Declare private things at h or c

[tonnot]

Hello,
I've seen some c++ code examples with private: section at .h file.
I dont understand the finality of it. If thery are private....
Can anyone give me some reason ?

Thanks

[filipe]

Do you know what a class or struct is? The private keyword is used to declare class members that are not accessible outside the class, hence, they are private to the class.

[quirkyusername]

if a member variable is private only its member functions can modify it

class A{
public:

void setPri(int i) { pri = i; }
int pub;

private:
int pri;

};

int main()
{
A a;
a.pub = 1; // fine
a.setPri(1); // fine
a.pri = 1; // error!
}

[filipe]

quirkyusername wrote:

if a member variable is private only its member functions can modify access it

[tonnot]

Thanks everybody
I'm talking about .h and .cpp 'model'.
I see a private section at h file.
The unique reason I can think of is a 'work team' reason, with the idea to inform other programmers.
But the same can be achieved putting it at the cpp file, isn't it ?
Another reason ? A compiler advantage ?
tHANKS

[Disch]

The private keyword has no place in the C++ language outside of a class or struct.

You can't declare part of a header as private. You can only declare part of a class as private.

[tonnot]

Disch:
And is it possible to have private elements for a class into cpp ? I think no . Only can have private elements for all the classes, isn't it ?
Thanks

[Disch]

I don't understand you're question.

It's not that complicated.

class YourClass
{
public:

  // anything put here will be public

protected:

  // anything put here will be protected

private:

  // anything put here will be private

};

There's nothing more to it.

[khattakjunaid]

can someone explain from line no. 6 to line no.18? please! :(

1. /*C Program to arrange 5 numbers in ascending order*/
2. #include<iostream.h>
3. #include<conio.h>
4. void main()
5. {
6. int i,j,temp,a[5];

7. cout<<"Enter 5 integer numbers\n";

8. for(i=0;i<5;i++)

9. cin>>a[i];

10. for(i=0;i<5;i++)
11. {
12. for(j=i+1;j<5;j++)

13. {
14. if(a[i]>a[j])
15. {
16. temp=a[j];
17. a[j]=a[i];
18. a[i]=temp;
19. }
20. }

21. }

22. cout<<"\n\nThe 5 numbers sorted in ascending order are\n";

23. for(i=0;i<5;i++)

24. cout<<"\t"<<a[i];
25. getch();
26. }

[jsmith]

6 - declare i, j, and temp as integer variables, and a as an array of 5 integers.
7 - output the message to the user
8-9 - loop 5 times, read integer input from keyboard and store integer value in successive positions in the array
10-21 - sort the array in ascending order using an incorrect (but still functional) bubble sort

[filipe]

tonnot wrote:

And is it possible to have private elements for a class into cpp ?

Class declarations are usually kept in header files, so they can be used across a number of .cpp files. But notice that the only difference between .h and .cpp files is conventional. They're both text files and all the #include directive does is copy and paste the text from the .h file into the .cpp file.

[khattakjunaid]

thankyou jsmith! :) can you give me a code for putting the five integers in ascending order using a correct bubble sort or without it?

[EzwRko23]

This is a well known inconsistency in the C++ OOP model, caused by the fact they wanted to keep C++ compatible with C. The private keyword is a compiler enforced comment saying, "please, whenever I access this member from outside, treat is as an error". Apart from that, the compiler does not make any other distinction between private/protected/public - internally all private and protected members are part of the **public** interface of the class, so they must be visible to any code using that class. Thus, they are usually placed in headers. If you change a private member signature, add/remove a private member, all the code depending on this class will need to be recompiled.

Note: This is a different behaviour than in true OOP languages (like Smalltalk or Java). In those languages private members are not part of the API/ABI. You can freely change them, and the changes are isolated from the rest of the world.

[kfmfe04]

And is it possible to have private elements for a class into cpp ? I think no . Only can have private elements for all the classes, isn't it ?

You could, using a static variable in a .cpp module. For example, you can have:

// in .hpp
class MyClass {
  private:
    static int g_MyClassInt;
};

// in .cpp
int MyClass::g_MyClassInt = 42;    // initialize 

or, you can have the equivalent.

// in .cpp
static int g_MyClassInt = 42;

I often prefer the latter construct over the former because I don't have to reveal my class private members in a header (and add additional compile dependencies between modules).

[tonnot]

kfmfe04 :

I often prefer the latter construct over the former because I don't have to reveal my class private members in a header (and add additional compile dependencies between modules).

This is the answer that I expected. Thanks