Default Constructors
A default constructor is a constructor that either has no parameters, or if it has parameters, all the parameters have default values.
If no user-defined constructor exists for a class A and one is needed, the compiler implicitly declares a constructor A::A(). This constructor is an inline public member of its class. The compiler will implicitly define A::A() when the compiler uses this constructor to create an object of type A. The constructor will have no constructor initializer and a null body.
The compiler first implicitly defines the implicitly declared constructors of the base classes and nonstatic data members of a class A before defining the implicitly declared constructor of A. No default constructor is created for a class that has any constant or reference type members.
A constructor of a class A is trivial if all the following are true:
- It is implicitly defined
- A has no virtual functions and no virtual base classes
- All the direct base classes of A have trivial constructors
- The classes of all the nonstatic data members of A have trivial constructors
If any of the above are false, then the constructor is nontrivial.
A union member cannot be of a class type that has a nontrivial constructor.
Like all functions, a constructor can have default arguments. They are used to initialize member objects. If default values are supplied, the trailing arguments can be omitted in the expression list of the constructor. Note that if a constructor has any arguments that do not have default values, it is not a default constructor.
A copy constructor for a class A is a constructor whose first parameter is of type A&, const A&, volatile A&, or const volatile A&. Copy constructors are used to make a copy of one class object from another class object of the same class type. You cannot use a copy constructor with an argument of the same type as its class; you must use a reference. You can provide copy constructors with additional parameters as long as they all have default arguments. If a user-defined copy constructor does not exist for a class and one is needed, the compiler implicitly creates a copy constructor, with public access, for that class. A copy constructor is not created for a class if any of its members or base classes have an inaccessible copy constructor.
The following code fragment shows two classes with constructors, default constructors, and copy constructors:
class X {
public:
// default constructor, no arguments
X();
// constructor
X(int, int , int = 0);
// copy constructor
X(const X&);
// error, incorrect argument type
X(X);
};
class Y {
public:
// default constructor with one
// default argument
Y( int = 0);
// default argument
// copy constructor
Y(const Y&, int = 0);
};
Copy Constructors
The copy constructor lets you create a new object from an existing one by initialization. A copy constructor of a class A is a non-template constructor in which the first parameter is of type A&, const A&, volatile A&, or const volatile A&, and the rest of its parameters (if there are any) have default values.
If you do not declare a copy constructor for a class A, the compiler will implicitly declare one for you, which will be an inline public member.
The following example demonstrates implicitly defined and user-defined copy constructors:
#include <iostream>
using namespace std;
struct A {
int i;
A() : i(10) { }
};
struct B {
int j;
B() : j(20) {
cout << "Constructor B(), j = " << j << endl;
}
B(B& arg) : j(arg.j) {
cout << "Copy constructor B(B&), j = " << j << endl;
}
B(const B&, int val = 30) : j(val) {
cout << "Copy constructor B(const B&, int), j = " << j << endl;
}
};
struct C {
C() { }
C(C&) { }
};
int main() {
A a;
A a1(a);
B b;
const B b_const;
B b1(b);
B b2(b_const);
const C c_const;
// C c1(c_const);
}
The following is the output of the above example:
Constructor B(), j = 20 Constructor B(), j = 20 Copy constructor B(B&), j = 20 Copy constructor B(const B&, int), j = 30
The statement A a1(a) creates a new object from a with an implicitly defined copy constructor. The statement B b1(b) creates a new object from b with the user-defined copy constructor B::B(B&). The statement B b2(b_const) creates a new object with the copy constructor B::B(const B&, int). The compiler would not allow the statement C c1(c_const) because a copy constructor that takes as its first parameter an object of type const C& has not been defined.
The implicitly declared copy constructor of a class A will have the form A::A(const A&) if the following are true:
- The direct and virtual bases of A have copy constructors whose first parameters have been qualified with const or const volatile
- The nonstatic class type or array of class type data members of A have copy constructors whose first parameters have been qualified with const orconst volatile
If the above are not true for a class A, the compiler will implicitly declare a copy constructor with the form A::A(A&).
The compiler cannot allow a program in which the compiler must implicitly define a copy constructor for a class A and one or more of the following are true:
- Class A has a nonstatic data member of a type which has an inaccessible or ambiguous copy constructor.
- Class A is derived from a class which has an inaccessible or ambiguous copy constructor.
The compiler will implicitly define an implicitly declared constructor of a class A if you initialize an object of type A or an object derived from class A.
An implicitly defined copy constructor will copy the bases and members of an object in the same order that a constructor would initialize the bases and members of the object.
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