Initializing blocks C++ & Java

Question

4.00/5 (1 vote)

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Hi all,

I read in some articles that there is something called Initializing blocks in java ; where we can perform some initializing assignements When the Class is loaded or an instance is created.

C#

class InitDemo
{
    static int y;
    int x;
     
     {
       y=10;
       x = 0;
     }
}

I am asking is there is such paradigme in C++ ?

Thank you.

Posted 9-May-12 6:08am

Schehaider_Aymen

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3 solutions

Solution 3

I would no call it a "paradigm"; this is just a tiny syntactic feature. It looks like StackOverflow already gave you an answer; please check it.

I need to add something to this answer. First, instance (non-static) fields are recommended to be initialized in C++ initialization statement, which applies only to constructors and is important fool-proof syntactic feature. See how x(0) initializes x:

C++

class InitDemo {
    static int y;
    int x;
public:
    InitDemo() : x(0) { }
};

// this line cannot be in a header file:
int InitDemo::y = 10;

The problem is initialization of a static field. Of course, you could initialize it in the constructor, but the result of it would not be what you want. Consider:

C++

class InitDemo {
    static int y;
    int x;
public:
    InitDemo() : x(0) {
        y = 10; // WARNING! Don't repeat at home (unless you know exactly what are you doing :-)
    }
};

It works at least once, but imagine what happens if you want to accumulate some value across all the instances of this class. In this case, each constructor would reset the modified value of y to 10, effectively cancelling the result of previous calculations.

So, what do you think? Looks ugly compared to Java? And in .NET, there are very handy static constructors perfect for the purpose. I personally consider C++ as a ridiculously archaic language (despite its apparent success), but there are too many C++ fans ready to scratch out my eyes for such words… I'm prepared for down-votes… :-)

—SA

Posted 9-May-12 7:46am

Sergey Alexandrovich Kryukov

Updated 9-May-12 8:23am

v3

Comments

Code-o-mat 9-May-12 14:14pm

Even though i am a C++ "fan", here's a 5 for your answer. :) (Btw I agree somewhat, in some things C++ is indeed a bit...old style)

Sergey Alexandrovich Kryukov 9-May-12 14:21pm

Great to hear from a reasonable software developer who certainly needs to have wider views. :-)
Thank you,
--SA

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Code-o-mat · Accepted Answer · 2012-05-09T06:21:00

Solution 1

In the/a header file:

class InitDemo
{
  static int y; //Declaration of static member variable 'y'
  int x; //Declaration of member variable 'x'
  
  InitDemo(); //Declaration of constructor
};

in the/a cpp file:

int InitDemo::y = 1; //Initialization of static member variable 'y'

InitDemo::InitDemo() //Implementation of constructor
{
  x = 2; //initialization of member variable 'x'
}

Does this answer your question?

Posted 9-May-12 6:21am

Code-o-mat

Comments

Schehaider_Aymen 9-May-12 12:26pm

I afraid that it is not what i am looking for, i am aware about the constructors.

below what is written in the article that i read:


Apart from methods and constructors, Initialization Blocks are the third place in a Java Program where operations can be performed.

in the Initializing block code is written between braces with no suffix


{ 
   y =10;
   x=0;
}

Code-o-mat 9-May-12 13:10pm

Then i guess, as Schehaider_Aymen also said, there's no such thing in C++ that i know of.

Sergey Alexandrovich Kryukov 9-May-12 13:49pm

Please also see my answer with some extra explanations.
--SA

Sergey Alexandrovich Kryukov 9-May-12 13:48pm

Correct, but not the best way to initialize x; I voted 4 -- please see my answer.
--SA

Code-o-mat 9-May-12 14:10pm

Thanks for your vote. What the "best" way is is really a question of personal taste. I avoided that form of initialization for the sake of clarity, since "x = 10" is pretty obvious, but "x(10)" might look confusing at first. I heard that at some places they even discourage you from using this form for initialization with simple types because at the first look it suggests that you are constructing some more complex object and it makes your code harder to decyphwer when others look at it. Nevertheless, i should have mentioned it at least.

Sergey Alexandrovich Kryukov 9-May-12 14:25pm

This is not just a personal taste, it might look more confusing, but for those who use it, it leave less chances for a accidental bug.
As to the old C++, C++ 11 is a pretty good step to modernization.
--SA

Aescleal 10-May-12 13:45pm

I've noticed that people who've never programmed in C have fewer problems with initialiser list syntax in C++ than those that have. Even with built in types you want to avoid the whole assignment thing going on it constructors just so people reading your code don't think it's a good idea with objects.

And, just for completeness, hopefully coming to a compiler near you soon, uniform initialisation syntax:

int a { 27 };

Looks ghastly but I'm sure I'll get used to it when I have something to practise with.

Cheers,

Ash

Schehaider_Aymen · Accepted Answer · 2012-05-09T06:49:00

Solution 2

Ok, based on another forum, i guess that such concept does'not exist in C++.

It's nice to learn new things :)

Posted 9-May-12 6:49am

Schehaider_Aymen