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Gluster Interview Question for Software Engineer / Developers

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6
Answers

4) How to initialize Constant variables in class( or object).

example 1: throws error "uninitialized member 'myclass::i' with 'const' type 'const int'
"

#include <iostream>
using namespace std;

class myclass{
public:
       const int i;

       myclass()
       {
            i = 10;  // here throws error
       }
};

int main()
{
      myclass m;
      cout<<m.i<<endl;
      return 0;
}

example 2: below piece of code works fine .

#include <iostream>
using namespace std;

class myclass{
public:
       const int i;

       myclass() : i(10)
       {

       }
};

int main()
{

      myclass m;
      cout<<m.i<<endl;
      return 0;
}

Can some one please explain me , why second example works fine ? . why not first example ?

- siva.sai.2020 February 13, 2011 | Report Duplicate | Flag |
Gluster Software Engineer / Developer C++

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1

of 1 vote

Apart from initialsing constant variables, is there any scenario where we should be using class initialsiation list instead of constructors?
Is it a proper method to use class initialsiation list instead of constructors for normal variables

- xing2785 February 28, 2011 | Flag Reply

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Yes, you can use the initialization list to initialize the normal (non-const, non-static) members. Infact, this is a more efficient way of initializing class members.

Other situations where you need to use the initialization list are:
1. Initializing references.

Class A;
Class B
{
  private:
     A& ref;
  public 
     B(const A& ref): ref(ref)
     {

     }
}

2. Need to initialize some members of the parent class (if the parent class has a parameterized constructor).
3. Const members.

- Anonymous May 03, 2011 | Flag

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0

of 0 vote

The const variable specifies whether a variable is modifiable or not. The constant value assigned will be used each time the variable is referenced. The value assigned cannot be modified during program execution. class is typically declared in a header file and a header file is typically included into many translation units. However, to avoid complicated linker rules, C++ requires that every object has a unique definition. That rule would be broken if C++ allowed in-class definition of entities that needed to be stored in memory as objects. A const variable has to be declared within the class, but it cannot be defined in it. We need to define the const variable outside the class.
-HTH

- hk18 February 13, 2011 | Flag Reply

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myclass() : i(10)
Here the i = 10 assignment in initializer list happens much before the class initilizaiton occurs.
 
myclass(
{
     i = 10
}
Here the assignment happens after initializing the class, once this is initialized you cant modify the const variable.

- Anonymous February 14, 2011 | Flag Reply

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A (non-static) const variable in a class is required to be initialized before initialization of the object of that class. The class initialization list is the only place where such a variable can be initialized cause it occurs prior to execution of the constructor.

- Anonymous February 21, 2011 | Flag Reply

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0

of 0 vote

This thing has been explained very nicely in Effective c++ by Scott Mayer. In the first case say
myclass(int a=10)
{
i = a; // Its assignment not initialization
}
Here First the default constructor is called and then the value is again assigned to i by calling the copy constructor, thus wasting the work done by default constructor. Initialization mean the value with which the variable is born. To ensure that we make use of member initialization list.
myclass(int a=10)::i(a)
{
// This is called initialization
}
This ensures that the variable is born with the given value. in previous case the value of constant was getting modified on being the copy constructor called. But the member initialization list solves that problem. This is very effective when we require to initialize the user defined data members.

- Anurag Jain June 30, 2011 | Flag Reply

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