Correct declaration of constructor of class in namespace

I have a class which I want to be in a namespace. I have been doing it like

namespace ns {
    class A;
}

class ns::A {
    ...
public:
    A();
};

and I define the constructor in a separate file like

ns::A::A() {
    ...
}

My question is about the correct way of defining the constructor. Is that the correct way, or should I add the namespace to the declaration?

namespace ns {
    class A;
}

class ns::A {
    ...
public:
    ns::A();
};

And if that’s the case, how is the constructor defined in a separate file?

>Solution :

how is the constructor defined in a separate file?

You can do it as shown below:

header.h

#ifndef HEADER_H
#define HEADER_H 

namespace NS 
{   
    //class definition
    class A 
    {
        public:
        
            //declaration for default constructor 
            A();
            
            //declaration for member function 
            void printAge();
        private:
             int age; 
    };
}
#endif

source.cpp

#include"header.h" 
#include <iostream>
namespace NS 
{
    //define default constructor 
    A::A(): age(0)
    {
        std::cout<<"default consttuctor used"<<std::endl;
    }
    
    //define member function printAge 
    void A::printAge()
    {
        std::cout<<age<<std::endl;
    }
}

main.cpp

#include <iostream>
#include"header.h"

int main()
{
    NS::A obj; //this uses default constructor of class A inside namespace NS 
    
    obj.printAge();
    return 0;
}

Also don’t forget to use header guards inside the header file to avoid cyclic dependency(if any).

The output of the program can be seen here

Some of the changes that i made include:

1. Added include guard in header file header.h.
2. Added declarations for the default constructor and a member function called printAge inside class A inside the header file.
3. Defined the default constructor and the member function printAge inside source.cpp.
4. Used constructor initializer list in the default constructor of class A inside source.cpp.

Method 2

Here we use the scope resolution operator :: to be in the scope of the namespace NS and then define the member functions as shown below:

source.cpp

#include"header.h" 
#include <iostream>

//define default constructor 
NS::A::A(): age(0)
{
    std::cout<<"default consttuctor used"<<std::endl;
}

//define member function printAge 
void NS::A::printAge()
{
    std::cout<<age<<std::endl;
}

The output of method 2 can be seen here