How to intitialize const data using constructor

Question

Hi, I've created a class which has three constructors, two integer members & one const int member. So for one constructor I'm using initializer list to assign const int member but I am getting error in other two constructors

Here is my code:

class base
{
public:
    base();
    base(const int _count);
    base(int a ,int b);
    ~base();
protected:
    int m_ia , m_ib;
    const int count;
};
base::base()
{
}
base::base(const int _count):count(_count)
{
}
base::base(int a , int b)
{
    m_ia = a ;
    m_ib = b;

}
base::~base()
{

}
void main()
{
    base *obj2 = new base(1000);
    getchar();
}

Number of Errors:2

1.'base::count' : must be initialized in constructor base/member initializer list at base()

2.'base::count' : must be initialized in constructor base/member initializer list at base(int a ,int b)

... initialise ``count`` in the other 2 constructors in the same way... It doesn't automatically initialise it to 0 just because you didn't say anything. — flight, Jan 04 '14 at 06:45
`void main` is not legal. The return type must be `int`. Also, if `base` is supposed to be a base class, that destructor should be virtual. `obj2` should also be a normal object in this example. Right now, it's leaking memory. Finally, why not use a constructor initializer list for your other members as well? There's really almost never any down side. — chris, Jan 04 '14 at 06:46

score 4 · Answer 1 · answered Jan 04 '14 at 06:48

You should probably make sure all your constructors are initializing all member variables, not just the one's being passed in as arguments. I'd rewrite your constructors as

base::base()
: m_ia()    // value initializes i.e. sets to 0
, m_ib()
, count()
{
}

base::base(const int _count)
: m_ia()
, m_ib()
,count(_count)
{
}

base::base(int a , int b)
: m_ia(a)
, m_ib(b)
, count()
{
}

And if you have a C++11 compiler that supports delegating constructors, you could create a 4 constructor that takes 3 arguments and have the other constructors delegate to that one.

base::base()
: base(0, 0, 0)
{
}

base::base(const int _count)
: base(0, 0, _count)
{
}

base::base(int a , int b)
: base(a, b, 0)
{
}

base::base(int a , int b, int count)
: m_ia(a)
, m_ib(b)
, count(count)
{
}

The last constructor can be made private if you don't want it to be part of the class interface.

bolov · Answer 2 · 2014-01-04T07:24:10.713

3

In c++11 you can have

protected:
    int m_ia , m_ib;
    const int count = 0;

It works in VS 2013.

edited Jan 04 '14 at 07:24

answered Jan 04 '14 at 06:53

bolov

72,283
15
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+1 It won't work in VS2012 (maybe if you install the CTP, but definitely not VS2012 proper) I forgot about non-static data member initializers, this is a better option than my answer. I'd also assign `m_ia` and `m_ib` to 0. – Praetorian Jan 04 '14 at 07:02

score 0 · Answer 3 · answered Jan 04 '14 at 07:04

0

As per standard C++ and you current code you have to Initialize your const variable in constructor using initialize list so code can be modified as below :

#include"iostream"

using namespace std;

class base
{
public:
    base();
    base(const int _count);
    base(int a ,int b, const int _count);
    ~base();
protected:
    int m_ia , m_ib;
    const int count;
};
base::base():count(0)
{
}

base::base(const int _count):count(_count)
{
}

base::base(int a , int b, const int _count):count(0)
{
    m_ia = a; 
    m_ib = b;
}

base::~base()
{

}
int  main()
{
    base *obj2 = new base(1000);
    getchar();
    return 0;
}

answered Jan 04 '14 at 07:04

Astro - Amit

767
3
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why should we initialize count in other two constructor with out any arguments – Jeggu Jan 04 '14 at 17:02
please check the - "http://stackoverflow.com/questions/10647448/why-constant-data-member-of-a-class-need-to-be-initialized-at-the-constructor" and read about const data members in class it will give you more clearity – Astro - Amit Jan 06 '14 at 01:24

How to intitialize const data using constructor

3 Answers3