C++基类与派生类的构造函数

缺省构造函数的调用关系

通过下面的例子，我们来看一下基类与派生的构造函数的调用顺序。创建时先基类后派生类。销毁时先派生类后基类。

#include <iostream>
#include <string>
using namespace std;

class CBase {
    string name;
    int age;
public:
    CBase() {
        cout << "BASE" << endl;
    }
    ~CBase() {
        cout << "~BASE" << endl;
    }
};

class CDerive : public CBase {
public:
    CDerive() {
        cout << "DERIVE" << endl;
    }
    ~CDerive() {
        cout << "~DERIVE" << endl;
    }
};

int main ( )
{
    CDerive d;

    return 0;
}

运行结果：
BASE
DERIVE
~DERIVE
BASE

有参数时的传递

当有参数时，参数必须传送给基类。注意例子中传递的方法(第8行、第19行)。

#include <iostream>
#include <string>
using namespace std;

class CBase {
    string name;
public:
    CBase(string s) : name(s) {
        cout << "BASE: " << name << endl;
    }
    ~CBase() {
        cout << "~BASE" << endl;
    }
};

class CDerive : public CBase {
    int age;
public:
    CDerive(string s, int a) : CBase(s), age(a) {
        cout << "DERIVE: " << age << endl;
    }
    ~CDerive() {
        cout << "~DERIVE" << endl;
    }
};

int main ( )
{
    CDerive d("小雅", 27);

    return 0;
}

运行结果：
BASE：小雅
DERIVE：27
~DERIVE
~BASE

祖孙三代的参数传递

当有三层继承时，参数要一层一层地传递下去（第30行、第19行、第8行）。

#include <iostream>
#include <string>
using namespace std;

class CBase {
    string name;
public:
    CBase(string s) : name(s) {
        cout << "BASE: " << name << endl;
    }
    ~CBase() {
        cout << "~BASE" << endl;
    }
};

class CDerive : public CBase {
    int age;
public:
    CDerive(string s, int a) : CBase(s), age(a) {
        cout << "DERIVE: " << age << endl;
    }
    ~CDerive() {
        cout << "~DERIVE" << endl;
    }
};

class CSon : public CDerive {
    string id;
public:
    CSon(string s1, int a, string s2) : CDerive(s1, a), id(s2) {
        cout << "SON: " << id << endl;
    }
    ~CSon() {
        cout << "~SON" << endl;
    }
};

int main ( )
{
    CSon s("小雅", 27, "8503026");

    return 0;
}

运行结果：
BASE：小雅
DERIVE：27
SON：8503026
~SON
~DERIVE
~BASE

多重继承的参数传递

多重继承时参数的传递方法和上面一样，要注意的是两个基类的顺序。决定2个基类的顺序是知27行。将27行的CBase1和CBase2的顺序交换一下，其结果中BASE1和BASE2的顺序也随之改变，与第30行无关。

#include <iostream>
#include <string>
using namespace std;

class CBase1 {
    string name;
public:
    CBase1(string s) : name(s) {
        cout << "BASE1: " << name << endl;
    }
    ~CBase1() {
        cout << "~BASE1" << endl;
    }
};

class CBase2 {
    int age;
public:
    CBase2(int a) : age(a) {
        cout << "BASE2: " << age << endl;
    }
    ~CBase2() {
        cout << "~BASE2" << endl;
    }
};

class CDerive : public CBase1, public CBase2 {
    string id;
public:
    CDerive(string s1, int a, string s2) : CBase1(s1), CBase2(a), id(s2) {
        cout << "DERIVE: " << id << endl;
    }
    ~CDerive() {
        cout << "~DERIVE" << endl;
    }
};

int main ( )
{
    CDerive d("小雅", 27, "8503026");

    return 0;
}

BASE1：小雅
BASE2：27
DERIVE：8503026
~DERIVE
~BASE2
~BASE1