C++中显式指定调用父类的构造函数

在C++语法中，派生类中的构造函数操作是一个重要知识点，构造函数是用来初始化对象的。

基类构造函数总是先于派生类构造函数被调用。因为基类是派生类的基础，所以必须先创建基类。如果不指定要使用的基类构造函数，编译器就安排调用默认的基类构造函数。

规则：创建对象时首先调用基类的构造函数，然后调用派生类的构造函数；而销毁对象时首先调用派生类的析构函数，然后调用基类的析构函数。 

#pragma once

#include <iostream>
#include <iomanip>
using namespace std;
//基类
class Box
{
public:
	Box()
	{
		cout << "不带参数的父类构造函数\n";
	}
	Box(double lv, double wv, double hv) : length{ lv }, width{ wv }, height{ hv }	//显式调用父类的构造函数
	{
		cout << "带三个参数的父类构造函数\n";
	}
	Box(double side) : Box{ side, side, side }
	{
		cout << "带一个参数的父类构造函数\n";
	}

	~Box()
	{
	}

	double getLength()
	{
		return length;
	}
	double getWidth()
	{
		return width;
	}
	double getHeight()
	{
		return height;
	}
	double volumn() const
	{
		return length * width * height;
	}

	friend ostream& operator<<(ostream& stream, const Box& box);

protected:
	double length{ 1.0 };
	double width{ 1.0 };
	double height{ 1.0 };

private:

};

inline ostream& operator<<(ostream& stream, const Box& box)
{
	stream << "Box(" << setw(2) << box.length << ","
		   << setw(2) << box.width << ","
		   << setw(2) << box.height << ")";
}

#pragma once
#include "Box.h"
//派生类
class Carton : public Box
{
public:
	Carton(double lv, double wv, double hv, const string desc) : Box{ lv, wv, hv }, material{ desc }
	{
		cout << "带四个参数的子类构造函数\n";
	}
	Carton(const string desc) : material{ desc }
	{
		cout << "带一个参数的子类构造函数\n";
	}
	Carton(double side, const string desc) : Box{ side }, material{ desc }
	{
		cout << "带两个参数的子类构造函数\n";
	}
	Carton()
	{
		cout << "不带参数的子类构造函数\n";
	}

private:
	string material{ "纸盒" };
};

#include "Carton.h"

int main()
{
	Carton carton1;
	cout << "---------------------\n";
	Carton carton2{ "One" };
	cout << "---------------------\n";
	Carton carton3{ 4.0, 5.0, 6.0, "Four" };
	cout << "---------------------\n";
	Carton carton4{ 2.0, "Two" };
	cout << "---------------------\n";

	cout << "carton1 volumn is " << carton1.volumn() << endl;
	cout << "carton2 volumn is " << carton2.volumn() << endl;
	cout << "carton3 volumn is " << carton3.volumn() << endl;
	cout << "carton4 volumn is " << carton4.volumn() << endl;
	getchar();

	return 0;
}