Class constructor (c++ newb)

Looking at the tutorial on classes, at the constructor part:

Following part in public class:

Quote:

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Computer();
//Constructor

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Member function:

Quote:

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Computer::Computer()
{ //Constructors can accept arguments, but this one does not
processorspeed = 0;
//Initializes it to zero
}

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My problem lies that, say I have 5 variables in the private section of the class. I need to initialize those values to 0 if there isn't already anything in there, but do I have to make 6 different member functions, accomodating the fact that they can either send in no variable, one, two, three, etc, or is there a way to make one and have it determine if anything has already been initialized.... I don't know if this makes much sense reading it, but help is appreciated.

Quote:

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if there isn't already anything in there

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do you mean, if the variables havn't been initialized?

Quote:

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that they can either send in no variable, one, two, three, etc

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sorry, i have no clue what you mean here.. :p

but..from what i can gather from you question, your asking if there is a way not to initalize the private data to 0 if it already has a value. Well, remember that the constructor runs automatically whenever their is an instance of an object, so when an object is created, there would be no point in checking if the data already contains a value, because the constructor is the first function that executes. Once it executes it doesn't run again, so, your safe from the constructor accidently overwriting data.

Er.. heres what I'm trying to mean in code by the 'sending in one, two three, etc'

Code:

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Payroll employee1(5,4,3,2,1); Payroll employee2(5,4,3,2); Payroll employee3 (5,4,3)

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And keep continuing that until you get down to where its not sending a variable. Do I need to make six member functions to cover the amount of combinations which can be sent in?

Code:

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class Computer
{
public:
        Computer(int ps = 0, int mem = 0, int ds = 0) :
                processorspeed(ps), memory(mem), drivespace(ds)
        { }
private:
        int processorspeed;
        int memory;
        int drivespace;
};

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This line:

Code:

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Computer(int ps = 0, int mem = 0, int ds = 0)

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is what you should focus on to have a constructor that takes different values. Just like any other function, the parameters with " = 0" are optional, meaning they will be 0 if the user doesn't specify them. The downside is that if the user wants to indicate memory but not processorspeed, they must add the 0 for processor speed also (like in this example):

Code:

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Computer myComputer(0, 128);

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which creates a computer with memory at 128 and processorspeed and drivespace at 0. Note that the default doesn't have to be 0, I just chose that.

Thats exactly what I needed to know :D

*Edit*

Except now I just realized I'm lost when you did this:

Code:

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processorspeed(ps), memory(0), drivespace(ds)

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That's called an initializer list. You don't have to use it if you don't want to. In general, before the body of your constructor (the stuff inside the braces) is run, all the member variables of the class are created. Then, inside the body of your constructor you set them to initial values that you want. For integers, that is not really a big deal. But when you get into more advanced programming, you will have other classes as members, and sometimes it will be inefficient to have those objects be created once, and then have them set their initial value later. By putting these things into the initializer list, they get constructed with those initial values immediately.

So in general, when you feel comfortable with it, it is a good idea to use the initializer list to set the initial values of your member variables. The syntax is like that of a constructor, which is why there are parentheses with the initial value inside.

[Edit] - Ohh, and i just realized I made a typo! It should be:

Code:

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processorspeed(ps), memory(mem), drivespace(ds)

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Oops! :o

I'm scared :eek: .... that all made sense to me :D