How can I do to transmit an argument

Hello I have this code which works perfectly :

main.cpp :

#include <iostream>
#include "class1.hpp"

int main(){
Class1 class1;
return EXIT_SUCCESS;
}

class1.cpp

#include <iostream>
#include "class1.hpp"

Class1 :: Class1(){

class2;





}

class1.hpp

#ifndef DEF_CLASS1
#define DEF_CLASS1

#include "class2.hpp"

class Class1{

public:
Class1();
Class2 class2;

};

#endif

class2.cpp :

#include <iostream>
#include "class2.hpp"

Class2 :: Class2(){

std::cout << "Test OK1" << std::endl;


}

class2.hpp :

#ifndef DEF_CLASS2
#define DEF_CLASS2

class Class2{

public:
Class2();

};

#endif

When I compile and I execute the code I get "Test OK1" so It is okay. Right now I want to put an argument to the class 2 like this class2(std::string a) How can I do this because I tried a lot of things without success unfortunately.

Thank you very much !