Programming Forums > Application Development > C++

clearbit · Jul 6th, 2005, 1:31 PM

ok its been a while since ive coded in C++ but im trying to learn about classes agian, and ive been reading a book ive had, just never had the time to read.(well till now). It talks about to initialize a data member(private) you have to make it the preamble to the constructor of the class..

For example

(Toggle Plain Text)

class Whatever
{
 : _xin (x)  //HERES WHERE MY QUESTIONS AT.
public:
  Construct(int xin) {std::cout<<xin;}
 ...
private:
const int _xin;
}

The question is can I just assign an integer to the _xin like this

(Toggle Plain Text)

: _xin = 1;

lame question i know but i dont have a compiler to test it right now..thnx.

Ancient Dragon · Jul 6th, 2005, 2:28 PM

Throw that book away immediately! :eek: just because someting is private does not mean it has to be initialized like that.

(Toggle Plain Text)

class MyClass
{
public:
	MyClass() {m_x = 0;}

private:
	int m_x;
};

The only time I initialize class variables is when it is a reference to something.

(Toggle Plain Text)

class MyClass
{
public:
	MyClass(int n) : m_r(n)
	{
		m_x = 0;
	}

private:
	int m_x;
	int& m_r;
};


int main(int argc, char* argv[])
{
	int x = 0;
	MyClass m(x);
	return 0;
}

clearbit · Jul 6th, 2005, 2:41 PM

opps sorry, this is what it actualy looks like in the book

(Toggle Plain Text)

class Whatever
{

public:
  Construct(int xin) 
   : _xin (x)  //HERES WHERE MY QUESTIONS AT.
{
  std::cout<<xin;
}
 ...
private:
const int _xin;
}

clearbit · Jul 6th, 2005, 2:42 PM

Whats wrong with initializing it like this?? Security flaw?? what?

Animatronic · Jul 6th, 2005, 2:51 PM

Nothing it wrong with it. Using an initialiser list is usually the preferred way to intialise members inside the class as it gives the greatest posible performance (members only get constructed once instead of a posible two times).

Ancient Dragon · Jul 6th, 2005, 2:53 PM

nothing "wrong" with it -- you said your book told you have to do it that way. That's not true. If you have a bunch of class members it would be too diffucult to read if you initialized them like that. Put the initializers inside the constructor's function is cleaner and easier to read.

Ancient Dragon · Jul 6th, 2005, 2:55 PM

Quote:

Originally Posted by Animatronic

Using an initialiser list is usually the preferred way

depends on who's doing the preferring

I have a class that has over 40 member data objects. I'm certainly not going to use an initializer list! :eek:

clearbit · Jul 6th, 2005, 6:22 PM

Quote:

Originally Posted by Ancient Dragon

nothing "wrong" with it -- you said your book told you have to do it that way. That's not true. If you have a bunch of class members it would be too diffucult to read if you initialized them like that. Put the initializers inside the constructor's function is cleaner and easier to read.

By initializers do you mean the preamble? and sorry, my book just said thats one way to do it, my bad for not specifying

grumpy · Jul 7th, 2005, 5:23 AM

What you (and your book) are describing as the "preamble" is more usually described in C++ as the initialiser (or initializer) list. The word "preamble" literally means "preliminary material".

As others have said, there is nothing wrong with initialising class members using the initialiser list, but there are alternatives should you wish to use them.

clearbit · Jul 8th, 2005, 10:30 PM

oh ok..thank you. I understand, I'll make a note in my book

Jul 6th, 2005, 1:31 PM	#1
clearbit Newbie Join Date: Jun 2005 Location: far..far..away Posts: 25 Rep Power: 0	preamble to constructor ok its been a while since ive coded in C++ but im trying to learn about classes agian, and ive been reading a book ive had, just never had the time to read.(well till now). It talks about to initialize a data member(private) you have to make it the preamble to the constructor of the class.. For example (Toggle Plain Text) class Whatever { : _xin (x) //HERES WHERE MY QUESTIONS AT. public: Construct(int xin) {std::cout<<xin;} ... private: const int _xin; } class Whatever { : _xin (x) //HERES WHERE MY QUESTIONS AT. public: Construct(int xin) {std::cout<<xin;} ... private: const int _xin; } The question is can I just assign an integer to the _xin like this (Toggle Plain Text) : _xin = 1; : _xin = 1; lame question i know but i dont have a compiler to test it right now..thnx.

Jul 6th, 2005, 2:28 PM	#2
Ancient Dragon PFO God In Training Join Date: Jun 2005 Location: near St Louis, MO. (USA) Posts: 600 Rep Power: 4	Throw that book away immediately! :eek: just because someting is private does not mean it has to be initialized like that. (Toggle Plain Text) class MyClass { public: MyClass() {m_x = 0;} private: int m_x; }; class MyClass { public: MyClass() {m_x = 0;} private: int m_x; }; The only time I initialize class variables is when it is a reference to something. (Toggle Plain Text) class MyClass { public: MyClass(int n) : m_r(n) { m_x = 0; } private: int m_x; int& m_r; }; int main(int argc, char* argv[]) { int x = 0; MyClass m(x); return 0; } class MyClass { public: MyClass(int n) : m_r(n) { m_x = 0; } private: int m_x; int& m_r; }; int main(int argc, char* argv[]) { int x = 0; MyClass m(x); return 0; }

Jul 6th, 2005, 2:41 PM	#3
clearbit Newbie Join Date: Jun 2005 Location: far..far..away Posts: 25 Rep Power: 0	opps sorry, this is what it actualy looks like in the book (Toggle Plain Text) class Whatever { public: Construct(int xin) : _xin (x) //HERES WHERE MY QUESTIONS AT. { std::cout<<xin; } ... private: const int _xin; } class Whatever { public: Construct(int xin) : _xin (x) //HERES WHERE MY QUESTIONS AT. { std::cout<<xin; } ... private: const int _xin; }

Jul 8th, 2005, 10:30 PM	#10
clearbit Newbie Join Date: Jun 2005 Location: far..far..away Posts: 25 Rep Power: 0	problem Solved oh ok..thank you. I understand, I'll make a note in my book Last edited by clearbit; Jul 8th, 2005 at 10:32 PM. Reason: ~SOLVED~

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Jul 6th, 2005, 2:42 PM	#4
clearbit Newbie Join Date: Jun 2005 Location: far..far..away Posts: 25 Rep Power: 0	Whats wrong with initializing it like this?? Security flaw?? what?

Jul 6th, 2005, 2:51 PM	#5
Animatronic Programmer Join Date: Jun 2005 Posts: 99 Rep Power: 4	Nothing it wrong with it. Using an initialiser list is usually the preferred way to intialise members inside the class as it gives the greatest posible performance (members only get constructed once instead of a posible two times).

Jul 6th, 2005, 2:53 PM	#6
Ancient Dragon PFO God In Training Join Date: Jun 2005 Location: near St Louis, MO. (USA) Posts: 600 Rep Power: 4	nothing "wrong" with it -- you said your book told you have to do it that way. That's not true. If you have a bunch of class members it would be too diffucult to read if you initialized them like that. Put the initializers inside the constructor's function is cleaner and easier to read.

Jul 7th, 2005, 5:23 AM	#9
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,261 Rep Power: 5	What you (and your book) are describing as the "preamble" is more usually described in C++ as the initialiser (or initializer) list. The word "preamble" literally means "preliminary material". As others have said, there is nothing wrong with initialising class members using the initialiser list, but there are alternatives should you wish to use them.

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Programming Forums > Application Development > C++

preamble to constructor