Why would someone provide an empty default constructor for a class?

Question

class complex {   float x,y; public:   complex(){} //constructor with no arguments //what is use of giving such constructor     complex(float z){x=y=z;}//constructor with 1 argument.     complex(float real,float imag)     {x=real;y=imag;}     friend complex sum(complex,complex);      friend void show(complex); };  complex sum(complex c1,complex c2) {   complex c3;   c3.x=c1.x+c2.x;   c3.y=c1.y+c2.y;   return (c3); }  void show (complex c) {   cout<<c.x<<"+j"<<c.y<<"\n"; } int main() {   complex p,q,r;   p=complex(2.5,3.9);   q=complex(1.6,2.5);   r=sum(p,q);    cout<<"p=";show(p);   cout<<"q=";show(q);   cout<<"r=";show(r);   return 0; } 

Answer 1

When you declare a class without any constructors, the compiler provides a default constructor (on demand) that default-initializes the members. (Note that default-initialization on built-ins such as float does nothing). However, once you define any constructor for the class, the default constructor is no longer automatically provided. Hence you must define your own default constructor if you still want one.