class person { std::string name; int age; public: person(const std::string& name, int age) : name(name), age(age) { } }; int main() { person a("Bjarne Stroustrup", 60); person b(a); // What happens here? b = a; // And here? } Why constructor with 2 argument parameter accepts copy object as parameter. We calling constructor with 1 argument person b(a) with different type and it works?
How ?
It doesn't. This line of code:
person b(a); invokes implicitly defined person's copy constructor. The one generated by a compiler. It would also invoke a copy constructor if you had:
person b = a; That constructor accepts one parameter of type person&, const person&, volatile person& or const volatile person&. In your case that would be the object a of type person. It does not call the following constructor:
person(const std::string& name, int age) 
person b(a); This calls the copy constructor of person that's generated by the compiler for you. The compiler generated version looks somewhat like this (naive version) :
person(const person& other) { name = other.name; age = other.age; } b = a; // And here? This calls the copy assignment operator which is also generated by the compiler for you which does more or less the same as the copy constructor in this case.
The compiler usually generates some constructors for you, including the copy constructor person::person(const person &) used by person b(a); and assignment operators, including the copy assignment operator person & person::operator=(const person &) used by b = a;.
A good learning exercise is that if you don't know what your program automatically do, implement it yourself to understand how things work. Here is a good website that explains the issue you have. It's about how the constructors works in C++
class person { std::string name; int age; public: person(const std::string& name, int age) : name(name), age(age) { } /** * If you do nothing this is what is automaticaly impremented */ personne(const person p) { this.name = p.name; // or p.getName() this.age = p.age; // or p.getAge() } person person::operator=(const person & p) { if(this == &p) return *this; this.name = p.name; // or p.getName() this.age = p.age; // or p.getAge() return *this; } }; int main() { person a("Bjarne Stroustrup", 60); person b(a); // copy constructor b = a; // assignment operator overloading } 
= default or = delete.
