C / C++ / MFC Discussion Boards

Re: Problem with return value of private variable within class

Cedric Moonen25-Mar-09 21:14

25-Mar-09 21:14

Michael Randolph wrote:
I thought once you instantiate the object it sets the variables that I have for parameters in the function? Where am I forgetting something? This should work as far as I can see.

I think you are seriously mistaken here: the compiler is no magic. You declare a constructor and pass two variables in that constructor. If you don't do anything with those values, the compiler won't magically assign them to your class members. That would be crazy...
You have to assign them yourself as the other poster already told you.

Michael Randolph wrote:
I thought once you instantiate the object it sets the variables that I have for parameters in the function?

Once again, the compiler is no magic, it can't guess what you want to do with the variables. What if you pass less variables than you have class members (or more) ? What if they are in different order ? In most of the cases, a class is much more than two member variables (sometimes around 100 members) and in general you pass only a few of them in the constructor. So, how will the compiler know tho which member variables he has to assign those ? That would be crazy...

Cédric Moonen
Software developer

Charting control [v1.5]
OpenGL game tutorial in C++

Re: Problem with return value of private variable within class

Niklas L26-Mar-09 1:25

Niklas L

26-Mar-09 1:25

Calling a constructor is no different from calling a normal function in this sense. The arguments supplied needs to be handled explicitly.

home

Re: Problem with return value of private variable within class

Stuart Dootson25-Mar-09 23:41

Stuart Dootson

25-Mar-09 23:41

Change the constructor to assign the constructor parameters to the members, either using initialiser syntax:

Point::Point(float f_x, float f_y, float f_z) : x(f_x), y(f_y), z(f_z)
{
   cout << "We're in the constructor with arguments " << (int)this /*unique identifier*/ << endl;
}<pre>

or explicit assignments

<pre>Point::Point(float f_x, float f_y, float f_z)
{
   x = f_x;
   y = f_y;
   z = f_z;
   cout << "We're in the constructor with arguments " << (int)this /*unique identifier*/ << endl;
}

As Cedric said - the compiler ain't magic - it doesn't look for similarities between parameter names and data member names and decide to assign parameters to data members automatically!

Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p

Re: Problem with return value of private variable within class

Michael Randolph26-Mar-09 19:14

Michael Randolph

26-Mar-09 19:14

ahh I see. That makes so much more sense! Thanks for your responses. My next question deals with inheritance. It appears that the = operator declaration is not being inherited from my class Point. I am getting this compiling error:

g++ -o inheritance inheritance.cpp
inheritance.cpp: In function ‘int main()’:
inheritance.cpp:165: error: no match for ‘operator=’ in ‘vect3 = Vector::operator+(Vector&)(((Vector&)(& vect2)))’
inheritance.cpp:135: note: candidates are: Vector& Vector::operator=(Vector&)

here is my source:

#include <iostream>

using namespace std;

class Point
{
public:
	Point(float f_x = 1.0, float f_y = 1.0, float f_z = 1.0);
	~Point();

	// return by reference
	void getXYZ(float &X, float &Y, float &Z);
	float getX();
	float getY();
	float getZ();
	
	void setXYZ(float X, float Y, float Z);
	void setX(float X);
	void setY(float Y);
	void setZ(float Z);	
	
	// overloading = operator to deal with pointers
	Point operator = (Point &p);

private: 
	float x, y, z;

protected:

};

// define constructor with args
Point::Point(float f_x, float f_y, float f_z)
{
	cout << "We're in the constructor with arguments " << (int)this /*unique identifier*/ << endl;
	x = f_x;
	y = f_y;
	z = f_z;
}

// define destructor
Point::~Point()
{
	cout << "Destructing now!!! " << (int)this /*unique identifier*/ << endl;
}

// public function to return private value of variable x
float Point::getX()
{
	return x;
}

// public function to return private value of variable y
float Point::getY()
{
	return y;
}

// public function to return private value of variable z
float Point::getZ()
{
	return z;
}

// public function to change value of private variable x
void Point::setX(float X)
{
	x = X;
}

// public function to change value of private variable y
void Point::setY(float Y)
{
	y = Y;
}

// public function to change value of private variable z
void Point::setZ(float Z)
{
	z = Z;
}

// public function to change value of all three private variables
void Point::setXYZ(float X, float Y, float Z)
{
	/* slower
	setX(X);
	setY(Y);
	setZ(Z);*/
	// faster
	x = X;
	y = Y;
	z = Z;
	
}

// public function to return values of x,y,z by reference
void Point::getXYZ(float &X, float &Y, float &Z)
{
	X = getX();
	Y = getY();
	Z = getZ();
}

// overloading operator << (output stream)
ostream &operator <<(ostream &stream, Point &p)
{
	stream << p.getX() << " " << p.getY() << " " << p.getZ();
	return stream;
}

// overloading operator >> (input stream)
istream &operator >>(istream &stream, Point &p)
{
	float x, y, z;
	stream >> x >> y >> z;
	p.setXYZ(x, y, z);
	return stream;
}

// overloading = operator
Point Point::operator =(Point &p)
{
	setX(p.getX());
	setY(p.getY());
	setZ(p.getZ());
	// passing back value rather than address
	return *this;
}
//------------------------------------------
// Point Class End
//------------------------------------------

class Vector : public Point
{
public:
	Vector(float X=0, float Y=0, float Z=0);
	Vector operator +(Vector &p);
};

// initiate X, Y, Z from Point Class with : operator
Vector::Vector(float X, float Y, float Z) : Point(X, Y, Z)
{
//anything extra can go here
}

Vector Vector::operator +(Vector &p)
{
	Vector outV;
	outV.setX(getX() + p.getX());
	outV.setY(getY() + p.getY());
	outV.setZ(getZ() + p.getZ());
	return outV;
}

int main()
{
	// instance of the Vector Class
	Vector vect1(1, 2, 3);

	Vector vect2(5, 4, 2);

	Vector vect3;

	vect3 = vect1 + vect2;

	cout << vect3 << endl;
	
}

Re: Problem with return value of private variable within class

Stuart Dootson26-Mar-09 21:32

Stuart Dootson

26-Mar-09 21:32

OK - a couple of issues there. Firstly, your operator= isn't inherited because the signature of operator= in Point doesn't conform to what the generated operator= in Vector wants to see - it wants this:

Point& operator = (Point const &p);

i.e. take a const reference and return a non-const reference. Having said that, returning Point by value rather than reference is also a conformant signature, as C++ function signatures depend on parameter types and CV qualification (for methods), not return value - but why return Point by value anyway?

Secondly, make your getters const methods - then they can be used on const references, as (now) taken by your operator=:

void getXYZ(float &X, float &Y, float &Z) const;
float getX() const;
float getY() const;
float getZ() const;

If you make those two changes, then your Vector's generated operator= will compile successfully.

Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p

Re: Problem with return value of private variable within class

Michael Randolph27-Mar-09 2:40

Michael Randolph

27-Mar-09 2:40

Thank you so much for your help. Looks like I have a bit of studying to do. I am not totally sure how this works but it does. Here is the fixed source in case someone needs to check it out.

#include <iostream>

using namespace std;

class Point
{
public:
	Point(float f_x = 1.0, float f_y = 1.0, float f_z = 1.0);
	~Point();
	
	void setXYZ(float X, float Y, float Z);
	void setX(float X);
	void setY(float Y);
	void setZ(float Z);

	// return by reference
	void getXYZ(float &X, float &Y, float &Z);
	float getX() const;
	float getY() const;
	float getZ() const;
	
	// overloading = operator to deal with pointers
	Point& operator =(Point const &p);

private: 
	float x, y, z;

protected:

};

// define constructor with args
Point::Point(float f_x, float f_y, float f_z)
{
	cout << "We're in the constructor with arguments " << (int)this /*unique identifier*/ << endl;
	x = f_x;
	y = f_y;
	z = f_z;
}

// define destructor
Point::~Point()
{
	cout << "Destructing now!!! " << (int)this /*unique identifier*/ << endl;
}

// public function to return private value of variable x
float Point::getX() const
{
	return x;
}

// public function to return private value of variable y
float Point::getY() const
{
	return y;
}

// public function to return private value of variable z
float Point::getZ() const
{
	return z;
}

// public function to change value of private variable x
void Point::setX(float X)
{
	x = X;
}

// public function to change value of private variable y
void Point::setY(float Y)
{
	y = Y;
}

// public function to change value of private variable z
void Point::setZ(float Z)
{
	z = Z;
}

// public function to change value of all three private variables
void Point::setXYZ(float X, float Y, float Z)
{
	/* slower
	setX(X);
	setY(Y);
	setZ(Z);*/
	// faster
	x = X;
	y = Y;
	z = Z;
	
}

// public function to return values of x,y,z by reference
void Point::getXYZ(float &X, float &Y, float &Z)
{
	X = getX();
	Y = getY();
	Z = getZ();
}

// overloading = operator
Point& Point::operator =(Point const &p)
{
	setX(p.getX());
	setY(p.getY());
	setZ(p.getZ());
	// passing back value rather than address
	return *this;
}

// overloading operator << (output stream)
ostream &operator <<(ostream &stream, Point &p)
{
	stream << p.getX() << " " << p.getY() << " " << p.getZ();
	return stream;
}

// overloading operator >> (input stream)
istream &operator >>(istream &stream, Point &p)
{
	float x, y, z;
	stream >> x >> y >> z;
	p.setXYZ(x, y, z);
	return stream;
}

//------------------------------------------
// Point Class End
//------------------------------------------

class Vector : public Point
{
public:
	Vector(float X=0, float Y=0, float Z=0);
	Vector operator +(Vector &p);
};

// initiate X, Y, Z from Point Class with : operator
Vector::Vector(float X, float Y, float Z) : Point(X, Y, Z)
{
//anything extra can go here
}

Vector Vector::operator +(Vector &p)
{
	Vector outV;
	outV.setX(getX() + p.getX());
	outV.setY(getY() + p.getY());
	outV.setZ(getZ() + p.getZ());
	return outV;
}

int main()
{
	// instance of the Vector Class
	Vector vect1(1, 2, 3);

	Vector vect2(5, 4, 2);

	Vector vect3;

	vect3 = vect1 + vect2;

	cout << vect3 << endl;
	
}

How can I remove vertical scrollbar from Microsoft web browser?

Md. Ali Naser Khan25-Mar-09 17:54

Md. Ali Naser Khan

25-Mar-09 17:54

I am using Microsoft web browser using Active X Control to display Power Point file. But when it is displaying in the web browser a vertical scroll bar is displaying. How can I remove this Scroll bar?

Re: How can I remove vertical scrollbar from Microsoft web browser? [modified]

Yajnesh Narayan Behera26-Mar-09 3:35

Yajnesh Narayan Behera

26-Mar-09 3:35

If the application is in MFC then you can call the method which hide scrollbar of your Activex control through IDispatch interface. I do n't remember exactly which property you have to set. In case it is pure c++, then you have to write your own helper class.

modified on Thursday, March 26, 2009 10:16 AM

"game" fast fingers

Aljaz11125-Mar-09 8:18

Aljaz111

25-Mar-09 8:18

Hi to everyone!

I am writing because i need help with making this "game". I am looking for anyone who is familiar with mfc more than i am to write me a code(or send me project) for following program:

- game randomly show characters and numbers from keyboard(ONLY ALPHABETIC and NUMBERS from 1-9) and player pushes appropriate buttons on keyboard
- characters must be shown on panel in specific timing
-there must be option to set up time for game with slider
- with progress bar time must be shown
- start and stop buttons for game
- on panel must be shown: HOW MANY BUTTONS PLAYER HIT and HOW MANY MISSED, how many buttons player didn't manage to hit in time and CHARACTER which must be pressed at the moment

I would be very grateful to anyone who would make this program alive;) This "game" or program is very important for me and must be made fast.

Thanks,
Best regards