C++構(gòu)造函數(shù)初始化順序詳解

2020-01-26 15:16:48

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1.構(gòu)造函數(shù)、析構(gòu)函數(shù)與拷貝構(gòu)造函數(shù)介紹

構(gòu)造函數(shù)

1.構(gòu)造函數(shù)不能有返回值
2.缺省構(gòu)造函數(shù)時(shí)，系統(tǒng)將自動(dòng)調(diào)用該缺省構(gòu)造函數(shù)初始化對(duì)象，缺省構(gòu)造函數(shù)會(huì)將所有數(shù)據(jù)成員都初始化為零或空
3.創(chuàng)建一個(gè)對(duì)象時(shí)，系統(tǒng)自動(dòng)調(diào)用構(gòu)造函數(shù)

析構(gòu)函數(shù)

1.析構(gòu)函數(shù)沒(méi)有參數(shù)，也沒(méi)有返回值。不能重載，也就是說(shuō)，一個(gè)類中只可能定義一個(gè)析構(gòu)函數(shù)
2.如果一個(gè)類中沒(méi)有定義析構(gòu)函數(shù)，系統(tǒng)也會(huì)自動(dòng)生成一個(gè)默認(rèn)的析構(gòu)函數(shù)，為空函數(shù)，什么都不做
3.調(diào)用條件：1.在函數(shù)體內(nèi)定義的對(duì)象，當(dāng)函數(shù)執(zhí)行結(jié)束時(shí)，該對(duì)象所在類的析構(gòu)函數(shù)會(huì)被自動(dòng)調(diào)用；2.用new運(yùn)算符動(dòng)態(tài)構(gòu)建的對(duì)象，在使用delete運(yùn)算符釋放它時(shí)。

拷貝構(gòu)造函數(shù)

拷貝構(gòu)造函數(shù)實(shí)際上也是構(gòu)造函數(shù)，具有一般構(gòu)造函數(shù)的所有特性，其名字也與所屬類名相同。拷貝構(gòu)造函數(shù)中只有一個(gè)參數(shù)，這個(gè)參數(shù)是對(duì)某個(gè)同類對(duì)象的引用。它在三種情況下被調(diào)用：

1.用類的一個(gè)已知的對(duì)象去初始化該類的另一個(gè)對(duì)象時(shí)；
2.函數(shù)的形參是類的對(duì)象，調(diào)用函數(shù)進(jìn)行形參和實(shí)參的結(jié)合時(shí)；
3.函數(shù)的返回值是類的對(duì)象，函數(shù)執(zhí)行完返回調(diào)用者。

【代碼】

復(fù)制代碼代碼如下:

/*
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. 繼承關(guān)系中構(gòu)造函數(shù)執(zhí)行順序

(1)任何虛擬基類(virtual)的構(gòu)造函數(shù)按照它們被繼承的順序構(gòu)造；
(2)任何非虛擬基類(non-virtual)的構(gòu)造函數(shù)按照它們被繼承的順序構(gòu)造；
(3)任何成員對(duì)象(data member)的構(gòu)造函數(shù)按照它們聲明的順序調(diào)用；
(4)類自己的構(gòu)造函數(shù)(self)。

【代碼】

復(fù)制代碼代碼如下:

/*
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.
*/

【代碼2】

復(fù)制代碼代碼如下:

/*
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

按照繼承順序：Base3

第一步：先繼承Base3,在初始化列表里找不到Base3(), 則調(diào)用Base3里的默認(rèn)構(gòu)造函數(shù)Base3()，打印"Base3 *"

(2)non-virtual

按照繼承順序：Base2，Base1

第二步：繼承Base2,在初始化列表中找Base2(b)，調(diào)用Base2的構(gòu)造函數(shù)Base2(2)，打印"Base2 2"

第三步：繼承Base1,在初始化列表中找Base1(a)，調(diào)用Base1的構(gòu)造函數(shù)Base1(1)，打印"Base1 1"

(3)data member

按照申明順序：b1,b2,b3