Special member functions
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Special member functions[1] in C++ are functions which the compiler will automatically generate if they are used, but not declared explicitly by the programmer. The special member functions are:
- Default constructor (if no other constructor is explicitly declared)
- Copy constructor
- Copy assignment operator
- Destructor
In these cases the compiler generated versions of these functions perform a memberwise operation. For example the compiler generated destructor will destroy each sub-object (base class or member) of the object.
The compiler generated functions will be public, non-virtual[2] and the copy constructor and assignment operators will receive const& parameters (and not be of the alternative legal forms).
Example
The following example depicts two classes: Explicit for which all special member functions are explicitly declared and Implicit for which none are declared.
<source lang="cpp"> class Explicit {
friend class Implicit;
string msg;
public:
Explicit(void) : msg("")
{
cout << "Default constructor " << msg << endl;
}
Explicit(const string& value) : msg(value)
{
cout << "Non-default constructor " << msg << endl;
}
Explicit(const Explicit& other) : msg(other.msg)
{
cout << "Copy constructor " << msg << endl;
}
Explicit& operator=(const Explicit& other)
{
cout << "Copy assignment operator " << msg << endl;
if (this != &other)
{
msg = other.msg;
}
return *this;
}
~Explicit(void)
{
cout << "Destructor " << msg << endl;
}
};
class Implicit : public Explicit {
int i;
void* p;
Explicit member;
public:
void Spew(void) { cout << "Implicit(" << msg << ", " << member.msg << ")" << endl; };
}; </source>
In this case the class Implicit has not explicitly defined the destructor and the compiler will create a destructor equivalently to this: <source lang="cpp"> // Sub-objects are destroyed in the opposite order to their construction Implicit::~Implicit() {
member.~Explicit(); // destroy member (void)p; // do nothing for p, void* has no destructor (void)i; // do nothing for i, int has no destructor ~Explicit(); // call the base class's destructor
} </source>
References
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