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copied!<p>If I define the following class</p> <pre><code>class X {}; </code></pre> <p>The compiler will define the following methods:</p> <pre><code>X::X() {} // Default constructor. It takes zero arguments (hence default). X::~X() {} // Destructor X::X(X const& rhs) {}; // Copy constructor X& operator=(X const& rhs) {return *this;} // Assignment operator. </code></pre> <p>Note:<br> The default constructor is not built if <strong>ANY</strong> constructor is defined.<br> The other methods are not built if the user defines an alternative.</p> <p>What is slightly more interesting is the default implementation when we have members and a base:</p> <pre><code>class Y: public X { int a; // POD data int* b; // POD (that also happens to be a pointer) Z z; // A class }; // Note: There are two variants of the default constructor. // Both are used depending on context when the compiler defined version // of the default constructor is used. // // One does `default initialization` // One does `zero initialization` // Objects are zero initialized when // They are 'static storage duration' // **OR** You use the braces when using the default constructor Y::Y() // Zero initializer : X() // Zero initializer , a(0) , b(0) , z() // Zero initializer of Z called. {} // Objects are default initialized when // They are 'automatic storage duration' // **AND** don't use the braces when using the default constructor Y::Y() :X // Not legal syntax trying to portray default initialization of X (base class) //,a // POD: uninitialized. //,b // POD: uninitialized. ,z // Not legal syntax trying to portray default initialization of z (member) {} // // Note: It is actually hard to correctly zero initialize a 'automatic storage duration' // variable (because of the parsing problems it tends to end up a a function // declaration). Thus in a function context member variables can have indeterminate // values because of default initialization. Thus it is always good practice to // to initialize all members of your class during construction (preferably in the // initialization list). // // Note: This was defined this way so that the C++ is backward compatible with C. // And obeys the rule of don't do more than you need too (because we want the C++ // code to be as fast and efficient as possible. Y::Y(Y const& rhs) :X(rhs) // Copy construct the base ,a(rhs.a) // Copy construct each member using the copy constructor. ,b(rhs.b) // NOTE: The order is explicitly defined ,z(rhs.z) // as the order of declaration in the class. {} Y& operator=(Y const& rhs) { X::operator=(rhs); // Use base assignment operator a = rhs.a; // Use the assignment operator on each member. b = rhs.b; // NOTE: The order is explicitly defined z = rhs.z; // as the order of declaration in the class. return(*this); } Y::~Y() { Your Code first } // Not legal code. Trying to show what happens. : ~z() , ~b() // Does nothing for pointers. , ~a() // Does nothing for POD types , ~X() ; // Base class destructed last. </code></pre>

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