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how to define a constructor with const members

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Join Date: Mar 2009

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ganmo

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how to define a constructor with const members

Apr 28th, 2009

Hello, I am having problem to define a constuctor of a class with private member which is also of a const type. When I try to compile it complains that the const members do not have initalized data. Hmm, anyway it works fine if I remove const. However is it possible to define a constructor with const members which is possible to set values to?

I know that const is values you can't change values on, but exercise says to use const. help is appreciated.

e.g. I want to create an object of a person

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 cpp Syntax (Toggle Plain Text) 
Bike bicycle (2, "Monark", "blue", 24.11);
Bike bicycle (2, "Monark", "blue", 24.11);

and here is the constructor code

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Bike::Bike(const int wheel, const string brand, string color, double wheelsize) {
    w = wheel;
    b = brand;
    c = color;
    ws = wheelsize;
 
}
Bike::Bike(const int wheel, const string brand, string color, double wheelsize) {
    w = wheel;
    b = brand;
    c = color;
    ws = wheelsize;

}

and here is the class file

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class Bike {
public:
    Bike (const int, const string, string, double);
private:
    const int w;
    const string b;
    string c;
    double ws;
};
class Bike {
public:
    Bike (const int, const string, string, double);
private:
    const int w;
    const string b;
    string c;
    double ws;
};

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Join Date: Oct 2007

Posts: 792

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siddhant3s siddhant3s is offline

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Re: how to define a constructor with const members

Apr 28th, 2009

Try this:

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 cpp Syntax (Toggle Plain Text) 
class Bike {
public:
    Bike (const int, const string, string, double);
private:
    const int w;
    const string b;
    string c;
    double ws;
};
Bike::Bike(const int wheel, const string brand, string color, double wheelsize):
w(wheel),
b(brand),
c(color),
ws(wheelsize)
{}
class Bike {
public:
    Bike (const int, const string, string, double);
private:
    const int w;
    const string b;
    string c;
    double ws;
};
Bike::Bike(const int wheel, const string brand, string color, double wheelsize):
w(wheel),
b(brand),
c(color),
ws(wheelsize)
{}

It is generally better to write constructor using member initialization(what I did) rather than assignment(what you did).
This approach is more efficient.
why? because in this approach you are saving the overhead to create wheel,brand,color,wheelsize. Instead you are just using those value just to initialize your data members

Last edited by siddhant3s; Apr 28th, 2009 at 3:23 pm.

Siddhant Sanyam
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Migrate to Standard C++ :When to tell your C++ Code is Non-Standard.
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MatEpp

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Re: how to define a constructor with const members

Apr 28th, 2009

Bad Post

Last edited by MatEpp; Apr 28th, 2009 at 3:31 pm. Reason: My apologies, I posted false information.

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siddhant3s siddhant3s is offline

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Re: how to define a constructor with const members

Apr 28th, 2009

MatEpp>>The names, wheel, brand, etc. are missing from
[edit]This was posted in the post above. But Maybe he realised his fault and then removed this remark marking it as "Bad Post"[/edit]
That is a prototype of the constructor. It is perfectly fine.
It just tells the compiler that " you will find the definition of the constructor somewhere in the code below".
You usually omit the name of the formal parameters when declaring prototype.

Last edited by siddhant3s; Apr 28th, 2009 at 3:35 pm.

Siddhant Sanyam
(Not posting much)
Migrate to Standard C++ :When to tell your C++ Code is Non-Standard.
Please Read before posting: How To Ask Questions The Smart Way