Re: Are members of a c++ class initialized to 0 by the default constructor ?

From:
"Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com>
Newsgroups:
alt.comp.lang.learn.c-c++,comp.lang.c++
Date:
Tue, 21 Jun 2011 16:33:03 +0200
Message-ID:
<itq9pb$tqt$1@dont-email.me>
* Skybuck Flying, on 21.06.2011 16:12:

My question is basically:

Are members of a c++ class initialized to 0 by the default constructor ?


Nope.

Unless you define one that does that.

But for a Plain Old Data type (which does not have a user-defined constructor)
you can ask for zero-initialization where you create an object, like

     p = new T(); // The parenthesis asks for value-initialization,
                     // which for a Plain Old Data type is zeroing.

For example:

class TSomeClass
{
    private:
        int mField1;
        int mField2;

    protected:

    public:
        TSomeClass();
        ~TSomeClass();
};

// constructor
TSomeClass::TSomeClass()
{

}


This constructor does not zero-initialize the members.

But you could ask for it, like

   TsomeClass::TSomeClass()
       : mField1()
       , mField2()
   {}

// destructor
TSomeClass::~TSomeClass()
{

}

Usage example:

class TSomeOtherClass
{
    private:
        TSomeClass mSomeClass();

    public:
        // default constructor here etc.

}

int main()
{
    TSomeClass vSomeClass();


Note that this is a function declaration, not a variable declaration.

    TSomeClass *vSomeClassArray;

    vSomeClassArray = new TSomeClass[100];

    return 0;
}

So will all class objects members be initialized to zero ?


Not guaranteed with your code, no. But the compiler may emit code to do it anyway.

I think you could ask for it like

   vSomeClassArray = new TSomeClass[100]();

but I'm not sure of the exact syntax here. Nor about whether it is supported by
Microsoft's compiler.

[snip]

The thing you need to keep clear in your mind is that for POD type, there is not
really any constructor. Then the initialization performed depends on the
instantiation expression. T versus T().

Cheers & hth.,

- Alf

--
blog at <url: http://alfps.wordpress.com>

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