Objects and Headers

Question

Hi...

I was wondering if there is a way to put objects of a class in a header... for example...

class blabla{

   }object;  

if i made a object there and then include the header in more than one sorce file, i get an error of multiply defined symbols found. I am new so don't understand fully everything. Also, I want to do something lke object.function() inside of a different function which is a member of a differnt class how can I do that?

Answer 1

Assuming you want a single object, in the header file just declare the object:

extern blabla object;

and then in one source file define it:

blabla object;

As for calling a method on an object from a different class, that is perfectly fine as long as the method is public.

class foo
{
public:
    void public_method();

private:
    void private_method();
};


void some_other_class::method(foo &f)
{
    f.public_method();  // this is fine

    f.private_method(); // this is not okay, private_method can only be called from
                        // within foo
}

There's also a third visibility (protected), which comes into play once you start dealing with inheritance.

Answer 2

You can find ways to do it (see the other answers), but just don't do it. As you said, you are new to C++, so better learn the good practices, and when you need global objects - create them in the source (cpp) file.

Besides it, try to avoid using global objects at all, and define your objects inside the classes or functions.

Answer 3

You could also make a use of the singleton pattern, if that fits your need in this case.

For your second question. You can always make a class object as a (private) member variable of the class you want to call object.function() from.

For example:

// File a.h, includeguards etc. left out for clarity
class A {
public:
   void func();
};

// File b.h
#include "a.h"
class B {
public:
   void func();
private:
   A object;
};

// File b.c
#include "b.h"
void B::func()
{
   object.func();
}

Hope that helps.

Answer 4

It's not a good practice to put definitions in header files as this would result to the error you encountered. Although there are ways to get around this (extern, header guard), it should be avoided as much as possible. Just remember, put declarations in header files, definitions in source files.

Answer 5

About your second question note you can also call a static method using the :: operator and the class name (without an instance) :

void AnOtherClass::method()
{
    TheFirstClass::static_method();
}