variable

[ripley]

Hello,

how can I use variable, which is set in one class in another? should it be private or public? Should I do that through reference? and if so, how? Thanks a lot for your answer.

[Bazzy]

Your question is too general, could you be a bit more specific?

[ripley]

Ok, so like this:
I have class A, where I have a varible a. My first question is: should this be variable private or public, if I want to use it in another class? or it doesnt matter?

class A
{
   public: 
       int a;
   private:
       void f() {
          int a = 10;
          // do something else;
        }
}          

now I have a class B, where I want to use this variable in B::f1() from class A with the value, which was set in function f();

and here I got stuck. cos I dont know, how to use it. should I use some get_a function?

[Bazzy]

You have three options:
1- Make 'a' to be public, this way you'll can access it from anywhere outside the class 'A'
2- Make 'a' private/protected and write a public getter function
3- Make 'a' private/protected and class 'B' friend of class 'A', in this way only 'B' and 'A' could access A::a

[ripley]

could you explain that to me a bit more, please? i d like to know, how to use 'a' in B in options you gaved. cos I tried to use 'a' which is public in B and it return error. thanks a lot

[Bazzy]

If 'a' isn't a static member you need an object of 'A' to access it in 'B'
eg:

class A { public: int a; };
class B
{
    void f1()
    {
         A obj;
         std::cout << obj.a; // needs an object
    }
};

class A { public: static int a; };
int A::a = 10; // static members need to be defined
class B
{
    void f1()
    {
         std::cout << A::a; // doesn't need an object
    }
};

Was this the error you got?

[ripley]

And if I make 'a' private? I didnt do it through "::" but through "." that was an error

[jRaskell]

Show your code that's getting the error. It's too hard trying to quess what you've written.

[Bazzy]

"." is to access object members, can you post the code you have in class B?

[ripley]

Yes, I understand now.

the code is:

class A 
{
public:
   unsigned char* f(const char* mess) {
       a = sizeof(mess);
       // do something with mess;
       return mess;
    }
      
   long int get_a() {return a;}
   void set_a(const long int p_a) {a = p_a;}
 
private:
   long int a;
}

class B
{
public:
   unsigned char* f1() 
   {   
        A a;
        unsigned char * mess2 = NULL;
        mess2 = new unsigned char[a.get_a()];
        memset(mess2, 0, a.get_a());    
        // do something with mess2
        return mess;
      }    
}

[Bazzy]

You are missing some semicolons ( Lines 15 and 19 )
Notice that since you didn't provide a constructor for A, a.a would be uninitialized on line 24

[ripley]

Yes, the value in B is unitialized. How can I initialize it?
the constructors for A is

A(const B&)

and for B is
B(const int)

I initialize them in main().

[Bazzy]

Give it a value in A's constructor
eg:

A::A ( const B &b ) : a ( 123 ) // here 'a' is initialized 
{
   //...
}

[ripley]

Yes, but a is initialized in f() , and I dont know the value of a on the beginig ...

[Bazzy]

You should at least initialize it to 0, is always bad having uninitialized variables
Then you need to call A::f before A::get_a

[ripley]

I already solved it. I think, I should read some c++ book, this are the basics I think. Thanks for your help and time :D.

[jsmith]

I think there are likely other semantic errors in your program, just based on what I'm seeing
there.

sizeof( mess ) on line 5 has constant value 4 (32-bit machine, 8 on 64-bit machine), since it is
asking how many bytes of memory are occupied by a pointer to (constant) character.

I think you are actually wanting the length of the C-string? In which case you'd use
strlen(), but seriously I'd considering moving to the more natural C++ STL types
std::string or std::vector.

Anyway, if you are going to use C-style strings, then a more appropriate type for a
would be size_t, not long int, since I don't know what it means to have a string of
length -42.

[ripley]

i had strlen() in my aplication (and semicolons too), I ve made mistake in example above. I changed the type to size_t.