C++中賦值函數(shù)和拷貝構造函數(shù)（舉例說明）

拷貝構造函數(shù)發(fā)生在對象還沒有創(chuàng)建，需要創(chuàng)建時

如：   MyClass obj1(1);  

            MyClassobj3 = obj1;

賦值操作符重載僅發(fā)生在對象已經執(zhí)行過構造函數(shù)，即已經創(chuàng)建的情況下

如：

                 MyClass obj1(1);

                 MyClass obj3;

                obj3 = obj1;

具體代碼如下：

class MyClass

{

public:

                MyClass( int i = 0)

                {

                                cout << i;

                }

                MyClass( const MyClass &x)

                {

                                cout << 2;

                }

                 MyClass &operator=(const MyClass & x)

                {

                                cout << 3;

                                 return *this ;

                }

                ~MyClass()

                {

                                cout << 4;

                }

};

int main()

{

                 MyClass obj1(1);

                 MyClass  obj2(2);

                 MyClass obj3;

                obj3 = obj1;

                 return 0;

}

輸出：1203444

先創(chuàng)建對象obj1，調用構造函數(shù)并初始化為1，輸出1

再創(chuàng)建對象obj2，調用構造函數(shù)并初始化為2，輸出2

創(chuàng)建對象obj3，調用構造函數(shù)，使用默認值為0，輸出0

obj1賦值給obj3時，調用賦值函數(shù)，輸出3

再依次析構，析構的順序和調用構造的順序相反

先析構obj3,在析構obj2，最后析構obj1

class MyClass

{

public:

                MyClass( int i = 0)

                {

                                cout << i;

                }

                MyClass( const MyClass &x)

                {

                                cout << 2;

                }

                 MyClass &operator=(const MyClass & x)

                {

                                cout << 3;

                                 return *this ;

                }

                ~MyClass()

                {

                                cout << 4;

                }

};

int main()

{

                 MyClass obj1(1);

                 MyClass   obj2(2);

                 MyClass obj3 = obj1;

                 return 0;

}

輸出：122444

依次創(chuàng)建對象obj1,obj2，調用兩次構造，分別輸出初始化的值1，2

MyClass obj3 = obj1;由于賦值的時候obj3還沒有創(chuàng)建，所以會調用拷貝構造函數(shù)，輸出2

再依次析構三個對象