Can the constructor be defined out of the class? - c++

Question

Why gcc cannot compile successful of the codes below?
Can the constructor be defined out of the class?
#include <string>
using std::string;
class Person{
public:
Person(const string &a, const string &b);
private:
string name, address;
};
Person::Person(const string &a, const string &b){
name(a);
address(b);
}
Thanks!

Answer 1

Because neither name nor address are callable. You probably meant to put them into a member-initializer-list.
Person::Person(const string &a, const string &b)
: name(a), address(b)
{
}

Answer 2

Your syntax is wrong:
Person::Person(const string &a, const string &b) : name(a), address(b) {}

Answer 3

You just wrote it wrong. It should be:
Person::Person(const string &a, const string &b) : name(a), address(b) { }
In principle, and very much in practice, too, you can and should define member functions outside the class definition to decouple the code base and reduce compile times.

Answer 4

This is called separation of implementation and declaration. It is actually a good idea to keep your implementations separately, in a cc or cpp file.
Thus, in your header:
//Person.h
#ifndef PERSON_H // <---- include header guards in your headers
#define PERSON_H
#include <string>
//using std::string; <--- you should remove this line, you don't want to import namespaces
// in your header file, or else they are imported in all
// files including this header
class Person{
public:
Person(const std::string &a, const std::string &b);
private:
std::string name, address; // qualify your names in the header
};
#endif
and your implementation file:
//Person.cpp
#include "Person.h"
using namespace std; // <---- if you wish, import the std namespace in your global namespace
// in the implementation file
Person::Person(const string &a, const string &b):
name(a), // <---- correct syntax of initializer lists
address(b)
{
}