Item 5 - Know what functions C++ silently writes and calls

Empty class is not an empty class. If you don't declare them yourself, compilers will declare their own versions of a copy contructor, a copy assignment operator, and a destructor. Furthermore, if you declare no constructors at all, compilers will also declare a default constructor for you. All these functions will be both public and inline. As a result if you write

class Empty{};

it's essentially the same as if you'd written this:

class Empty {
public:
    Empty(){...}                                // default constructor
    Empty(const Empty& rhs){...}                // copy constructor
    ~Empty(){...}                               // destructor — see below for whether it's virtual
    Empty& operator=(const Empty& rhs){...}     // copy assignment operator
};

These functions are generated only if they are needed. The following code will cause each function to be generated:

Empty e1;                    // default constructor;
                             // destructor
Empty e2(e1);                // copy constructor
e2=e1;                       // copy assignment operator

Note: The generated destructor is non-virtual unless it's for a class inheriting from a base class that itself declares a virtual destructor (in which case the function's virtualness comes from the base class).

As for the copy constructor and copy assignment operator, the compiler -generated versions simply copy each non-static data member of the source object over to the target object.

Example:

Consider a NamedObject template that allows to associate names with objects of type T:

template<typename T>
class NamedObject {
public:
    NamedObject(const char* name, const T& value);
    NamedObject(const std::string& name, const T& value);
    ...
private:
    std::string nameValue;
    T objectValue;
};

Because a constructor is declared in NamedObject, compilers won't generate a default constructor. But NamedObject declares neither copy constructor nor copy assignment operator, so compilers will generate those functions (if needed). Look, then, at this use of copy constructor:

NamedObject<int> no1("Smallest prime Number",2);
NamedObject<int> no2(no1);                            // calls copy constructor

The compiler-generated copy assignment operator usually behave coorect when the resulting copde is both legal and has a reasonable chance of making sense. If either of these tests fails, compilers will refuse to generate an operator= for your class.

Example:

Suppose NamedObject were defined like this, where nameValue is a reference to a string and objectValue is a _const_T:

template<typename T>
class NamedObject{
public:
    // this ctor no longer takes aconst name, because nameValue
    // is now a reference-to-non-const string. The char* constructor
    // is gone, because we must have a strng to refer to.
    NamedObject(std::string& name, const T& value);
    ...                        // as above, assume no operator= is decalared
private:
    std::string& nameValue;    // this is now a reference
    const T objectValue;       // this is now const
};

Now consider what should happen here:

std::string newDog("Persephone");
std::string oldDog("Satch");

NamedObject<int> p(newDog, 2);         // when I originally wrote this, our dog Persephone was about to have her second birthday
NamedObject<int> s(oldDog, 36);        // the family dog Satch (from my childhood) would be 36 if she were still alive
p=s;                                   // what should happed to the data members in p?

Before the assignment, both p.nameValue and s.nameValue refer to string objects, though not the same ones. How should assignment affect p.nameValue? After the assignment, should p.nameValue refer to the string referred to by s.nameValue, i.e., should the reference itself be modified? If so, that breaks new ground, because

C++ doesn't provide a way to make a reference refer to a different object.

Alternatively, should the string object to which p.nameValue refers be modified, thus affecting other objects that hold pointers or references to that string, i.e., objects not directly involved in the assignment? Is that what the compiler-generated copy assignment operator should do?

Because of this, C++ refuses to compile the code. If you want to support copy assignment in a class containing a reference member, you must define the copy assignment operator yourself. Compilers behave similarly for classes containing const members (such as objectValue in the modified class above). It's not legal to modify const members, so compilers are unsure how to treat them during an implicitly generated assignment function. Finally, compilers reject implicit copy assignment operators in derived classes that inherit from base classes declaring the copy assignment operator private. After all, compiler-generated copy assignment operators for derived classes are supposed to handle base class parts, too, but in doing so, they certainly can't invoke member functions the derived class has no right to call.

Things to Remember:

• Compilers may implicitly generate a class's default constructor, copy constructor, copy assignment operator, and destructor.