關於使用朋友函數的複數

Question

我想通過使用朋友函數和構造函數來初始化值，但得到錯誤'複雜1 ::複雜1（）的未定義引用'可以有人建議我去哪裏錯了。

#include<iostream> 
using namespace std; 

class complex1 
{ 
    float real,img; 
public: 
    complex1(); 
    complex1(float a,float b) 
    { 
     real=a; 
     img=b; 
    } 
    friend complex1 sum(complex1,complex1); 
    friend void display(complex1); 
}; 

complex1 sum(complex1 c1,complex1 c2) 
{ 
    complex1 c3; 
    c3.real=c1.real+c2.real; 
    c3.img=c1.img+c2.img; 
    return c3; 
} 

void display(complex1 c) 
{ 
    cout<<c.real<<"+j"<<c.img; 
} 

int main() 
{ 
    complex1 c1(100.9,200.9); 
    complex1 c2(50.9,50.9); 
    complex1 c=sum(c1,c2); 
    display(c);  //display and sum is given directly because it is friend 
    return 0; 
} 

Answer 1

它與您的friend功能的友善無關。您聲明構造函數：

complex1(); 

但您從未定義它。然後你在complex1 c3;的sum中使用它。您需要定義默認的構造函數：

complex1() { 
    //do something 
} 

Here is a live example.