virtual base class constructors must be provided by the most derived class of the object being constructed. As a result, a virtual base class lacking a default constructor requires that all classes derived from that class — no matter how far removed — must know about, understand the meaning of, and provide for the virtual base class's constructors' arguments. Authors of derived classes neither expect nor appreciate this requirement.

Because of the restrictions imposed on classes lacking default constructors, some people believe all classes should have them, even if a default constructor doesn't have enough information to fully initialize objects of that class. For example, adherents to this philosophy might modify EquipmentPiece as follows:

class EquipmentPiece {
public:
  EquipmentPiece(int IDNumber = UNSPECIFIED);
  ...

private:
  static const int UNSPECIFIED;          // magic ID number value
                                         // meaning no ID was
};                                       // specified

This allows EquipmentPiece objects to be created like this:

EquipmentPiece e;                         // now okay

Such a transformation almost always complicates the other member functions of the class, because there is no longer any guarantee that the fields of an EquipmentPiece object have been meaningfully initialized. Assuming it makes no sense to have an EquipmentPiece without an ID field, most member functions must check to see if the ID is present. If it's not, they'll have to figure out how to stumble on anyway. Often it's not clear how to do that, and many implementations choose a solution that offers nothing but expediency: they throw an exception or they call a function that terminates the program. When that happens, it's difficult to argue that the overall quality of the software has been improved by including a default constructor in a class where none was warranted.

Inclusion of meaningless default constructors affects the efficiency of classes, too. If member functions have to test to see if fields have truly been initialized, clients of those functions have to pay for the time those tests take. Furthermore, they have to pay for the code that goes into those tests, because that makes executables and libraries bigger. They also have to pay for the code that handles the cases where the tests fail. All those costs are avoided if a class's constructors ensure that all fields of an object are correctly initialized. Often default constructors can't offer that kind of assurance, so it's best to avoid them in classes where they make no sense. That places some limits on how such classes can be used, yes, but it also guarantees that when you do use such classes, you can expect that the objects they generate are fully initialized and are efficiently implemented.