C++拾遗(三)

1. declaration and definition

Any variable can be defined for only once. On the contrary, declaration is different. Extern declaration is not definition and does not allocate memory for the variable.

    2.   cin and getline

getline ignore space, so in case I wanna input something like space inside my string, getline is for sure a better choice.

    3.   something about vector

Vector is a useful class template, in order to use it, we need to include certain headfiles. Actually, to ensure we can have a continuous piece of memory, we tend to create an empty vector first, can increase its element dynamically.

    4.   about constructor

#include <iostream>
using namespace std;
class Complex{
    double real;
    double imag;
public:
    //无参构造函数
    Complex(void)
    {
        real = 0.0;
        imag = 0.0;
        cout<<"Constructor with no parameter gets called!"<<endl;
    }
    //一般构造函数
    Complex(double r, double i)
    {
        real = r;
        imag = i;
        cout<<"Normal constructor gets called!"<<endl;
    }
    //复制构造函数
    Complex(const Complex &s)
    {
        real = s.real;
        imag = s.imag;
        cout<<"Copy constructor gets called!"<<endl;
    }
    //类型转换构造函数
    Complex(double r)
    {
        real = r;
        imag = 0.0;
        cout<<"Type transform constructor gets called!"<<endl;
    }
    //等号运算符重载
    Complex& operator=(const Complex &s)
    {
        cout<<"Equality operator overload!"<<endl;
        if(this == &s)
        {
            return *this;
        }
        this->real = s.real;
        this->imag = s.imag;
        return *this;
    }
};
int main()
{
    Complex a,b;                       //constructor with no parameter
    Complex c(1.2,3.4);                //normal constructor
    Complex d = Complex(1.2,3.4);      //normal constructor
    Complex e = c;                     //copy constructor
    a = c;                             //a is already constructed, = operator overload
    b = 5.2;                           //type transform constructor
    Complex f(c);                      //copy constructor

}

this little piece of code outputs:

Constructor with no parameter gets called!
Constructor with no parameter gets called!
Normal constructor gets called!
Normal constructor gets called!
Copy constructor gets called!
Equality operator overload!
Type transform constructor gets called!
Equality operator overload!
Copy constructor gets called!
请按任意键继续. . .

Complex test1(const Complex& c)
{
    return c;
}

Complex test2(const Complex c)
{
    return c;
}

Complex test3()
{
    static Complex c(1.0,5.0);
    return c;
}

Complex& test4()
{
    static Complex c(1.0,5.0);
    return c;
}
int main()
{
    Complex a,b;                       //constructor with no parameter
    //Complex c(1.2,3.4);                //normal constructor
    //Complex d = Complex(1.2,3.4);      //normal constructor
    //Complex e = c;                     //copy constructor
    //a = c;                             //a is already constructed, = operator overload
    //b = 5.2;                           //type transform constructor 
    //Complex f(c);                      //copy constructor
    //// 下面函数执行过程中各会调用几次构造函数，调用的是什么构造函数？

    test1(a);      //copy copy
    test2(a);      //copy

    b = test3();   //normal copy = overload
    b = test4();   //normal overload

    test2(1.2);    //type transform, copy
    // 下面这条语句会出错吗？
    test1(1.2);     
    test1( Complex(1.2 ));