making initializer list work without parameters

[programmy]

normally I think that a constructor with an initializer list would work if
there were parameters involved but I am reading the book
SFML Game Development with examples and saw a header file
that did not include a parameter in the constructor,tried
recreating the whole thing but it didn't work

SFML example:

:::::::Game.h::::::::

#pragma once
#include "Window.h"
#include "World.h"
#include "Snake.h"


class Game {
public:
	Game(); <-no parameters
	~Game();

	void HandleInput();
	void Update();
	void Render();

	sf::Time GetElapsed();
	void RestartClock();

	Window* GetWindow();
private:
	Window m_window;
	sf::Clock m_clock;
	float m_elapsed;

	World m_world;
	Snake m_snake;
	
};


:::::::::::::Game.cpp file::::::::::::


#include "Game.h"


////////the Game constructor works without any parameters


Game::Game() : m_window("Snake", sf::Vector2u(800, 600)),  m_snake(m_world.GetBlockSize()), m_world(sf::Vector2u(800, 600))
{
	m_clock.restart();

	
	
	m_elapsed = 0.0f;

	
}

Game::~Game() {}




:::::::::::my code when I tried recreating it:::::::::

:::::::Mars.h:::::::::

#ifndef MARS_H
#define MARS_H
#include "Mercury.h"
#include "Venus.h"

class Mars
{
    public:
        Mars();



        virtual ~Mars();

    protected:

    private:
        Mercury m_mercury;
        Venus m_venus;

};

#endif // MARS_H


::::::Mars.cpp:::::::

#include "Mars.h"


#include <iostream>

using namespace std;


 //// how to make this work?I already defined getBlockSize(); in the Venus class

Mars::Mars()
: m_mercury(m_venus.getBlockSize())  
{

}



Mars::~Mars()
{
   
}

[Repeater]

What does

it didn't work

mean?

[Manga]

It could be that the program did not compile due to some error, or maybe it did compile but he did not get the result he was hoping for.

[Repeater]

Yup. Could be all sorts of things. Maybe the OP will come back and tell us. We're not psychic.

[Manga]

Damn it Jim! I'm a coder not a psychic!

[icy1]

If he needed to do more work to initialize his Venus object (that is, Venus has no default constructor), then he'd have a problem with his getBlockSize() call. Solution would be to properly initialize his Venus object before using it for other stuff:

#include <iostream>

class Mercury
{
public:
    Mercury(int block_size) {}
};

class Venus
{
public:
    Venus(int someparam) {}  // 1. If this line is here (no default constr)
    int GetBlockSize() { return 500; }
};

class Mars
{
public:
    Mars() : 
        venus_(24),  // 2. Then something like this would be needed
        mercury_(venus_.GetBlockSize()) 
    {        
    }

private:
    Mercury mercury_; 
    Venus venus_;
};

int main() 
{
    Mars m;

    return 0;
}

[Manga]

I get the feeling that OP is trying to initialize m_mercury member vaiables when he initates a Mars object. And he is trying to use some data provided by m_venus which is a member of Mars and not initialized yet.

This remindes me of a chicken and egg senario. Which came first?

What you can't see here is that Venus has a member called Mars.

[programmy]

forgot to mention that the error that pops out
is
error: no matching function to call to 'Mercury::Mercury(int)'

what I meant to find out is how did the SFML example make the

m_snake(m_world.GetBlockSize()) work ? if you take a look at Game::Game() at Game.cpp

and Game(); at Game.h no parameters are present for the initiliazer list , so how did it work?,

when I tried doing the same thing

with m_mercury(m_venus.getBlockSize()) I get an error

[Peter87]

Note that members are initialized in the order they are defined in the class definition, not in the order they appear in the member initializer list. This means that m_mercury is initialized before m_venus (even in icy1's code) which is a problem because you use m_venus to initialize m_mercury.

error: no matching function to call to 'Mercury::Mercury(int)'

This means that there is no Mercury constructor that takes an int as argument.

[Manga]

#include <iostream>

using namespace std;

class Venus
{
   char a;
    char b;
    
public:
   //A constructor for Venus. It takes parameters!
   Venus(char x, char y) : a(x), b(y) {}//Sets a to equal x and b to equal y.
   
      void geta() {
       cout << a;   
      }
       void getb() {
       cout << b;   
      }
       
};


class Mars
{
    Venus venus;
    int one;
public:
   Mars(); //This is a default constructor. No parameters.

   void showStuff() {
    cout << one << endl;
    venus.geta();
    cout << "\t";
    venus.getb();
   }
};

//Here we define the constructor and use an initailization list
Mars::Mars() : venus('A', 'B') {//Our Venus constructor is called.
    one = 1;
}




int main() 
{
   Mars m;
   
   m.showStuff();

    return 0;
}

[programmy]

figured it out, all had to do is specify an int in the Mercury constructor and it worked! thanks!