1.构造函数、析构函数与拷贝构造函数介绍
构造函数
1.构造函数不能有返回值
2.缺省构造函数时,系统将自动调用该缺省构造函数初始化对象,缺省构造函数会将所有数据成员都初始化为零或空
3.创建一个对象时,系统自动调用构造函数
析构函数
1.析构函数没有参数,也没有返回值。不能重载,也就是说,一个类中只可能定义一个析构函数
2.如果一个类中没有定义析构函数,系统也会自动生成一个默认的析构函数,为空函数,什么都不做
3.调用条件:1.在函数体内定义的对象,当函数执行结束时,该对象所在类的析构函数会被自动调用;2.用new运算符动态构建的对象,在使用delete运算符释放它时。
拷贝构造函数
拷贝构造函数实际上也是构造函数,具有一般构造函数的所有特性,其名字也与所属类名相同。拷贝构造函数中只有一个参数,这个参数是对某个同类对象的引用。它在三种情况下被调用:
1.用类的一个已知的对象去初始化该类的另一个对象时;
2.函数的形参是类的对象,调用函数进行形参和实参的结合时;
3.函数的返回值是类的对象,函数执行完返回调用者。
【代码】
/* version: 1.0 author: hellogiser date: 2014/9/25 */#include "stdafx.h" #include <iostream> using namespace std;
class point { private: int x, y; public: point(int xx = 0, int yy = 0) { x = xx; y = yy; cout << "Constructor" << endl; } point(const point &p) { x = p.x; y = p.y; cout << "Copy Constructor" << endl; } ~point() { cout << "Destructor" << endl; } int get_x() { return x; } int get_y() { return y; } };
void f(point p) { // copy constructor cout << p.get_x() << " " << p.get_y() << endl; // destructor }
point g() { point a(7, 33); //constructor return a; // copy constructor temp object }
void test() { point a(15, 22); // constructor point b(a); //(1) copy constructor cout << b.get_x() << " " << b.get_y() << endl; // 15 22 f(b);// (2) copy constructor b = g(); // (3) copy constructor cout << b.get_x() << " " << b.get_y() << endl; // 7 33 }
int main() { test(); return 0; } /* Constructor Copy Constructor 15 22 Copy Constructor 15 22 Destructor Constructor Copy Constructor Destructor Destructor 7 33 Destructor Destructor */
2. 继承关系中构造函数执行顺序
(1)任何虚拟基类(virtual)的构造函数按照它们被继承的顺序构造;
(2)任何非虚拟基类(non-virtual)的构造函数按照它们被继承的顺序构造;
(3)任何成员对象(data member)的构造函数按照它们声明的顺序调用;
(4)类自己的构造函数(self)。
【代码】
/* version: 1.0 author: hellogiser date: 2014/9/27 */#include "stdafx.h" #include <iostream> using namespace std;
class OBJ1 { public: OBJ1() { cout << "OBJ1\n"; } };
class OBJ2 { public: OBJ2() { cout << "OBJ2\n"; } };
class Base1 { public: Base1() { cout << "Base1\n"; } };
class Base2 { public: Base2() { cout << "Base2\n"; } };
class Base3 { public: Base3() { cout << "Base3\n"; } };
class Base4 { public: Base4() { cout << "Base4\n"; } };
class Derived : public Base1, virtual public Base2, public Base3, virtual public Base4 { public: Derived() : Base4(), Base3(), Base2(), Base1(), obj2(), obj1() { cout << "Derived.\n"; } protected: OBJ1 obj1; OBJ2 obj2; };
void test() { Derived aa; cout << "This is ok.\n"; }
int main() { test(); return 0; } /* Base2 Base4 Base1 Base3 OBJ1 OBJ2 Derived. This is ok. */
【代码2】
/* version: 1.0 author: hellogiser date: 2014/9/27 */#include "stdafx.h" #include <iostream> using namespace std;
class Base1 { public: Base1(int i) { cout << "Base1 " << i << endl; } };
class Base2 { public: Base2(int i) { cout << "Base2 " << i << endl; } };
class Base3 { public: Base3() { cout << "Base3 *" << endl; } };
class Derived : public Base2, public Base1, virtual public Base3 { public: Derived(int a, int b, int c, int d, int e) : Base1(a), b2(d), b1(c), Base2(b) { m = e; cout << "Derived.\n"; } protected: Base1 b1; Base2 b2; Base3 b3; int m; };
void test() { Derived aa(1, 2, 3, 4, 5); cout << "This is ok.\n"; }
int main() { test(); return 0; } /* Base3 * Base2 2 Base1 1 Base1 3 Base2 4 Base3 * Derived. This is ok. */
分析:
(1) virtual
按照继承顺序:Base3
第一步:先继承Base3,在初始化列表里找不到Base3(), 则调用Base3里的默认构造函数Base3(),打印"Base3 *"
(2)non-virtual
按照继承顺序:Base2,Base1
第二步:继承Base2,在初始化列表中找Base2(b),调用Base2的构造函数Base2(2),打印"Base2 2"
第三步:继承Base1,在初始化列表中找Base1(a),调用Base1的构造函数Base1(1),打印"Base1 1"
(3)data member
按照申明顺序:b1,b2,b3
第四步:构造b1,在初始化列表中找b1(c),调用Base1的构造函数Base1(3),打印"Base1 3"
第五步:构造b2,在初始化列表中找b2(d),调用Base2的构造函数Base1(4),打印"Base2 4"
第六步:构造b3,在初始化列表中找不到b3(),调用Base3的构造函数Base3(),打印"Base3 *"
(4)self
第7步:执行自己的构造函数体,输出"Derived."