Defining static const int in class

Question

1.00/5 (2 votes)

See more:

Hi, I have a little problem in defining static const int in this code snippet

C++

#include <iostream>

class foo
{
public :
    static const int f = 5; 
};

void bar(const int& b)
{

}

int main()
{
    bar(foo::f); //undefined reference to foo::f
}

I have found a simple solution

C++

#include <iostream>

class foo
{
public :
    enum {f = 5};
};

void bar(const int& b)
{

}

int main()
{
    bar(foo::f); //works fine !!
}

why the first code does not work and the second one does work ? I need some explanation about what happens behind the scene, please.

Thanks, Sam.

Posted 27-Aug-15 2:37am

Samuel Shokry

Updated 27-Aug-15 2:39am

v2

Add a Solution

Comments

Philippe Mori 27-Aug-15 12:38pm

Are you using an old compiler?

Samuel Shokry 28-Aug-15 17:43pm

I'm using code block 13.12 IDE

Afzaal Ahmad Zeeshan 27-Aug-15 20:37pm

In my case, the above code works perfectly.

[no name] 27-Aug-15 20:48pm

Proving what?

Afzaal Ahmad Zeeshan 27-Aug-15 20:50pm

Proving that the member is accessible. I tried in Visual Studio.

[no name] 27-Aug-15 20:53pm

Which visual studio? Actually it does not prove the member is accessible. It proves that at some point the compiler has a const value. Read my answer (preferably before downvoting) and this https://msdn.microsoft.com/en-us/library/b1b5y48f(v=vs.140).aspx.

2 solutions

Solution 2

It is a variable which belongs to the class and not to object(instance)
Static variables are initialized only once , at the start of the execution . These variables will be initialized first, before the initialization of any instance variables
A single copy to be shared by all instances of the class
A static variable can be accessed directly by the class name and doesn’t need any object!

Static variable are class level variables, they can't be declared inside a method, if declared class will not compile.

class foo
{
public :
static const int f;
};
const int foo::f = 5;

void bar(const int& b)
{

}

int main()
{
bar(foo::f);
}

Works finely and compiles without error

Posted 31-Aug-15 1:28am

Chetan Gowda

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

User 59241 · Accepted Answer · 2015-08-27T03:38:00

Nothing magic here. One is permitted syntax one isn't. For const member variables must be declared in class and defined outside. http://www-01.ibm.com/support/knowledgecenter/SSLTBW_1.12.0/com.ibm.zos.r12.cbclx01/cplr038.htm[^]

So:

C#

class foo
{
public :
    static const float f;
};

const float foo:f = 5.0f;

in c++11 you could do:

C#

class foo
{
public :
    static constexpr float f = 5.0f;
};

However some C++ compilers allow the omission of an external definition for int but it is advised to have one if you intend taking the address later. This is because the compiler may remove the variable and just keep the value.
See here: http://stackoverflow.com/questions/1312241/using-a-static-const-int-in-a-struct-class[^]

This is what you discovered. You did not mention your compiler but that is crucial here.

For further explanation look here:http://stackoverflow.com/questions/11300652/static-data-member-initialization[^]

Quote
"On the other hand, a static member variable is not contained within an instance of the class, it exists independently of any single instance and exists from the start of the program, at a fixed address. In order for a static member variable (or any other global object) to get a unique address the linker must see exactly one definition of the static variable, in exactly one object file, and assign it an address.

Because a static variable needs exactly one definition in exactly one object file, it doesn't make sense to allow that definition to be provided in the class, since class definitions typically exist in header files and are included in multiple object files. So although you can provide an initializer in the class, you still need to define the static data member somewhere"

Enums do not exist in memory they are entirely handled by the compiler:
http://stackoverflow.com/questions/1206579/memory-location-of-enum-value-in-c[^]