构造函数

基本概念

为什么需要他

语法演示

构造函数的基本应用

//以前的你是这样的
#include<bits/stdc++.h>
using namespace std;
class Complex
{
    public:
        double real,imag;
        void getComplex(void);    
}; 
void Complex::getComplex(void)
{
    if(imag==0.0) std::cout<<real<<endl;
    else if(imag>0.0) std::cout<<real<<"+"<<imag<<"i"<<endl;
    else  std::cout<<real<<imag<<"i"<<endl;
}
int main(void)
{
    Complex c1;
    Complex *c2=new Complex;
    c1.real=3,c1.imag=4;
    c2->real=7,c2->imag=8;
    c1.getComplex();
    c2->getComplex();
    return 0;
}

//现在是这样的
#include <bits/stdc++.h>
using namespace std;
class Complex
{
    public:
        double real, imag;
        void getComplex(void);
        Complex(double r, double i = 0);
        Complex(const Complex &c1, const Complex &c2);
};
void Complex::getComplex(void)
{
    if (imag == 0.0)
        std::cout << real << endl;
    else if (imag > 0.0)
        std::cout << real << "+" << imag << "i" << endl;
    else
        std::cout << real << imag << "i" << endl;
}
Complex::Complex(double r, double i)
{
    real = r;
    imag = i;
}
Complex::Complex(const Complex &c1, const Complex &c2)
{
    real = c1.real + c2.real;
    imag = c1.imag + c2.imag;
}
int main(void)
{
    Complex c1(2);                   //OK
    Complex *c2 = new Complex(3, 4); //OK
    Complex c3;                      //Error
    Complex *c4 = new Complex;       //Error
    c1.getComplex();  //2
    c2->getComplex(); //3+4i
    Complex c3(c1, *c2);
    c3.getComplex(); //5+4i
    return 0;
}

构造函数在数组中的应用

构造函数没有参数或只有一个参数

#include<bits/stdc++.h>
using namespace std;
class CSample
{
    private:
        int x;
    public:
        CSample(void);
        CSample(int);
};
CSample::CSample(void)
{
    std::cout<<"Constructor No.1 Called"<<endl;
}
CSample::CSample(int n)
{
    x=n;
    std::cout<<"Constructor No.2 Called"<<endl;
}
int main(void)
{
    cout<<"Step 1"<<endl;
    CSample s1[2];//未声明参数
    cout<<"Step 2"<<endl;
    CSample s2[2]={4,5};//两个对象全部声明参数
    cout<<"Step 3"<<endl;
    CSample s3[2]={2};//仅有一个对象声明参数
    cout<<"Step 4"<<endl;
    CSample *s4=new CSample[2];//未声明参数
    return 0;
    //Output:
    /*Step 1
    Constructor No.1 Called
    Constructor No.1 Called
    Step 2
    Constructor No.2 Called
    Constructor No.2 Called
    Step 3
    Constructor No.2 Called
    Constructor No.1 Called
    Step 4
    Constructor No.1 Called
    Constructor No.1 Called*/
}

构造函数有两个（多个参数）

#include<bits/stdc++.h>
using namespace std;
class CSample
{
    private:
        int x,y;
    public:
        CSample(void);//无参构造函数
        CSample(int);//单参构造函数
        CSample(int,int);//双参构造函数
        void getSample(void);
};
CSample::CSample(void)
{
    std::cout<<"Constructor No.1 Called"<<endl;
}
CSample::CSample(int n)
{
    x=n;
    std::cout<<"Constructor No.2 Called"<<endl;
}
CSample::CSample(int m,int n)
{
    x=m;
    y=n;
    std::cout<<"Constructor No.3 Called"<<endl;
}
void CSample::getSample(void)
{
    std::cout<<x<<ends<<y<<endl;
}
int main(void)
{
    CSample s1[3];//无参初始化
    int a=0,b=0;
    for(int i=0;i<3;i++)
    {
        cin>>a>>b;//1 2 3 4 5 6
        s1[i]=CSample(a,b);//双参初始化，注意这里CSample(a,b)生成的是临时对象，赋给s1[i]后
        s1[i].getSample();                                        //自动消亡，下面同理
    }
    for(int i=0;i<3;i++)
    {
        cin>>a;//7 8 9 
        s1[i]=CSample(a);//单参初始化 且是全部重新初始化，不单单是在原来的基础上把x改掉
        s1[i].getSample();
    }
    for(int i=0;i<3;i++)
    {
        s1[i]=CSample();//无参初始化
    }
    CSample *s2[3];//三个对象指针
    for(int i=0;i<3;i++)
    {
        std::cin>>a>>b;//1 2 3 4 5 6
        s2[i]=new CSample(a,b);//new初始化，注意这里生成的反而不是临时对象，赋值后不会立刻消亡
        s2[i]->getSample();
    }
    delete [] s2;
    return 0;
}
/*Output    
Constructor No.1 Called
Constructor No.1 Called
Constructor No.1 Called
Constructor No.3 Called
1 2
Constructor No.3 Called
3 4
Constructor No.3 Called
5 6
Constructor No.2 Called
7 -1(不定值)
Constructor No.2 Called
8 -1
Constructor No.2 Called
9 -1
Constructor No.1 Called
Constructor No.1 Called
Constructor No.1 Called
Constructor No.3 Called
1 2
Constructor No.3 Called
3 4
Constructor No.3 Called
5 6
*/