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Help with static members

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Help with static members

 
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  #1
Dec 26th, 2008
Hi, I was making a pogram to demostrate static variables and I don't know why my compiler (VS 2005) is giving me these crazy errors that my book im using said I shouldn't be getting. Here is my program, please help:
header:
C++ Syntax (Toggle Plain Text) 
  1. class number{
  2. private:
  3. 	 static int num;
  4. public:
  5. 	 int getnum()
  6. 	 {return num;}
  7. 	void setnum(int innum)
  8. 	{num=innum;}
  9. };
body (.cpp)
C++ Syntax (Toggle Plain Text) 
  1. #include<iostream>
  2. #include"headerr.h"
  3. using namespace std;
  4. int main()
  5. {
  6. 	int innum=0;
  7. 	class number test;
  8. 	test.setnum(10);
  9. 	cout<<test.getnum();
  10. system("pause");
  11. return (0);
  12. }

This code returns the errors:
body.obj : error LNK2019: unresolved external symbol "private: static int number::num" (?num@number@@0HA) referenced in function "public: static int __cdecl number::getnum(void)" (?getnum@number@@SAHXZ)
1>C:\Documents and Settings\Computer stuff man\Desktop\test progams\classes 3.0\Debug\classes 3.0.exe : fatal error LNK1120: 1 unresolved externals
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Re: Help with static members

 
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  #2
Dec 26th, 2008
You need to define num as well as declare it:
C++ Syntax (Toggle Plain Text) 
  1. class number{
  2. private:
  3. 	 static int num;
  4. public:
  5. 	 int getnum()
  6. 	 {return num;}
  7. 	void setnum(int innum)
  8. 	{num=innum;}
  9. };
  10.  
  11. int number::num;
I'm here to prove you wrong.
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Re: Help with static members

 
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  #3
Dec 26th, 2008
Does it matter where you do this (int number::num) because I tried it out in the main and it works now but why? It doesn't seem to do anything other than maybe introduce it into a new block however you put it in the header and it still works. So what does it really do ?
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Re: Help with static members

 
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  #4
Dec 27th, 2008
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Originally Posted by CPPRULZ View Post
Does it matter where you do this (int number::num) because I tried it out in the main and it works now but why? It doesn't seem to do anything other than maybe introduce it into a new block however you put it in the header and it still works. So what does it really do ?
It doesn't matter where you do it, as long as you obey the one-definition rule (ODR). Typical projects link together object multiple object files to create an executable, and each object file is created from a single source file. If you have two or more source files in your project, then only one of them can define number::num.

Placing the definition in your header file will violate the ODR if more than one source file in your project #include's it.
Right 98% of the time, and don't care about the other 3%.
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Re: Help with static members

 
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  #5
Dec 27th, 2008
My real question now is why do I need the syntax number::num; - what does it do physically in the program.
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Re: Help with static members

 
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  #6
Dec 27th, 2008
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Originally Posted by CPPRULZ View Post
My real question now is why do I need the syntax number::num; - what does it do physically in the program.
Narue answered that already.

The "static int num;" within the body of class number declares the static member. The statement outside the class body "int number::num;" defines it.

Declaring a static member of a class means that it is known to exist, so code can reference it. Defining a static means allocating storage for it so, when a program is linked, all references in code to that static member resolve to the same object (i.e. the same area of physical memory).
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Re: Help with static members

 
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  #7
Dec 27th, 2008
So, just to check, int number::num; clears a place to store data at a certain data location-just like when you declare a class variable you give all of the variables in the class a place in physical memory and an adress however the adress holds NULL.
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Re: Help with static members

 
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  #8
Dec 27th, 2008
Line 2 of your second code snippet : #include"headerr.h"
Two rr's?
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Re: Help with static members

 
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  #9
Dec 27th, 2008
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Originally Posted by CPPRULZ View Post
So, just to check, int number::num; clears a place to store data at a certain data location-just like when you declare a class variable you give all of the variables in the class a place in physical memory and an adress however the adress holds NULL.
Sorry, that statement strikes me as gibberish.

The declaration "static int num;" within the body of class number simply tells the compiler that a single integer named number::num exists somewhere in memory.

The definition "int number::num;" ensures that memory is actually allocated to hold (or represent) that single integer.
Right 98% of the time, and don't care about the other 3%.
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