[C++学习笔记]构造器析构器与this指针

1.       构造器：类里一种特殊类型的方法。

（1）       构造器名字和它所在类名字一样。

（2）       系统会在创建某个类的新实例后立即自动调用这个类的构造器。

（3）       构造器永远不返回任何值。

#include <iostream> class Rectangle { public: unsigned width,height; //Constructor: Rectangle(unsigned x=0,unsigned y=0); void setSize(unsigned x,unsigned y); unsigned area(); unsigned perimeter(); bool isSquare(); protected: private: }; //Define the constructor: Rectangle::Rectangle(unsigned x,unsigned y) { width = x; height = y; } void Rectangle::setSize(unsigned x,unsigned y) { width=x; height=y; } unsigned Rectangle::area() { return (width*height); } unsigned Rectangle::perimeter() { return(width+width+height+height); } bool Rectangle::isSquare() { if (width==height) { return true; } else{ return false; } } int main() { unsigned width=25; unsigned height=25; //Rectangle myRectangle; //myRectangle.setSize(width,height); Rectangle myRectangle(width,height); std::cout<<"Area is: "<<myRectangle.area()<<"./n"; std::cout<<"Perimeter is: "<<myRectangle.perimeter()<<"./n"; std::cout<<"The rectengle "; if (myRectangle.isSquare()) { std::cout<<"is also"; }else { std::cout<<"is not"; } std::cout<<" a square"<<"./n/n"; return 0; }

2.       析构器：销毁对象

#include <iostream> #include <string> #include <fstream> class StoreQuote{ public: std::string quote,speaker; std::ofstream fileOutput; StoreQuote(); //Destructor ~StoreQuote(); void inputQuote(); void inputSpeaker(); bool write(); }; StoreQuote::StoreQuote() { fileOutput.open("quotes.txt",std::ios::app); } //Define the destructor StoreQuote::~StoreQuote() { fileOutput.close(); } void StoreQuote::inputQuote(){ std::getline(std::cin,quote); } void StoreQuote::inputSpeaker(){ std::getline(std::cin,speaker); } bool StoreQuote::write(){ if (fileOutput.is_open()) { fileOutput<<quote<<"|"<<speaker<<"/n"; return true; }else { return false; } } int main() { StoreQuote quote; std::cout<<"Enter a quotation(without quotation marks):/n"; quote.inputQuote(); std::cout<<"Enter the person to whom quotes is attributed:/n"; quote.inputSpeaker(); if (quote.write()) { std::cout<<"Written!/n"; }else{ std::cout<<"The data could not be written!/n"; return 1; } return 0; }

3.       this指针：代表当前对象（因为某个类并不知道你将使用什么名字去创建一个该类的对象）。用途是避免二义性，尤其在引用变量和属性的时候。使用原则：如果代码不存在二义性，就不必使用this指针。

#include <iostream> class Rectangle { public: unsigned width,height; //Constructor: Rectangle(unsigned width=0,unsigned height=0); void setSize(unsigned width,unsigned height); unsigned area(); unsigned perimeter(); bool isSquare(); protected: private: }; //Define the constructor: Rectangle::Rectangle(unsigned width,unsigned height) { this->width = width; this->height = height; } void Rectangle::setSize(unsigned width,unsigned height) { this->width = width; this->height = height; } unsigned Rectangle::area() { return (width*height); } unsigned Rectangle::perimeter() { return(width+width+height+height); } bool Rectangle::isSquare() { if (width==height) { return true; } else{ return false; } } int main() { unsigned width=25; unsigned height=25; //Rectangle myRectangle; //myRectangle.setSize(width,height); Rectangle myRectangle(width,height); std::cout<<"Area is: "<<myRectangle.area()<<"./n"; std::cout<<"Perimeter is: "<<myRectangle.perimeter()<<"./n"; std::cout<<"The rectengle "; if (myRectangle.isSquare()) { std::cout<<"is also"; }else { std::cout<<"is not"; } std::cout<<" a square"<<"./n/n"; return 0; }