member variable initialization

[George2]

Hello everyone,


I am wondering in the following code, member variable a in class B is not put in the initialization list or constructor of B directly, but it is initialized. How and when member variable a of class B is created and initialized? Is constructor of B invokes constructor of a?

Output is,

In constructor A
In constructor B

#include <iostream>
 
using namespace std;
 
class A
{
public:
 
    A()
    {
        cout << "In constructor A" << endl;
    }
};
 
class B
{
public:
 
    A a;
 
    B()
    {
        cout << "In constructor B" << endl;
    }
};
 
int main()
{
    B b;
 
    return 0;
}
 
 

thanks in advance,
George

[weaknessforcats]

The sequence of events is:

1) compiler allocates memory for B data members
2) compiler calls constructors for data members:
- calls A constructor if specified on B initilizer list
- calls A default constructor if no A constructor specified on B initializer list
3) compiler calls B constructor

[George2]

Thanks weaknessforcats,


So, I think it is good code if we do not put member variable in initialization list of constructor and even the body of constructor, right?

The sequence of events is:

1) compiler allocates memory for B data members
2) compiler calls constructors for data members:
- calls A constructor if specified on B initilizer list
- calls A default constructor if no A constructor specified on B initializer list
3) compiler calls B constructor

regards,
George

[weaknessforcats]

So, I think it is good code if we do not put member variable in initialization list of constructor and even the body of constructor, right?

That's false. The member initialization list is used to intialize the members. If you have const member X that needs to be initialized to a value the user enters then you would:

class MyClass
{
   const int X;
   public:
   MyClass(int arg);
};
 
MyClass::MyClass(int arg) : X(arg)
{
 
}
 

The initialization list is the only way to initialize a const data member. Later, when using inheritance, the initialization list is used to call you base class coinstrcuctor.

In short, use the initialization list whenever possible since the values in the list will be used when the member variable is being created.

[George2]

Thansk weaknessforcats,


I do not agree that const member can not be initialized if it is not in the constructor initialization list. In the following code, const member has a default constructor, and it could be intialized.

Could you review it please? If I am wrong, please feel free to correct me. :-)

#include <iostream>
 
using namespace std;
 
class A
{
public:
    A()
    {
        cout << "Initialized" << endl;
    }
};
 
 
class MyClass
{
   A _a;
   public:
   MyClass();
};
 
MyClass::MyClass()
{
 
}
 
int main()
{
    MyClass instance;
 
    return 0;
}
 

regards,
George

That's false. The member initialization list is used to intialize the members. If you have const member X that needs to be initialized to a value the user enters then you would:

class MyClass
{
   const int X;
   public:
   MyClass(int arg);
};
 
MyClass::MyClass(int arg) : X(arg)
{
 
}
 

The initialization list is the only way to initialize a const data member. Later, when using inheritance, the initialization list is used to call you base class coinstrcuctor.

In short, use the initialization list whenever possible since the values in the list will be used when the member variable is being created.

[weaknessforcats]

I don't see a const member in your example.

[George2]

Sorry weaknessforcats,


Here is the code. You can see const member could be initialized even if it is not in the initialization list.

#include <iostream>
 
using namespace std;
 
class A
{
public:
    A()
    {
        cout << "Initialized" << endl;
    }
};
 
 
class MyClass
{
   const A _a;
   public:
   MyClass();
};
 
MyClass::MyClass()
{
 
}
 
int main()
{
    MyClass instance;
 
    return 0;
}
 

I don't see a const member in your example.

regards,
George

[weaknessforcats]

That's fine. But how do you intialize that constant if the value isn't known until run time???

What you show is a compile time initialization.

[George2]

Sorry weaknessforcats,


I do not quite understand what do you mean. Could you show some pseudo code please?

That's fine. But how do you intialize that constant if the value isn't known until run time???

What you show is a compile time initialization.

regards,
George

[weaknessforcats]

Here you go:

class A
{
    private:
       const int data;
    public:
       A(int arg);
       int GetData();
};
int A::GetData()
{
     return this->data;
}
int main()
{
     cout << "Enter your constant value: ";
     int value;
     cin >> value;
     A  obj(value);
     cout << "A created with " << obj.GetData() << endl;
}
 

Now write the A constructor to initialize the A::data member.

[George2]

Thanks weaknessforcats,


I think you mean in your sample below, we must put the intialization of const member variable data into constructor's initialization list, right?

Here you go:

class A
{
    private:
       const int data;
    public:
       A(int arg);
       int GetData();
};
int A::GetData()
{
     return this->data;
}
int main()
{
     cout << "Enter your constant value: ";
     int value;
     cin >> value;
     A  obj(value);
     cout << "A created with " << obj.GetData() << endl;
}
 

Now write the A constructor to initialize the A::data member.

regards,
George

[weaknessforcats]

That's correct.

The initialization list is used to call constrcutors on any of your member variables. Some classes don't have default constructors and without a feature like this, there would be no way to have an object of these classes be a data member.

Also, you use the initialization list in inheritance so you can call your base class constructor. That constructor must execute before your derived class constructor starts.

[George2]

Thanks for your clarification, weaknessforcats!


I agree this time. :-)

That's correct.

The initialization list is used to call constrcutors on any of your member variables. Some classes don't have default constructors and without a feature like this, there would be no way to have an object of these classes be a data member.

Also, you use the initialization list in inheritance so you can call your base class constructor. That constructor must execute before your derived class constructor starts.

regards,
George