Returning reference to member object

[tech]

Hi, I want to return a reference to member object
provided in class Test, so have
provided a getter function GetObj() and declared it
const. However i'm getting a
compiler error that i need to return reference to const
obj but if i do do this
i can't call non const member functions eg
GetObj().NonConstFoo()
How do i get round this problem. I woul think that
GetObj is logically const as
it does not change the object.This is on MSVC++ V8.0

class Obj
{

};

class Test
{
public:
Obj& GetObj() const
{
return m_obj;
}

private:
Obj m_obj;
};

 

[Saeed Amrollahi]

               Hi, I want to return a reference to member object
provided in class Test, so have
              provided a getter function GetObj() and declared it
const. However i'm getting a
               compiler error that i need to return reference to const
obj but if i do do this
              i can't call non const member functions eg
GetObj().NonConstFoo()
              How do i get round this problem. I woul think that
GetObj is logically const as
               it does not change the object.This is on MSVC++ V8.0

               class Obj
        {

        };

        class Test
        {
        public:
                Obj& GetObj() const
                {
                        return m_obj;
                }

        private:
                Obj m_obj;
        };

Hi
const member functions is a kind member functions that you can't
change the state of object through them.
GetObj() is const. so you can't change the state of Test object (here
is m_Obj) via it. on the other hand
it returns the reference to data member. so accord to reference
concept it is ready to change. for example
you can call a non-const member function of class Obj:
class Obj {
void Change() { // ... } // non-const
};

Test t;
t.GetObj().Change();
clearly, this is a contradiction. So before any use Test class
compiler complains.
There are two workarounds:
1. If you intend to change the m_Obj, declare GetObj() as an ordinary
member function.
2. If you intend to return a copy of m_Obj, change the return type to
Obj (return the value of m_Obj)

Good luck
Regards,
Saeed Amrollahi

 

[Krzysztof Czaiñski]

class Obj {};

class Test
{
public:

You need 2 versions of this method: for const and non-const use of the
returned reference:

const Obj& GetObj() const

{
return m_obj;
}

Obj& GetObj()

 

[Federico Zenith]

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Hi, I want to return a reference to member object provided in class Test, so have
provided a getter function GetObj() and declared it const. However i'm getting a
compiler error that i need to return reference to const obj but if i do do this
i can't call non const member functions eg GetObj().NonConstFoo()
How do i get round this problem. I woul think that GetObj is logically const as
it does not change the object.This is on MSVC++ V8.0

Nope, it is _not_ logically const, because Obj is indeed a part of Test.
If you change Obj, you change Test as well.

You either have to return a const Obj&, or you do not declare GetObj to
be const, or you make both versions.

Cheers,
- -Federico
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