【14】建構式和解構式

建構式、默認建構式、拷貝建構式、深拷貝、淺拷貝、解構式、常成員函式，blabla一坨概念，云里霧里，今天我們就來好好理一理這些概念...

1、建構式

#include "iostream"
#include "string"

using namespace std;

class base {
public:
	int a;
	base() {}
	base(int m_a) :a(m_a) {
		cout << "base()" << endl;
	}
	base& operator+(base &b);
	void operator=(base &a);
};

class derive : public base {
public:
	int a;
	int b;
	derive(int a) :a(a) {
		cout << "init a" << endl;
	}
};

base& base::operator+(base &b)
{
	b.a= this->a + b.a;
	return b;
}

void base::operator=(base& a)
{
	this->a = a.a;
}

int main()
{
	base a(5);
	derive b(1);
	cout << b.base::a << endl;
	cout << b.a << endl;
}

對于這種父類子類有相同成員變數的情況，定義子類物件時，會通過命令空間來保留兩份a，那么如何初始化基類的a值呢？

#include "iostream"
#include "string"

using namespace std;

class base {
public:
	int a;
	//base() {}
	base(int m_a) :a(m_a) {
		cout << "base()" << endl;
	}
	base& operator+(base &b);
	//void operator=(base &a);
	void printHello() {
		cout << "this is a test" << endl;
	}
};

class derive : public base {
public:
	int a;
	int b;
	derive(int base_a, int a, int b) :a(a),b(b),base(base_a) {
		cout << "init a" << endl;
	}
};

base& base::operator+(base &b)
{
	b.a= this->a + b.a;
	return b;
}


int main()
{
	base a(5);
	derive b(1,2,3);
	cout << b.base::a << endl;
	cout << b.a << endl;
}

如何父類只有帶參的建構式，則子類建構式中必須顯式呼叫父類的帶參建構式，如上，

默認建構式：

2、拷貝建構式

#include "iostream"
#include "string"

using namespace std;

class base {
public:
	int a;
	//base() {}
	base(int m_a) :a(m_a) {
		cout << "base()" << endl;
	}
	base& operator+(base &b);
	//void operator=(base &a);
	void printHello() {
		cout << "this is a test" << endl;
	}
};

class derive : public base {
public:
	int a;
	int b;
	derive(int base_a, int a, int b) :a(a),b(b),base(base_a) {
		cout << "init a" << endl;
	}
};

base& base::operator+(base &b)
{
	b.a= this->a + b.a;
	return b;
}


int main()
{
	base a(5);
	derive b(1,2,3);
	cout << b.base::a << endl;
	cout << b.a << endl;
	/*************** 拷貝建構式 *****************/
	derive k = b;
	cout << "k.a = " << k.base::a << endl;
}

當類中沒有定義拷貝建構式時，編譯器會默認提供一個拷貝建構式，進行成員變數之間的拷貝，(這個拷貝操作是淺拷貝)

3、常成員函式

#include "iostream"
#include "string.h"

using namespace std;

class base {
public:
	int a;
	base() {
		cout << "base()" << endl;
	}
	base(int m_a) : a(m_a) {
		cout << "base(" << m_a << ")" << endl;
	}
};

class derive : public base {
public:
	int a;
	int b;
	derive(int m_a, int m_b, int m_c) : a(m_a),b(m_b),base(m_c) {
		cout << "derive(" << m_a << "," << m_b << "," << m_c << ")" << endl;
	}
	void printHello();
	void printHello() const;
};

void derive::printHello()
{
	cout << "printHello()" << endl;
}

void derive::printHello() const
{
	cout << "printHello() const" << endl;
}

int main()
{
	const derive d(1,2,3);
	d.printHello();
	cout << "d.base::a = " << d.base::a << endl;
	return 0;
}

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parallels@parallels-vm:~$ g++ main.cpp -o main;./main
base(3)
derive(1,2,3)
printHello() const
d.base::a = 3