non-const static member variale initialization

Originally Posted by George2

Code:

Class Foo{
public:
    int i;
    Foo(): i(100)
    {
    }
};

Where do you think is declaration and where do you think is definition of variable i?

The declaration is where it says "int i;"
The definition as such doesn't exist. The member is defined as part of the larger object whenever a variable of type Foo is defined.

I do not understand what do you mean "still required"?

As in: just because the initializer is in the class body, doesn't give you the right to omit the definition.

Yes, but what is the location if Foo::i?

I don't actually KNOW why it is, I'm only guessing to the reason - if you really want to know, you are probably best asking Bjarne or someone on the C++ standards committee.

--
Mats

Thanks Mats,

Originally Posted by matsp

Yes, but what is the location if Foo::i?

Mats

I think all static variable should be stored to some global storage, maybe the same as where global variables are stored?

Thanks CornedBee,


Could you show us some code about what do you mean definition of a member variable (not declaration), I am confused.

Originally Posted by CornedBee

The declaration is where it says "int i;"
The definition as such doesn't exist. The member is defined as part of the larger object whenever a variable of type Foo is defined.


As in: just because the initializer is in the class body, doesn't give you the right to omit the definition.

regards,
George

A class declaration is the abstraction of an object, it's not the object itself.
I'm pretty sure we all agree about that.
Every public or private variable IMO should be initialized when the istance is created: the way I prefer is using the class constructor method.
c++ = logic, and so it behaves. I believe that c++ developers try to avoid any logical exception, when possible.

Thanks carlorfeo,


I agree with your below comments. We are clear about what is member variable declaration and member variable initialization. But unclear what is about member variable definition.

Could you show us some code about which line of code do you think is member variable definition please? :-)

Originally Posted by carlorfeo

A class declaration is the abstraction of an object, it's not the object itself.
I'm pretty sure we all agree about that.
Every public or private variable IMO should be initialized when the istance is created: the way I prefer is using the class constructor method.
c++ = logic, and so it behaves. I believe that c++ developers try to avoid any logical exception, when possible.

regards,
George

Code:

Foo fooInstance;

This one.

I think all static variable should be stored to some global storage, maybe the same as where global variables are stored?

Yes, but the specific compilation unit matters, too.

Though I don't think that's the actual issue. At least not for static initialization. I think the issue here is simply that by disallowing initialization at the point of declaration, you effectively make it impossible to accidentally give different declarations different values (something that would be possible and nearly undetectable otherwise).
For dynamic initialization, there's also the order of initialization to worry about.

Thanks CornedBee,


1.

Originally Posted by CornedBee

Code:

Foo fooInstance;

This one.

I have read again the whole post, but I can not find this line of code. I think you mean where is the definition of class instance, right?

But where is the definition of a specific data member variable of a class, like where is the definition of member variable i (not declaration)?

Code:

Class Foo {
public:
    int i;
    ...
};

Originally Posted by CornedBee

Though I don't think that's the actual issue. At least not for static initialization. I think the issue here is simply that by disallowing initialization at the point of declaration, you effectively make it impossible to accidentally give different declarations different values (something that would be possible and nearly undetectable otherwise).
For dynamic initialization, there's also the order of initialization to worry about.

I do not understand what is "different declarations different values" and "the order of initialization to worry about"? Could you show more description of some pseudo code please?


regards,
George

Originally Posted by George2

I have read again the whole post, but I can not find this line of code.

Correct.

I think you mean where is the definition of class instance, right?

But where is the definition of a specific data member variable of a class, like where is the definition of member variable i (not declaration)?

The member is part of the object, so the definition of the object is the definition of the member. That's what I said here:

The member is defined as part of the larger object whenever a variable of type Foo is defined.

But apparently you really need to be told everything twice.

I do not understand what is "different declarations different values" and "the order of initialization to worry about"? Could you show more description of some pseudo code please?

The order of initialization issue is too complicated for me to come up with a quick example.

The multiple declarations issue occurs when a stupid programmer doesn't refer to the canonical header file for getting a declaration. I'll use normal globals because it's shorter than whole classes, but the issue is the same:

Code:

// foo.hpp
extern int GLOBAL = 100;

--------------------------
// foo.cpp
#include "foo.hpp"
int GLOBAL;

--------------------------
// idiot.cpp

// I'm cool, yeah.
extern int GLOBAL = 200;

Because the compiler never sees the two declarations at the same time, it can't detect the mismatch. Thus, it's easier to simply forbid initialization at the declaration, of which there can be multiple, and limit it to the definition, of which there can be only one.

Thanks CornedBee,

Originally Posted by CornedBee

Code:

// foo.hpp
extern int GLOBAL = 100;

--------------------------
// foo.cpp
#include "foo.hpp"
int GLOBAL;

--------------------------
// idiot.cpp

// I'm cool, yeah.
extern int GLOBAL = 200;

Because the compiler never sees the two declarations at the same time, it can't detect the mismatch. Thus, it's easier to simply forbid initialization at the declaration, of which there can be multiple, and limit it to the definition, of which there can be only one.

I am confused what your samples are about,

1. In foo.hpp, you define variable GLOBAL? Not declaration since you initialize there;
2. In foo.cpp, you include foo.hpp, you define GLOBAL and then define another GLOBAL variable, there will be duplicated compile error?
3. In idiot.cpp, you define another GLOBAL variable. The compile can pass.

What is your sample about to say?


regards,
George

The lines that have the extern keyword are declarations. In my example, they contain forbidden initializers. This shows that, if declarations could have initializers, these might differ, without the compiler noticing.

Thanks CornedBee,


1.

Originally Posted by CornedBee

The lines that have the extern keyword are declarations. In my example, they contain forbidden initializers.

I understand this.

2.

Originally Posted by CornedBee

This shows that, if declarations could have initializers, these might differ, without the compiler noticing.

What do you mean differ?


regards,
George

Originally Posted by George2

What do you mean differ?

In one place is

static int = 200;

in the other

static int = 300;

Hi vart,


We are talking about extern, not static right?

Originally Posted by vart

In one place is

static int = 200;

in the other

static int = 300;

regards,
George

Originally Posted by George2

Hi vart,


We are talking about extern, not static right?




regards,
George

I'm talking about static member of the class.
You can have multiple declarations of the same class in different parts of the project.
If you put the initialization of static members inside declaration - you got the problem CornedBee has mentioned

Thanks vart,


My question is answered.

Originally Posted by vart

I'm talking about static member of the class.
You can have multiple declarations of the same class in different parts of the project.
If you put the initialization of static members inside declaration - you got the problem CornedBee has mentioned

regards,
George